Investigating Ruminal Nitrogen Metabolism

Nitrogen losses from dairy production have negative impacts on the environment and on production economics. In Northern Europe, dairy cattle are mainly fed forage-based diets supplemented with protein feeds such as rapeseed meal. This thesis sought to develop and evaluate new methods for forage protein evaluation and to improve the value of locally produced protein feeds. Ruminal metabolism of the grass silage soluble nitrogen (SN) fraction and of Nlabelled ammonia was modelled based on degradation kinetics determined in vivo. Kinetic models were developed to provide an optimal fit between predicted and observed values of N atom excess in different rumen N pools. Microbial N synthesis was around 20% greater with soluble non-ammonia N (SNAN) than with ammonia N, proving that SNAN stimulates microbial growth. The protozoal contribution to direct protein degradation in cows fed at high intake was rather small, with around 95% of protozoal N originating from bacterial N. An estimated 12.5% of SNAN escaped ruminal degradation. The in vitro degradation rates of SN fractions were very similar to those observed in other in vitro studies, but much lower than those measured in vivo. Immediate predicted microbial uptake of SN in grass silage was similar to that observed in vivo (20% and 14%, respectively), but significantly greater for red clover SN fractions (> 50%). Proportionally more grass silage insoluble N was degraded (detected) to ammonia N than of red clover, and of dried forages than of silage. Dried forage had a higher concentration of utilisable crude protein (uCP) than silage. Heat treatment of legume seeds (field bean, lupin and peas) increased uCP concentrations, but high temperatures (over 140°C in oven or 120°C in autoclave for more than 30 min) also significantly increased the indigestible N fraction in the feeds. The in vitro method shows potential for estimating uCP concentrations, but requires further development to estimate degradation of proteins in vivo. Using N-labelled forages is a promising method for future protein metabolism studies.