Nutritional strategies to reduce nitrogen excretion in beef cattle

Ruminant production systems have a negative impact on the environment and are a major contributor to atmospheric ammonia (NH3) emissions. Therefore, there is a need to investigate dietary strategies and novel feed additives for ruminants that reduce nitrogen (N) excretion. The two overall aims of this thesis are: 1) To investigate the effects of low crude protein (CP) diets, different carbohydrate sources and novel feed ingredients on nutrient intake and digestibility, ruminal fermentation, and N excretion in beef heifers 2) To evaluate novel feed additives on ruminal fermentation parameters in vitro using the rumen simulation technique (RUSITEC). To achieve these aims four experiments were carried out. The first experiment evaluated the effect of supplementing grass silage based diets with differing carbohydrate sources. Offering a carbohydrate source that is rapidly degraded in the rumen, such as, rolled barley did not alter ruminal NH3 concentrations nor reduced N excretion in beef heifers. The second experiment investigated the effects of chitosan differing in molecular weight, at different inclusion levels on the manipulation of rumen fermentation in vitro using RUSITEC. Chitosan inclusion had little impact on rumen fermentation in vitro. The third experiment also investigated the effects of chitosan inclusion at two levels of dietary CP on nutrient intake and digestibility, ruminal fermentation, and N excretion in beef heifers. Chitosan inclusion increased ruminal NH3 concentrations in high CP diets while having no effect on N excretion. Finally, the fourth experiment evaluated the effects of supplementing diets with different CP levels with different brown seaweed species on rumen fermentation in vitro using RUSITEC. Inclusion of brown seaweeds reduced NH3 concentrations in both high CP and low CP diets, but not sufficiently to affect microbial fermentation. The findings of this thesis indicate that dietary strategies (altering CP levels and carbohydrate sources) and inclusion of brown seaweeds offer potential to mitigate against N excretion in cattle, while acknowledging that further research is required.