Effects of post-ruminal fiber fermentation on digestion and nitrogen balance in lactating dairy cows

Ammonia (NH3) losses from dairy manure reduce environmental quality. Development of strategies that shift some nitrogen (N) excretion from urine to feces should reduce manure NH3 volatilization. Three experiments were conducted to quantify the shift in manure N excretion due to increasing large intestinal carbohydrate fermentation in dairy cows.; In experiments 1 and 2, three and six lactating cows, respectively, were infused abomasally with 0 to 1 kg/d citrus pectin, a fermentable fiber source. One kg/d pectin infusion shifted 22 g/d N from urine to feces, reducing urinary N output by 12%. These data support the hypothesis that increasing post-ruminal fermentation in dairy cows might reduce manure NH3 volatilization. However, ruminal NH3 and urinary purine derivatives tended to decrease with abomasal pectin. Pectin may have reduced urea recycling to the rumen and rumen microbial protein production, suggesting that post-ruminal fermentation may increase a cow's crude protein requirements.; Experiment 3 was designed to test whether increasing post-ruminal fermentation increases rumen degradable protein (RDP) requirements for lactating cows. Eight cows were used in a replicated 4x4 Latin square with a 2x2 factorial arrangement of treatments. Cows were fed diets balanced to be adequate or 28% deficient in RDP. Abomasal infusion with 0 or 1 kg/d of fructan was used to increase intestinal microbial growth. Abomasal fructan shifted 23 g/d N from urine to feces with no effect on milk production. Although blood urea N and milk urea N were decreased by abomasal fructan, fructan did not affect ruminal NH3 or urinary purine derivative excretion. Results suggested that increasing hindgut fermentation did not limit N available to ruminal microbes, even on diets predicted to be severely limiting in rumen NH 3.; Overall, 1 kg/d abomasal fiber shifted approximately 23 g/d N from urine to feces without adversely affecting production. In published studies, large intestinal digestion of organic matter by lactating dairy cows ranges from -0.3 to 3.0 kg/d. Based on this range, manipulating diets to increase large intestinal fermentation can decrease urinary N excretion by up to 70 g/d.