Dietary protein and non-protein nitrogen effects on fermentation and nitrogen dynamics in sheep consuming wheaten hay

Abstract

The experiments reported in this thesis were conducted to investigate the effect of different sources of supplementary N for ruminants fed on low-quality feed (chaffed wheaten straw (6.9 MJ ME/kg dry matter, 9.1 % CP)). Degradability of the wheaten straw in the rumen, whole–gut digestibility and efficiency of microbial N synthesis were determined in unsupplemented sheep and sheep supplemented with two sources of dietary N. Rumen degradability of dry matter (DM) and protein in the wheaten straw were examined both 'in vivo' and 'in sacco'. Ammonia production in the rumen from N supplements and its assimilation into rumen bacteria and soluble protein were determined by tracer methods using intraruminally administered ¹⁵(NH₄)₂ SO₄. Methane (CH₄) and whole–body carbon dioxide (CO₂) production were determined while the sheep were confined in respiration chambers and their energy expenditure was predicted from their CO₂ production. Whole–body CO₂ and ruminal CO₂ production were also measured using intraruminal and intravenous injections of ¹³C-bicarbonate. ... The work reported in this thesis suggests that when low-medium quality roughages are the main source of feed, supplementation with N sources is advantageous. There was no apparent benefit from providing N to the rumen as rumen degradable true protein (whey) relative to providing a NPN source such as urea. Supplementation with either of these N sources increased the concentrations of rumen ammonia to above the minimum level required by microbes (50 mg N/), improving both the efficiency of microbial growth (YATP), and the total quantity of microbial protein synthesized in the rumen. Supplementary N may be expected to improve production of animals with a high protein requirement, such as in lactating animals and young growing animals, although this was not tested in this study. From this study it can be inferred that during critical dry periods, where low-medium quality roughages are the main source of feed, strategic N supplementation should be practised to sustain ruminants and avoid loss of productivity as a result of compensatory weight gain required after a dry season. It is concluded that further studies should be made in ruminants consuming diets with lower N:ME ration than in the current study to determine if enhanced microbial growth efficiency occurs even when DM digestibility is unaffected as this would offer another advantage to N supplementation.