Interpretive Summary: Optimum grape pomace proportion in feedlot cattle diets
By Anne Zinn
A study recently published in the Journal of Animal Science investigated the optimum proportion of grape pomace in finishing cattle diets. The dietary inclusion of grape pomace in ruminant diets can reduce reactive nitrogen and methane emissions and enhance the shelf life and the beneficial fatty acids content of meat because of its high content of polyphenolic compounds, but inclusion beyond an unknown threshold can also compromise nutrient supply and growth performance. Because of this, it was hypothesized that increasing the dietary proportion of grape pomace would result in a decrease in nutrient supply in finishing cattle and aimed to determine the optimal proportion. The responses to grape pomace inclusion were measured by assessing nutrient intake and apparent total tract digestibility, ruminal fermentation characteristics, microbial protein production, route of reactive nitrogen excretion, and serum fatty acid profile.
Results demonstrated that dietary inclusion of grape pomace affected metabolic parameters linked to nutrient supply in feedlot steers. These responses were mostly quadratic with the impairment of apparent total tract nutrient dry matter, organic matter, neutral detergent fiber, and reactive nitrogen digestion, estimated microbial reactive nitrogen supply, and reactive nitrogen retention occurring at the 30% but not at the 15% inclusion level. In addition, increasing the dietary proportion of grape pomace resulted in an increase in excretion of insoluble bound forms of reactive nitrogen in feces, which suggests greater stability of manure reactive nitrogen. These results suggest that nutrient supply could be compromised beyond a 15% dietary inclusion level of grape pomace, leading to possible restricted growth performance in feedlot cattle. Further research is warranted because there are indications that drying could limit the efficacy of the polyphenolic compounds from grape pomace in modulating ruminal metabolism