Feeding diets varying in forage proportion and particle length to lactating dairy cows: I. Effects on ruminal pH and fermentation, microbial protein synthesis, digestibility, and milk production

摘要

Physically effective neutral detergent fiber (peNDF)content of dairy cow diets was modified by varyingthe theoretical chop length of alfalfa silage and forage:concentrate (F:C) ratio, and effects on nutrient intakes,ruminal fermentation, site and extent of digestion,microbial protein synthesis, and milk production wereevaluated. Estimates of dietary peNDF contents werecompared with recommendations, and predictions ofruminal pH from peNDF and the recently developedphysically adjusted neutral detergent fiber (paNDF)system were compared with observed pH. The experimentwas designed as a triple 4 × 4 Latin square using12 mid-lactating dairy cows with 4 intact, 4 ruminallycannulated, and 4 ruminally and duodenally cannulatedcows. Site and extent of digestion and microbial proteinsynthesis were measured in a single 4 × 4 Latin square.Treatments were a 2 × 2 factorial arrangement; 2 forageparticle lengths (FPL) of alfalfa silage (short and long)were combined with low (35:65) and high (60:40) F:Cratios [dry matter (DM) basis]. The peNDF contentswere determined by multiplying the proportion (DMbasis) of total mixed ration retained on 2 (8 and 19 mm;peNDF_(8.0)) or 3 (1.18, 8, and 19 mm; peNDF_(1.18)) sievesof the Penn State Particle Separator by the neutraldetergent fiber content of the diet. The dietary peNDFcontents ranged from 10.7 to 17.5% for peNDF_(8.0) orfrom 23.1 to 28.2% for peNDF_(1.18). Interactions betweenF:C ratio and FPL content were few. Increasing peNDFcontent of diets by increasing F:C ratio decreased DMintake, milk yield, and milk protein yield, whereas apparenttotal-tract DM digestibility and milk efficiencyimproved. Increasing F:C ratio improved ruminal pHstatus but decreased total volatile fatty acid concentrationand microbial protein synthesis. Increasing peNDFcontent of diets via dietary FPL increased mean ruminalpH, but did not affect DM intake, total-tractdigestibility, or milk production. The results indicatethat feeding dairy cows a low F:C diet helps increaseDM intake, milk production, and microbial proteinsynthesis, but may adversely affect feed digestibilityand milk efficiency due to increased risk of subacuteruminal acidosis. Increased FPL improved ruminal pHstatus, but had minimal effects on feed intake, ruminalfermentation, nutrient digestibility, and milk production.The results indicate a trade-off between reducingthe risk of subacute ruminal acidosis and maximizingruminal fermentation, feed digestibility, and milkproduction of dairy cows. The paNDF model showedimprovement in the predictability of ruminal pH overthe peNDF model, but the accuracy of predictions varieddepending upon the diet and ruminal fermentationvariables considered in the equations.