title: Lactation performance of mid-lactation dairy cows fed ruminally degradable protein at concentrations lower than national research council recommendations.
abstract: The aim of this study was to test whether feeding of diets containing lower proportions of ruminally degradable protein (RDP) but with a constant proportion of ruminally undegradable protein (RUP) alters feed intake, milk production and yield, and the apparent efficiency of N utilization by mid-lactation dairy cows. During the covariate period (d 1 to 28), 40 mid-lactation cows (36 Holstein and 4 Jersey x Holstein cross-breds) were fed a common diet formulated to contain 11.3% of diet dry matter (DM) as RDP. During the treatment period (d 29 to 47), cows were randomly assigned to 1 of 4 diets formulated to contain 11.3, 10.1, 8.8, or 7.6% RDP, whereas ruminally undegradable protein remained constant at 7.1% of DM. All diets contained 47.5% forage and 52.5% concentrate on a DM basis. Dry matter intake was significantly reduced for the 7.6% RDP diet. The lowest RDP content was associated with a trend for reduced milk yield. Dietary RDP had no effect on body weight or milk fat, protein, and lactose contents. Milk protein yield was not affected by RDP level; however, milk fat yield decreased linearly as dietary RDP was reduced. Concentrations of plasma essential amino acids were unaffected, whereas milk urea-N concentrations decreased linearly as dietary RDP content was reduced. The apparent efficiency of N utilization for milk N production increased from 27.7% on the 11.3% RDP diet to 38.6% on the 7.6% RDP diet. The dietary RDP requirement of cows in this study was apparently met between 15.9 and 14.7% dietary crude protein. Milk production was not significantly affected by the 8.8% RDP (15.9% crude protein) diet even though the NRC (2001) model predicted that RDP supply was 87% of that required, suggesting the current NRC recommendations for RDP may be overestimated for mid-lactation dairy cows in this study.
authors.affiliation: Department of Dairy Science, Virginia Polytechnic Institute and State University, Blacksburg 24061, USA.
authors.forename: J
authors.initials: J
authors.lastname: Cyriac
authors.forename: A G
authors.initials: AG
authors.lastname: Rius
authors.forename: M L
authors.initials: ML
authors.lastname: McGilliard
authors.forename: R E
authors.initials: RE
authors.lastname: Pearson
authors.forename: B J
authors.initials: BJ
authors.lastname: Bequette
authors.forename: M D
authors.initials: MD
authors.lastname: Hanigan
mesh_terms: D000596:Amino Acids / Q000097:blood; D000821:Animal Feed / Q000032:analysis; D000818:Animals; D001835:Body Weight; D002417:Cattle / Q000378:metabolism*; D003612:Dairying; D004032:Diet / Q000662:veterinary*; D004044:Dietary Proteins / Q000378:metabolism*; D004435:Eating; D005260:Female; D006287:Health Planning Guidelines; D007774:Lactation / Q000502:physiology*; D016018:Least-Squares Analysis; D008892:Milk / Q000737:chemistry / Q000378:metabolism; D009307:National Academy of Sciences, U.S.; D009584:Nitrogen / Q000378:metabolism; D011247:Pregnancy; D011897:Random Allocation; D012417:Rumen / Q000378:metabolism*; D014481:United States
chemical_list: D000596:Amino Acids; D004044:Dietary Proteins; D009584:Nitrogen