View of Supply of nutrients and productive responses in dairy cows given diets based on restrictively fermented silage

Supply of nutrients and productive responses ⁱⁿ dairy

cows given diets based ^on restrictively ^fermented silage

Pekka Huhtanen

AgriculturalResearch Centreof^Finland,Animal ProductionResearch, FIN-31600Jokioinen, Finland, e-mail:pelcka.huhtanen@mtt.fi

Theobjectiveof this paper is to review research which has evaluated thefeedingofdairycows with dietscontaining large proportions of grass silage. In Finland, milk production systems evolved are based onthe use ofrestrictively fermented silages. Higher potential yields, smallerproductionrisks than with cerealgrains, shortgrazing period and high digestibilityof grasses growninnorthernlati- tudeshave facilitatedthisdevelopment.Factors affecting nutrient supplyfrom these diets are dis- cussed.Digestibility is determinedmainly bythe stage ofmaturityatharvestingand it is not marked- lyaffected by the level of energy andprotein supplementation. Intake ofgrasssilage is influenced both by digestibility and fermentation characteristics. Efficiency of microbial synthesis ishigh in animalsgivendiets basedonrestrictivelyfermentedsilagebutrumenfermentation pattern is charac- terisedby low molarproportions ofpropionate. Production responses to additional concentrateare relatively small, especiallywhen the amount of concentrate exceeds 10kg day1^.Highsubstitution of silage drymatter (DM), negativeassociative effects ondigestion andpartitioning of energy towards body tissues account for smallproduction responses. Protein supplementation hasconsistently in- creased milkprotein yield but responses do not appear to be related to the level ofmilkproduction, silagecrudeproteincontent, amount of concentrateorstage of lactation. The^newproteinevaluation systemprovides anaccuratepredictionofprotein yieldwith thetypicalFinnish dairycowdiets. The high slopes (ca. 0.5)between protein supply andmilkprotein yield withinexperimentssuggest that protein supplyissuboptimaland protein supplementsare^{used with}ahigh efficiency.

Key words: energy, milkproduction, nutrientsupply, protein,response tonutrients, silage

ntroduction

Domesticated ruminantsarewell adaptedtodif- ferent environmental conditions and diets. Milk productionsystems adopted in different climat-

ic zones employ contrasting input levels. High

concentratedietsareused in manyWesterncoun- tries,whereas dairycowrations fed in New Zea- landaremainly comprised (90%) of grazed pas- ture. In Finland grazedpasture cannotcontrib- ute more than 25-30% of annual energy intake

©Agriculturaland Food Science inFinland ManuscriptreceivedFebruary 1998

Voi7(1998):219-250

owingtoashort(100-120 days) grazing period.

Long indoor feeding period and a large year- around demand of milk products have increased the importance ofconserved forages in dairycow diets. High concentrate costsand a shortage of domestic protein supplements have favoured the development of forage-based feeding systems.

Invention of silage additives by professor A.I.

Virtanen established_a firm background for si- lage-based feeding _systems. Although he was granted a Nobel prize in 1945 for these inven- tions, silage production didnot increase in Fin- land until the

1960 s when

flail-type forage har- vestersbecame widely available. This generated anextensive research programme into grassland production, ensiling, and feeding and supple- mentation of grass silage. This research pro- gramme ledtothe development ofasilage-based

‘GreenLine’ feedingsystem,which isnowwide- ly adopted in Finland.

There are severalreasons why grass silage- based feeding ^system has been adopted in Fin- land. Higher yield potential with smaller produc- tion risks of grasses compared with cereal grains has favoured grass production. In field experi- mentsdry matter (DM)yields of timothy(Phleum pratense) and meadow fescue ⁽Festuca ^prat- ensis) in North Ostrobothnia (64°40' N) and North Savo(63°10' N)aregenerally from 9000 to 10 000 kg/ha for 1-3-year old swards(Nis- sinen and Hakkola 1995,^Rinne, K.etai. unpub- lished).At the_sameresearch stations DM yields of barleyare ca. 5000 kg/ha. Despite lowercon- tentsof metabolisable energy (ME) and higher storage losses of grass silage than with grain, feed energy yield per hectare is considerably higher from grass than from grain production, especially in the middle and northern parts of Finland. Higher crude protein ^contentof grass was also considered as an advantage when pro- tein supplementswere ^not widely available.

Winter feeding systemsbasedongrasssilage provide several advantages compared with hay- based systems. Silage making is much less de- pendentonweather conditions than hay making and harvesting losses are generally smaller for silage than hay(Pitt 1991). When conservedas

silage, the swards can be harvestedatan earlier stageof growth and therefore the nutritional val- ueof silage is higher.

Owingtothe long winter feeding period and high building costsmilk production mustbe in- tensive to be profitable.However, because of high cereal grain and other concentrate prices, intensive milk production in Finland has been basedonproduction and feeding ofhigh-quality forages rather than increasing the proportion of concentrates in the diet. This strategy has been successful since annual milk yield in controlled herds increased from 5859 kg in 1987to 7186 kg in 1997 without marked increase in the pro- portion ofconcentrates in the diet[ca.350 g (kg DM

1

^{) ].}In addition tothe high nutrientcontent of silage, agood hygienic quality has even al- lowed production of Emmenthal cheese in Finn- ish dairies from milk produced by silage-fed cows. Milk quota, adopted in Finland in 1985, should encourage farmers toreduce unitcosts ofproduction andconcentrateinputs. Incontrast, subsidy policies of the European Union (EU) have considerably reduced the relative price of concentrates in Finland. Biased price ratios of concentrates toforages in relationtotheir^natu- ral production costscould jeopardise the sustain- able milk production system in Finland. The impacts of concentrate-basedsystems onanimal welfare and environmental issues would be del- eterious.Furthermore,the image ofmilkcurrent- ly held by consumers as being natural and healthy would also be threatened, if high _con- centratediets_arefed todairy_cows.

Research interests during the last ^two dec- ades have focusedonoptimisation of forage_con- servation, determinationof nutrient supply, sup- plementation of grass silage-based ^diets, opti- misation of diet composition to improve nutri- ent utilisation and milk composition and to re- duce environmental pollution. Factors affecting nutritive value and supplementation of grass si- lage have been discussed in several reviews (Tho- mas and Thomas 1985, Thomas and Rae 1988, Lampila ^et al. 1988, Chamberlain et al. 1989, Harrison et al. ^1994, van Vuuren etal. 1995).

This paper will focus more specifically on nu- Seminar in honour

of

the 100th anniversary

of

^MTT

trient supply and response tonutrients with di- etsbasedonrestrictively fermented silages wide- ly used in Finland.

Nutrient supply ^from grass silage-based ^diets

The amount of nutrients absorbed from the di- gestive tract depends _on the contentof digesti- ble nutrients in the feed and DM intake. Most of the variation in net energy content ofafeed is associated with digestibility, while methane and urinary losses and differences in the utilisation of ME account for relatively little variation.

Digestibility

Fermentation

Chemical composition and digestibility of grass silage _are affected by maturityat harvest _more than by othermanagementfactors such asparti- cle size, DM atharvest and harvesting system.

The effect of ensiling on digestibility is often small provided that silage is not badly ferment- ed and effluent production is minimised. Well- preserved silagescanbe classifiedas high-lac- tateorrestrictively fermented silages. The former canbe produced by natural fermentation with- outthe use of additives or as aresult of using bacterial inoculants, enzymes or low rates of formicorother acid acids. Restrictively ferment- ed silagesaregenerally produced by high appli- cationrates of formic acid (FA) during ensiling.

Digestibility of high-lactate silages has beenre- ported ^tobe similar^to that of restrictively fer- mented formic FA-treated silages (Jaakkola

1992. Harrison ^et al. 1994). Huhtanen ^et al.

(1997a)and Heikkilä^etal.(1998a)reported that organicmatter (OM) digestibility in dairycows fed inoculant-treated silage washigher than that treated withFA. Alowerdigestibility of neutral detergent fibre (NDF) of FA-treated silage ap-

peared to account for this difference. Enzymes have been usedassilage additives inan attempt to supply morefermentable substrates for lactic acid bacteria. Earlier it was also assumed that enzymes would improve silage digestibility, thereby permitting later harvesting and higher DM yields without reduced digestibility. How- ever,studies reviewed by Jaakkola⁽¹⁹⁹²⁾show- ed that cell wall degrading ^enzymes did not improve OM digestibility and in fact decreased cell wall digestibility. As aresult of preferential in silo degradation of the highly digestible cell wall material by enzymes, less digestible resi- due is left for ruminal digestion. Harrison etal.

(1994)reported that enzymetreatmentimproved silage DM digestibility in one-third of thecom- parisons.

Maturity

The decline in digestibility of grass with pro- ceeding stageof growth is faster^atnorthern lat- itudes(Deinum etal. 1981). For example, in the UK the decline in OM digestibility is typically 3 g kg ¹perday whereas in Finland it is approx- imately 5 g kg

1

^perday. The faster decline in digestibility atnorthern latitudes is relatedto a morerapidstemdevelopment and^tosmaller pro- portion of leaf DM (Deinum etal. 1981).How- ever,atthesamemorphological stagethe digest-

ibility of the whole grass crop is higheratnorth- ern latitudes. Lower temperature, long day and high solar radiation wouldpromote DM accu- mulation, without much effecton ageing and digestibility(Van Soest^etal. 1978).At50% and 70% contentof leaf sheaths and stems in vitro digestibility of timothy was 80 and 170 g kg’

1

higher in Tromsö (69°N) than in Wageningen (52°N) (Deinum et al. 1981).

Lignification in the cell wall fraction increases with plant maturity, which reduces its digestibil- ity and, consequently, energy value of grasses.

The lowertemperaturereduces lignification, and therefore the digestibility of plant material is maintainedatahigh level(700-750 gkg’

1

^)even

when the NDFcontent is ca. 600 g (kg DM)'1 .

Themeandecline of digestibility in four studies conducted at 60-61 °N with first-cut silages Vol.7^(1998): 219-250.

was 5.4 g kg"

1

^{perday (Fig. 1).Therate of de-}

cline isnotalways linear being generally slower

atthe beginning of the growingseasonwhen the rate of DM accumulation is slow. The fastest daily declines in OM digestibility of 8 to 10 g kg"

1

^perday often occurred during therecommen- ded harvesting time. OM digestibility ^(OMD) was positively correlated with silage crude pro- tein (CP) content and negatively with NDF in four studies using 18 silages:

OMD (g kg

1

^{) = 532 +} 1.393 xCP (g kg

1

⁾

(R²⁼0.696; SEest.32.8) (1) OMD (g kg"

1

^{) = 1115 -} 0.669 x NDF (g kg ^') (R²⁼0.744; SE_est.30.1) (2) Although within studies the relationship be- tween digestibility and NDFor CPcontentwas relatively _strong, prediction of silage D-value [g digestible OM (kg DM)'] from chemicalcom- position during harvesting time will leadto sub- stantial errors. Prediction errors of32.8 and 30.1 g kg ^'are unacceptable for ration formula- tion. In practice, prediction errors arelikelytobe muchgreatersince N fertilisation and otherman- agementfactorsarestandardisedbetter in research conditions. Improved methods for predicting silage D-value than that currently adopted based on grass CP and crude fibre contentareurgently required. It is essential that silage D-valuecan be accurately predicted from_astanding crop in^or- dertoreachan optimal balance between silage quality and DM yield. This is especially impor- tant at northern latitudes where the rates of changes in both DM yield and digestibility are very fast for first-cut swards. Surprisingly, the present data indicated that silage OMD could be more accurately predicted by number of days elapsed from May 1 (R²⁼0.845)than by chem- ical composition.

Total diet OM digestibility of grass silage- based diets can exceed 800 g kg'

1

^{in growing}

cattle fedatthe level between70to80 g DM per kg live weight(LW)⁰⁷⁵(Table 1).The proportion of grass silagewas 700 g (kg ^DM)'

1

and that of barley-rapeseed mixture(800:200) 300 g (kg DM) ^1. Digestibility of cell wall carbohydrates

was also very high indicating a high potential NDF digestibility. When determined by 12 day ruminal in situ incubation,potential NDF digest- ibility of grasscut atthe recommended _stage of maturity for silagecanexceed900 g kg ^'(Rinne etal. 1997, Huhtanen and Vanhatalo 1997).The decline in potential digestibility of^NDFis slow at the beginning of the growing season but ac- celerates later (Huhtanen and Jaakkola 1994, Huhtanen and Vanhatalo 1997). In contrast,the rate of digestion declines faster ^atearly stages

of the growing season.

Supplementation

Owingtothe high potential digestibility of grass silage and negative associative effectson diges- tion (refertoMould 1988),OM digestibility of the total diet is not affected by the amount of

concentratesupplements in dairycows(Table 2).

Onlyat alow level of supplementation did OM digestibility increase _asexpected, but _at higher levels of supplementation OM and especially NDF digestibility slightly decreased when the amount ofconcentrate was increased. Reduced cell wall digestibility is partly due to adverse effects of lowrumenpHon cellulolysis and_part- ly to the intrinsic characteristic of silage and concentrate NDF the former generally being moredigestible. Asaresult of negative associa- tive effectson digestion, the increase in ME in- take calculated from digestible OM ^(DOM) in- take was markedly lower than that estimated from feed table values (Tuori etal. 1996).The ME contentof incremental increases in DM in- take was 10.6 MJ kg"

1

^, i.e. only 73.5% of the predicted increase[14.5 MJ (kg^DM)'

I

^].Exclud-

ing the study using 3 and 6 kg ofconcentrate from thedata,the ME contentof incremental DM intake was only 9.8 MJ (kg DM) ' or68.4% of predicted increases.

Protein supplementation has often increased total diet digestibility, although dietary CP con- tenthas exceeded the level consideredtobe op- timal for ruminal cell wall digestion (Oldham

1984). Supplementation with soya bean meal (SBM) increased total diet DM digestibility by 0.25 g kg'

1

^per 1 g increase in concentrate CP Seminar in honour

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^{the 100th}anniversary

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^MIT

content (Gordon etal. 1981),which corresponds

to0.60 g kg ^'per 1 g increase in dietary CPcon- tent.In^contrast, supplementation of diets based on grass silage and cereal grains with rapeseed meal(RSM), which is the^most common dairy cowprotein supplement inFinland, had _no ef-

fect ontotal OM(0.723 vs. 0.725) orNDF di- gestibility (0.635vs.^0.643) in 13 comparisons.

Since digestibility of RSM is lower than that of SBM, smaller responses to RSM thanto SBM supplementation may merely reflect differences in protein supplement digestibility. Improved

Table 1.Digestibility of grasssilage-based(700 gsilage DM kg 1total DM) dietsincattle.

Ref. Composition(gkg^‘)

CP NDF

Digestibility (gkg 1⁾

OM NDF Cellulose

Khalili and Huhtanen 1991 Jaakkola and Huhtanen 1992 Rinne et al. 1997

183 501 813 774 846

179 484

187 409

837 824 869

409 821 757 818

Rinne et al. 1997 167 497 816 765 821

DM^-drymatter CP⁼crudeprotein

NDF⁼neutraldetergentfibre OM⁼organic matter

Fig. 1.Relationshipbetween silage organic matter (OM) digestibility in sheepwithdays afterMay I,crudeprotein and neutraldetergentfibre (NDF)content. [Data from stud- ies Rinne et^al, 1995(�), 1997(■),unpublished (�) and Tuon et al. 1992^(□)].

Vol. 7(1998): 219-250.

Table2,Effects of the amount of concentrateondietdigestibilityand marginalincreasesin ME intake.

Ref.¹ Concentrate(kg day^{') OM}dig(gkg ^') NDFdig(gkg ^') AME/ %of

Low High Low High Change⁴ Low High Change⁴ ADM1²expected⁵

(1) 3 6 684 699 5.0 658 635 -7.7 14.06 93.7

(2) 7 10 763 760 -1.0 715 693 -7.3 10.16 75.4

(3) 7 10 715 712 -1.0 665 629 -12.0 9.78 76.5

(4) 9 15 735 725 -1.7 650 593 -9.5 10.67 68.3

(5) 6 12 746 725 -3.5 670 589 -13.5 8.42 53.5

Mean⁵ 729 724 -0.43 672 628 -10.00 10.62 73.5

Mean⁶ 740 731 -1.79 675 626 -10.58 9.76 68.4

1 References: Saarisalo et al. 1997(1), Rinne et ai.1995(2), Shingfield et ai. unpublished (3), Heikkilä1994(3)^?Heikkilä et ai. 1998 b⁽⁵⁾

Change in MEintake (MJ) per kg increase intotalDMintake

3 Change in MEintake (MJ) perkgincreaseintotalDMintakeas%ofexpectedincrease calculated from feed table values ordigestibility measurementdetermined insheep

4 ChangeperkgincreaseinconcentrateDMintake

5 Mean of all five studies

6 Mean of studies exluding study⁽¹⁾inwhichasmaller amount of concentratewasused OM⁼organicmatter

NDF⁼neutraldetergentfibre ME⁼metabolisable energy DMI⁼drymatterintake

OM digestibility with increasing level of fish meal inclusion(Heikkilä etal. 1998^a) is in line with this suggestion.

Feed intake

Feeding levels ofhigh producing dairycows are 3-4 times higher than required formaintenance, i.e. much higher than that ofother domesticated animals used for food production. Animal per- formance is determined mainly by intake of di- gestible energy(DE) and nutrients. A large pro- portion of the variation (60-90%) in DE intake between animals and feedscanbe accounted for by differences in feed intake. Only 10%_to 40%

of this variability is related todifferences in di- gestibility (see Mertens 1994). Feed intake is regulated by physical and physiological mecha- nisms. Physiological regulation, often called metabolic regulation,operates when animalsare fed highly digestible, palatable and low-fill diets and intake is limited by energy demand of animals. Physical factors regulate intake when

the intake is limited by the physical capacity of the digestivetract (Mertens 1994).

Digestibility

Physical limitation of reticulo-rumen has gener- ally been accepted _tobe the major factor limit- ing intake of forage diets. Thomas and Cham- berlain (1982) suggested that physical factors regulate the intake of well-preserved highly di- gestible grass silages. The decrease in silage^DM intake when water-filled bladderswere inserted into the_rumen(Parhan and Thomas 1978) sup- portsthis hypothesis.However, morerecentdata fromrumen evacuation studies(Gasa et

al.

^1991,

Bosch etal. 1992, Rinneetal. 1996)suggestthat factors other thanrumenfill limit the intake of early-cut highly digestible silages. Rumen fill in terms of total digesta, DM orNDFwas smaller forcowsfed early-cut than for those fed late-cut silage. Using digestion and passage kinetic pa- rameters and rumen pool size of NDF derived from the study of Rinne^etal. (1996) inamech- anistic rumen model (Huhtanen, unpublished) predicted that the intake potential of high digest- Seminar in honour

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the I OOth anniversary

ofMTT

ibility silages(D-values730 and 737 g DOM (kg DM)

1

^were 3.2 and 3.6 kg day ¹ higher than measured values in lactating dairy cows.Simu- lations_werebased_onthe assumption that intake of late-cut silage (D-value 637) waslimited by rumen NDF fill. Analysis of data from produc- tion trials carried out at the University of Hel- sinki from 1985^to 1993 alsosuggeststhatener- gy demandwas more likelytobe limiting total DM intake thanrumenfill. In all studies (47 di- ets),grasssilage_wasfed ad libitum andconcen- tratescomprised approximately 400 g kg' of the total DM. Regression equations indicate that in- creased DM intake provided almost all theextra energyrequired for higher milk production.

DM intake (kg) ⁼-9.68 ⁺0.0307 x LW (kg) ⁺ 0.407 x milk (kg)^(R2=0.901)

DM intake (kg) ⁼-10.10 ⁺0.0300 xLW(kg) ⁺ 0.421 x ECM (kg) (R²⁼0.929)

In addition large increases in silage and total DM intake in feeding experiments during thepast 20 years clearly suggestthat the intake of grass silage-based diets has notbeen limited by ru- men fill. For example, data of Heikkilä etal.

(1998 b) indicated that the highest mean silage DM intake of 16.0 kg day

1

^{with6 kg day}

1

^of

concentratewashigher than the total DM intake (13.9 kg day¹⁾ in studies carried out in 1970- 1975(Ettala etal. 1978).Datafrom milk record- ing from practical farms also indicate that mean milk yields exceeding 9000 to 10 000 kg year

1

from diets containing 35% of totalDMintakeas concentrates (Kaustell,K., unpublished).

Intake potential of grass silage is determined by intrinsic characteristics of the grass ensiled.

Silage DM intake is positively related^todigest- ibility, although this relationship is notalways soclearaswith dried forages. Themeanincrease in silage DM intakewas 0.16 kg^(SE0.018) per 10 g kg

1

increase in D-value (Harrisonetal._un- published). Although the regression equation of Rook etal. (1991) suggested that the relation- ship between silage D-value and DM intake is curvilinear, other published studies (e.g. Rinne etal. 1995)indicate that this relationship is lin-

ear up to 740 g kg

1

^.The large variation in the effects of silage D-valueonintakecanoften be associated with differences in silage fermenta- tion characteristics. Early-cut grasses are often moredifficultto ensile duetoahigher buffering capacity, and therefore the higher intakepoten- tial ofan early-cut silage isnot always realised due topoor fermentation quality. Improved di- gestibility and enhanced intake lead to an in- crease in total ME intake byca. 0.3 MJ per 1 g kg'

1

increase in D-value. In addition^toincreased ME intake, utilisation of ME for milk produc- tion andenergy retention have been reportedto be higher with early-cut than late-cut silage (Gor- don _etal. 1995).

Silage

fermentation

^quality

In addition_tophysical and physiologicalfactors, psychogenic modifiers suchas taste, smell, ^tex- ture and visual appeal caninfluence feed intake (Mertens 1994).Modifications of carbohydrate and nitrogen fractions during ensiling produce fermentation end-products which decrease silage DM intake compared with fresh forages. Depres- sions in intake have been highly variable ^(1- 64%) in sheep (Demarquilly 1973). However, providing that the quality of fermentation is good, the effects of ensilingonintakearegener- ally very small in cattle. In two recent studies (Keady and Murphy 1993) and Cushanan and Mayne(1995)the intake of FA-treated silagewas not significantly lower than the intake of fresh forage. A review of Finnish and Swedish studies comparing barn-dried hay and FA-treated silage made from thesame sward indicatedno differ- ence in forage DM intake (Huhtanen

1993

^b).

Variable effects of ensiling on forage DM intakearemostlikelytobe relatedto extentand typeof in silo fermentation. Earlier studies based on correlations calculated from individual ob-

servations (Wilkins et al. 1971, Ettala and Lampila 1978)clearly demonstrate the adverse effects of fermentation end-productsonintake.

Analyses of the Agricultural Research Centre of Finland(MTT) data by Huhtanen^(1993b) using 49 treatmentmeans from 14 dairy cow studies indicated that withinanexperimentthe totalacid Vol. 7(1998): 219-250.

contentwas the best predictor of silage DM in- take. The effects of lacticacid, volatile fattyac- ids (VFA) and ammoniaon silage DM intake were curvilinear. At low concentrations, lactate had _a positive effect _on silage intake. This is probably related tothe secondary fermentation ofsomelow lactate silagesasindicated by high concentrations of ammonia and VFA. At higher concentrations,lactic acid hada negative effect onsilage DM intake but there is little evidence that lactic acid per se limits intake since con- centrations of ammonia and VFA are often in- creased in extensively fermented silages. Recent studies with wilted silage (Jaakkolaetal. 1995, Heikkiläetal. 1998^a)suggestthat the effects of lactic acid_aresmall provided that secondary fer- mentation and proteolysis are prevented. The effects of ammonia and VFA on silage DM in- take appeartobe curvilinear withgreaterdepres- sions occurringatlow concentrations (Huhtanen

1993

^{b). Using a}larger datasetof250 treatment means,Khalili^etal. (unpublished) derived sim- ilar relationships between silage fermentation end-products and intake. Use of this larger data- setindicated that the linear effects of total acids and ammonia caused depressions in intake of 13.7gDM per 1g(kg DM)

1

^and 14.3 gDM per 1 g (kg ^N)

1

^.These values_arelower thanrespect- ive values of24.7 g per g (kg DM) ¹and 18.5 g DM per 1 g (kg ^N)'1 derived exclusively from Finnish data. The adverse effect of lactate on silage DM intakewas smaller than that of VFA when both independent variableswere used in thesame model(11.9and 21.4 g).

The mechanism of silage DM intake depres- sion with increasing concentrations of fermen- tation acids is notwell understood. Reduced in- take may simply be duetopsychogenic factors, i.e. fermentation products, which impair silage

‘palatability’. Silage fermentation products are also potential feed-back controllers of physio- logical intake regulation. Badly fermented si- lages decrease rumen motility (Clancy et al.

1977),which will induce a greater rumenfill at thesamelevel of intake. GreaterrumenNDF fill withoneof the enzyme-treated silages than with FA-treated silage (Jaakkolaetal. 1991)supports

this suggestion. Higher intake ofrestrictively fer- mented than of extensively and/or badly ferment- ed silage may also be associated with improved post-ruminal balance of amino acidsto energy as aresult of_moreefficient of_rumenmicrobial N synthesis in therumen.Positive effects of RSM supplementation and a greatereffect ofpostru- minal than ruminal casein infusion (Khalili et al.

1995

^{a) tendtosupport}this suggestion. How- ever,the adverse effects of lactic acid onsilage intake have notbeen overcome by postruminal casein infusion (Choung and Chamberlain

1993

^{a) or}by fish meal supplementation (Heik- kilä^etal. 1998^a).

Supplementation

Grass silage is in ^mostcasesfed with energy and protein supplements. Energy supplements de- crease silage DM intake but generally increase total DM intake. Ettala and Lampila (1978) re- portedadecrease of0.64 kg in silage DM intake perkg increase inconcentrateDMintake witha meanbarley DM intake of2.1 kg. Inaliterature review (Ryhänen etal. 1996)involving 121es-

timates of DM substitutionrates (SR; decrease in silage DM intake per increase in concentrate DM intake) themean SR_was 0.39 with _a high standard deviation (SD; 0.30). In recent studies

atMTT mean SR in 16 comparisons was 0.53 (SD0.12) when theamount ofconcentrate var- ied between 3 and 15 kg day^{1 .} Generally, SR tends _toincrease with increases in concentrate intake,forage digestibility and silage fermenta- tion quality. MTT data indicated that SR was 0.51 when the highestamountofconcentratedid notexceed 10 kg day¹and0.61 when it exceed- ed 10 kg. The effects of silage D-value are not always consistent. Rinneetal. (1995) observed that SR was greatest with the lowest D-value si- lage. In arecent study, Shingfieldetal. (unpub- lished)obtained SR values of 0.34,0.51 and 0.41 foruntreated,formic acid-treated and inoculated

silage demonstrating the importance of silage fermentation.

IncreasingconcentrateCP contentenhances silage DM intake. In studies with RSM, Tuori (1992) reporteda meanincrease of0.027 kg si- Seminar in honour

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^{the 100th}anniversary

ofMTT

lage DM per g kg ^'additional CP in the diet. In later studies reviewed for this paper, thecorre- sponding response was0.029. These values are greaterthan themeanresponse of 0.012 observed for fish meal supplementation^(Huhtanen 1993

a,

Heikkiläet al.

1998

a) but similar to those_re- ported for SBM for supplementation of grass si- lage and barley diets(Chamberlain etal. 1989).

Marginal increases in ME intake are approxi- mately 0.30 MJ per 1 g kg*

1

rise in dietary CP content. Increases in silage DM intake caused by protein supplementation have been suggest- edto result from a more efficient ruminal cell wall digestion (Oldham 1984) when the intake is limited by fill. However, asdiscussed before the effects of RSM on NDF digestibility are marginal atbest, and cannot therefore explain increased silage intake. Greater intake responses would be expected with late-cut or low CP silages because on such diets the supply ofru- men-degradable N ismore likely tolimit diges- tion.However,Rinneetal.(1995) noted that im- provements in silage DM intake with RSM tend- ed^tobe greater with early-cut than late-cut si- lages (0.75, 1.09,0.32 and 0.49 kg day

1

^{for si-}

lages with D-values of 739, 730, 707 and 639, respectively). Duodenal casein infusion has been shown toincrease silage DM intake more than ruminal infusion (0.3 vs.0.6 kg day^{1 )} suggest- ing that post-ruminal metabolism may also in- fluence DMintake(Khalili ^etal. 1995^a).Rumen NDF fill was increased by duodenal infusion suggesting thatrumen fill was not limiting in-

take during ruminal infusionsorthatcows were able to increase capacity when the balance be- tweenamino acids and energy was improved.

Cows given duodenal casein infusion also in- creased total chewing time despite spending 930 min day¹chewing_oncontrol diets of silage alone (Khalili etal. 1995^b). Small increases in silage DM intake with ruminal casein infusion were associated with _afasterrateof ruminal NDF di- gestion.

Rumen fermentation

The rumen fermentation pattern in dairy cows given diets based _on restrictively fermented silage is characterised by _asmall molar propor- tion of propionate and high molar propor- tions of acetate and/or butyrate in total VFA (Table 3). The ratios ofacetate topropionate and that of acetate plus butyrate to propionate are very high, which may explain the high Finnish milk fatcontent (44gkg'^1).Variation in fermen- tation parameters is small supporting the sug- gestion of Chamberlain and Choung (1993) that with grass silage-based dietsrumen fermenta- tionpattern is mainly controlled by silage type.

It is also noteworthy that despite high feeding levels of dairy cows, the molar proportion of propionate is not higher than that in growing cattle fed similar dietsatalower level of intake (e.g. Jaakkola and Huhtanen 1993, Jaakkolaet al. 1993, Rinne ^etal. 1997).

Table3.Pattern ofrumen fermentation of dairy cowsgivendiets based onrestrictively fermented grass silage(data derived from Finnish studies).

N Mean SD Maximum Minimum

Acetate (mmol mol') 34 665 20.2 696 624

Propionate(mmol mol ^') 34 165 10.9 187 142

Butyrate(mmol mol 1^{) 34 133 10.7 158 113}

A/P1 34 4.05 4.89 3.53 0.36

(A+B)/P² 34 4.85 0.38 5.73 4.23

1^{Acetate/Propionate}

2(Acetate⁺Butyrate)^/Propionate

Vol. 7(1998): 219-250.

Manipulating the proportions ofrumenVFA in animals fed diets based onrestrictively fer- mented grass silage is difficult. According to review of Huhtanen (1987a), increasing the pro- portion ofbarley-basedconcentratefrom0to750 g(kgDM)

1

^{did not}affect the proportion of pro- pionate in_{rumen VFA,} butacetatedecreased and butyrate increased with supplementation level.

Evenatvery high levels ofsupplementation [750 g(kg DM)'] molar proportions of propionate did not increase(Thomas etal. 1980, Jaakkola and Huhtanen 1993). Elevated butyrate with in- creasedconcentratemay be relatedtohigher_ru- men protozoal numbers (Chamberlain ^et al.

1983. Jaakkola and Huhtanen 1993).

Replacing barley with unmolassed sugar beet pulp (Huhtanen 1988, Murphy etal. 1993) or barley fibre (Huhtanen 1992) and sometimes delaying the harvesting of grass for silage(Rinne et al. 1996, 1997) have increased propionate mainly at the expense of butyrate. These re- sponses ^arerather unexpected since propionate tendedtoincrease with dietary NDF content. In addition, these responsesare, atleast partly, as- sociated with increases in protozoal number with diets containing less NDF. Replacing barley with oats (Vanhatalo etal.

1995

^a)or including RSM orSBM in the diet^(Aronenand Vanhatalo 1992, Tuori etal. 1993)had no effectson rumen fer- mentationpattern, whereas large inclusions of rapeseed oil markedly enhanced ruminal propi- onate(Tesfa 1993), probablyas aresult of par- tialrumendefaunation.

Rumen fermentationpattern isgreatly influ- enced by the extent of silage fermentation. A review of data from studies conductedoncattle showed_aclose positive relationship between the silage lactic acid concentration and the molar proportion of propionate in rumen VFA and a negative relationship between silage lactate and the lipogenic ^to glucogenic ratio in _rumenVFA (van Vuuren etal. 1995). Similar relationships were found in sheep(Martin, P.A. etal. 1994).

In their study, differences between silages in ru- menfermentationpattern weremaintained irre- spectiveconcentratecomposition. The effects of lactateon ruminal propionate have been very

consistent but water soluble carbohydrates (WSC) in restrictively fermented silages have sometimes increased acetate (Vanhatalo et al.

1992, Huhtanen et al.

1997

a) and sometimes butyrate(Jaakkola etal. 1991, 1993). Therea- sons for different fermentation_pattern of WSC are ^not known but it may be related to rumen methanogenic capacity, since contrary tobu- tyrateproduction methanogenesis isaprerequi- site ofacetate (Demeyer and Van Nevel 1975).

The effects of silage lactate and WSC on rumen fermentationpatterns have been consistent with those observed when supplements of lactate (Jaakkola and Huhtanen 1992)or sucrose (Syr- jälä 1972, Khalili and Huhtanen 1991)weregiv- entoanimals fed silage-based diets.

Protein supply

Protein requirements of ruminant animals are metfrom^twosources,microbial protein synthe- sised in therumenand feed protein escapingru- men degradation. With grass silage and cereal grain-based diets microbial protein is the major source of amino acids absorbed from the small intestine, since CP of silage, barley and oat is rapidly and extensively degraded in the rumen.

Despite larger contribution of microbial protein to the total supply of amino acids than that of undegraded feed protein, the main focus ofpro-

tein evaluation has beenon estimating ruminal protein degradability and intestinal digestibility of undegraded feed protein. This is probably due to nylon bag and mobile bag methods being handy toolstoestimate theseparameters. Stud- ies of Varvikko(1986)and Vanhatalo(1995)have shown several problems with these methodolo- gies. Less attention is paid to determining mi- crobial protein production and factors influenc- ing the efficiency of microbial protein synthesis (MPS),probably because the methods available for determining MPSaremuchmorecomplicat- ed, labour intensive and expensive. However, unlesswe canpredict differences in the efficien- cy ofMPS, limited real progress in protein ra- tioning systems can be expected.

Seminar in honourofthe 100th anniversary

of

^MTT

It has often been claimed that the efficiency of MPS is lower in animals given silage diets than those fed fresh or dried forage, although there is little direct evidenceto support this.

Threereasons have been suggested for the low- erefficiency of MPS with silage diets:a lower energy supply forrumen microbes from silage fermentation products than fromWSC; anasyn-

chronous release of energy and N from silage;

and thenatureof silage nitrogenous constituents (Thomas and Thomas 1985).Our studiesexam- ining the effects of silage additives have con- firmed that the reduced supply of energy from silage fermentation products is the mainreason for low efficiencies of MPS. Jaakkola ^et al.

(1991) reported a lower efficiency of MPS for diets based on untreated^orenzyme-treated si- lage than that based onformic acid treated si- lage. Gradual increase in FA application from 0 to6 litres pertonnereduced theextentof in silo fermentation and proteolysis, which clearly in- creased microbial protein production from 49.0 to65.4 g N day

1

(Jaakkola etal. 1993). In a re- centstudy by Huhtanenetal.(1997a), microbi- al protein production was 10%greater ⁱⁿcows given FA-treated silage than in those given in- oculated silage. Intraruminal infusion ofincreas- ing amounts of lactic acid did not affect MPS (Jaakkola and Huhtanen 1992) indicating that lactic acid is of littleor no value as an energy sourceforrumen microbes, which is in accord- ance with the theoretical considerations of Chamberlain(1987).

Jaakkola and Huhtanen (1992) compared the effects of barn-dried hay and FA-treated silage each fed with three levels of barley-based ^sup- plementon ruminal N metabolism. Efficiency of

MPS 117.3vs. 14.8 g N (kg^DOM)

1

^{] wassignif-}

icantly higher with silage than hay diets. Duo- denal feed protein flow tendedtobegreaterwith haydiets,which partially compensated for low- erMPS.In three feeding experimentscowsgiv- ensilage-based diets producedonaverage 4.8%

more milk protein than those given barn-dried hay made from thesame grass sward(Heikkilä 1993).Both studies demonstrate that the protein value of restrictively fermented silage isnot in-

ferior tobarn-dried hay. Suggestions of lower protein value of ensiled forages apply only to extensively and/or badly fermented silages.

SilageN is extensively degraded in the silo topeptides, amino acids and ammonia. Soluble N in silage is rapidly degraded in the^rumen, whereas the^amountof energy available is limit- ed because of WSC fermentation. Therefore it waslogical to conclude that asynchronous sup- plies of energy and Ncanlimit the efficiency of MPS with silage diets. Syrjälä (1972) found that sucrosesupplementationwas moreefficient than starch orcellulose in decreasingrumen ammo- nia concentration in sheep and attributed this^to afaster release of energy from sucrose. Cham- berlainetal. (1985) suggested that increases in rumen protozoal population and consequently, increased intraruminal recycling of N explains the less efficient utilisation of starchy supple- mentsfor ruminal MPS. Khalili and Huhtanen (1991) showed thatsucrose supplementationwas effective in decreasing rumen ammonia N and increasing MPS,but continuous infusion ofsu- crose stimulated MPS more than twice-daily feeding, despite the latter providing a moresyn- chronous supply of energy and N. Feeding su- crosetwice daily rapidly reducedrumenpH and increased lactic acid concentration. At low pH

‘energy spilling’ increases because ^rumenATP is used tomaintain intracellular pH ^atahigher level than in rumen fluid (Strobel and Russel 1986).Inaddition,theacrylate pathway provides much less energy for microbes than other path- ways.Further, depression in cell wall digestion caused by a low mean pH decreases microbe energysupply. Chamberlain and Choung (1995) discussed the importance of synchronisation of energy andN release and concluded that there is no convincing evidence ofaneed for close syn- chronisation of energy and N release in theru- men^toensureefficient MPS. The best approach might be to attempt toachieve an even pattern ofrumen energy supply (Henningetal. 1993).

In the data reviewed by Chamberlain and Choung(1995)the increase inMPSof22 gkg '

for carbohydrate supplements indicated that sug- ars had no additional value compared with mi- Vol. 7(1998):219-250.

crobial protein production from the basal diet.

However,the increase in theamountoffermenta- ble substrate is overestimated owingto adverse effects of sugar supplements onruminal diges- tion of the basal diet. The effects of sugar sup- plementation in the form of molasses have been disappointing both in lactating dairycows(Huh- tanen

1987

b) and growing cattle (Huhtanen and Hissa 1996). From the previous discussion itcan be concluded that asynchronous supply ofener- gy andN from silage doesnotlimit the efficien- cy of MPS.

Rumen ammonia concentrations of animals fed twice dailyatrestricted levelscanbe subop- timal for optimal microbial growth for extended periods of time. Therefore using protein supple- ments may improve the efficiency of MPS by providinga moreconstantsupply of nitrogen for rumenmicrobes.However, the effects of protein supplementation on ruminal N metabolism are not conclusive. Substantial increases in rumen MPS with protein supplementation(Rooke ^etal.

1987, Rooke and Armstrong 1989)support the suggestion that when grass silage is supplement- ed with easily degradable carbohydrates, micro- bial growth is stimulated by additional amino acids and peptides. Incontrast,RSM supplemen- tation did not increase MPS in growing cattle (Aronen and Vanhatalo 1992)orlactating dairy cows (Ahvenjärvi _etal. 1997). Bacteria digest- ing easily degradable carbohydrates in particu- lar areconsidered_tobenefit from N in the form of amino acids and peptides (Russel etal. 1992).

Increased MPS in response toruminal casein infusion in cows given _a diet based onred clo- ver-grass silage as a sole feed (Khalili et al.

1995^a)suggests that undersome circumstances bacteria using forage carbohydrates can also benefit from supplementary high quality protein.

Variable responses of protein supplements on ruminal MPS may be related tothe form of sol- uble^Nin silage. For example, Jacobs and McAl- lan (1992) founda greaterincrease in MPS with supplementary RSM with enzyme- than FA- treated silage. The proportion ofpeptides is much higher in FA-treated than untreated orinoculat- ed silage (Nsereko 1996). High marginal MPS

(38-58 gkg’¹fermentableOM)responses tore- stricting in silo fermentation(Jaakkola et al.

1993) suggest that improvements of MPS as- sociated with restriction of fermentation proba- bly involve changes in N fractions_aswell_asin- creasesin the supply of sugars. These valuesare considerablygreaterthan those reported for sup- plementary carbohydrates (Chamberlain and Choung 1995). However,when proteolysis was notincreased in extensively fermented silage, the decrease in microbial Noutputcould be entirely explained by decreases in fermentable OM (Huh- tanen etal.

1997

^a).

Itseems that the efficiency of MPS is high- estwhen moderateamounts ofconcentrates are fed (Jaakkola and Huhtanen 1993). Increasing the proportion of concentrate in the diet from 250to500 g kg’

1

^{enhancedMPS, whereasafur-}

ther increase to750 g kg

1

decreased it slightly.

When moderateamountsofconcentrates are fed, differentsources of carbohydrates provide a more^constantsupply of energy maintainingru- menpHatoptimal levels and avoiding wide fluc- tuations. The effects of the composition ofener- gysupplementsonMPS have been small. There was nodifference between barley and unmolas- sed sugar beet pulp^(SBP)in their effectsonMPS and neitherpartial replacement of barley orSBP with sugar beet molasses didnotinfluence MPS (Huhtanen 1988). Barley and_oats had _asimilar effect_onMPS when fed either with grassorred clover silage (Vanhatalo etal. 1995^b).Replace-

ment of barley with barley fibre decreased ru- menprotozoal number and increased the efficien- cy of MPS but not the duodenal flow of micro- bial protein (Huhtanen 1992).Similarly, supple- mentation of grass silage and barley diet with rapeseed oil decreasedrumen protozoa and in- creased the efficiency of MPS but microbial N flowtoduodenumwasnotincreased, duetore- duced ruminal OM digestibility (Tesfa 1993).

The results of thesetwo studies imply that im- proved efficiency of MPS cannotnecessarily be interpretedas increasing the supply of amino acids when it is related to reduced OM diges- tion in therumen.

Replacement ofbarley with fibrous by-prod- Seminar in honour

of

the I OOth anniversary

ofMTT

ucts such as unmolassed sugar beet pulp^(Huh- tanen 1988, Murphy etal. 1993)and barley fi- bre (Huhtanen 1992) increased the flow of feed protein escaping ruminal degradation. Howev- er, the quantity of amino acids absorbed from the small intestine was markedly changed due to higher faecal N output with fibrous supple- ments. RSM supplementation had noinfluence on microbial N flow in dairy^cows, but the amountof feed protein escaping ruminal degra- dation increased significantly (Ahvenjärvi etal.

1997). Heat-moisture treated RSM did not in- creaseeither feedortotal N flow compared with untreated RSM despitea large difference in ru- minal N degradability determined by in situ in- cubations.

Response ^{to nutrients}

Energy supply

Rationing system used by the Finnish farmers advisory service proceeds from a desired milk production. This is used to calculate the feed input required ^to meet corresponding require- ments. Generally, fixed and often restricted amountsof silage arefed while energy and pro- tein supplements allocated according to milk yield. To optimise the profitability of milk pro- duction it is importanttoknow the relationship between marginal changes in input and induced changes in animal performance.

Increasing the amount ofconcentrate in the diet is the most common strategy ^to increase energy intake and energy density of the diet.

Marginal responsestoincreasedconcentratesup- ply and incremental ME differ considerably from theoretical values calculated from changes in input and average efficiencies of energy utilisa- tion. In alarge datasetincluding 121 compari- sons milk yield increased by 0.77 kg per 1 kg DM extra concentrate (Ryhänen et al. 1996).

However, the responseswerehighly variable(SD

=0.54) but markedly consistent for continuous

and change-over experiments (0.84 vs. 0.77).

Marginal responses ^to additional concentrate decreased with the level of supplementation be- ing 0.94, 0.80 and 0.60 when daily concentrate DM allowance was below 5.0, from5.0 to 7.3 and above7.3 kg, respectively. However, mar- ginal responses of 0.10 and 0.09 kg of milk and energy correctedmilk (ECM)per MJ additional ME areconsiderably lower than 0.194 kg ECM predicted from theoretical approaches (Tuoriet al. 1996).

Marginal responses to increased amount of concentrate wereevaluated from fiverecent stud- ies conducted atMTT(Heikkilä 1994, Rinneet

al. 1995, Saarisalo et al. 1997, Heikkilä et al.

1998^{b, Shingfieldet}al. unpublished). Data in- cluded 16comparisons. Otherfactors, precision- chopped_vsround- baled silages(Heikkilä 1994),

silage digestibility(Rinne etal. 1995),level of RSM supplementation (Saarisalo et al. 1997), forage preservation method and silage fermen- tation (Shingfieldetal. unpublished) and spring- and autumn-harvested silage (Heikkilä et al.

1998

^b),were also assessed. In allstudies, total diet OM digestibilitywasdetermined using acid insoluble ash as an internal marker. ME intake wasestimated using feed table values andasthe product of 16(MJkg^')x⁽DOM (kg). Datawere further divided into low and high concentrate levels the latter including studies in whichcon- centratesupplementationwas 12or 15 kg day¹ (Heikkilä 1994,Heikkiläetal. 1998^{b).The other} studies had concentratelevels of3 to 6 and 7to

10 kg day^{1 .}

The effects ofconcentrate supplementation onfeedintake, milk yield and milk composition areshown in Table4. Themeansubstitution^rate was 0.53 (SD 0.12) and it was higher(0.61 vs.

0.51) at higherconcentrate levels. Substitution rate in this dataset was higher than themean value of 0.39 basedonthe data of Ryhänen ^etal.

(1996), which may also explain smaller produc- tion responses [0.56 (SD0.19) _vs. 0.77 kg milk per 1 kg extra concentrate DM]. Milk and ECM yield responses were lower ^at high (0.45 and 0.43) than low levels ofconcentrate (0.60 and 0.68), In earlier studies(Ettala etal. 1978)the Vol.7^(1998): 219-250.