Abstract

The objective of this study was to determine the effects of supplementation of protein deficient diet with increasing amounts of urea-N on feed intake, milk yield, rumen fermentation, and nutrient digestibility in dairy cows. The hypothesis was that low rumen ammonia-N concentrations provide suboptimal conditions for rumen microbes and these conditions can be alleviated by urea-N that increases rumen ammonia, N concentrations. To evaluate this hypothesis, the diet was formulated slightly deficient with respect to rumen-degradable protein. To supplement the diet with rumen degradable N, 5 levels of urea-N (0, 17, 33, 49, and 66 g/d) were continuously infused into the rumen of 5 dairy cows according to a 5 x 5 Latin square. Increasing levels of urea-N infusion increased N intake and N excretion in urine and feces in a linear manner and tended to increase milk and milk protein yields. Feed intake and fiber digestibility were not affected by urea-N infusion levels. Rumen ammonia-N concentrations remained low (3.5 mg/100 mL) and did not respond to urea-N infusions levels between 0 to 49 g/d, whereas the highest level of urea-N (66 g/d) increased rumen ammonia-N concentration to 5.1 mg/100 mL(quadratic effect). These observations suggested that rumen microbes efficiently captured ammonia-N from rumen fluid until sufficient intracellular ammonia-N concentrations were attained, after which ammonia-N concentrations started to increase in extracellular rumen fluid. In contrast, milk urea-N concentrations increased in a curvilinear manner (cubic effect) from 4.4 to around 6 mg/100 mL for the medium levels of urea-N and then to 7.9 mg/100 mL for the highest level of urea-N infusion. The current results indicated that 18% of supplementary N intake was secreted in milk and 53% in urine. In spite of low rumen ammonia-N concentrations observed for the basal diet, it was estimated that only 43% of supplementary N was captured by rumen microbes. Estimated true digestibility for supplementary N (93%) provided further evidence that urea-N stimulated microbial N synthesis. The current results indicate that rumen ammonia-N concentration was an insensitive indicator of N deficiency at low levels of diet CP, whereas milk urea-N was responsive to diet CP concentrations at all urea-N infusion levels.

Keywords

urea; dairy cow; nitrogen; rumen

Published in

Journal of Dairy Science
2018, Volume: 101, number: 10, pages: 9004-9015
Publisher: ELSEVIER SCIENCE INC

SLU Authors

• Huhtanen, Pekka

  • Department of Agricultural Research for Northern Sweden, Swedish University of Agricultural Sciences
    

Sustainable Development Goals

SDG2 End hunger, achieve food security and improved nutrition and promote sustainable agriculture


UKÄ Subject classification

Animal and Dairy Science


Publication identifier

DOI: https://doi.org/10.3168/jds.2018-14617

Permanent link to this page (URI)

https://res.slu.se/id/publ/96625