Can nutritional grouping lower feed costs and enteric methane when diets are optimized with modest ingredient changes?

<h2>Abstract</h2> Nutritional grouping (NG), where cows are fed in groups based on similar nutrient requirements, has been used as a strategy to improve the precision of nutrition delivery and thus economic returns. Enteric methane, one of the largest on-farm sources of greenhouse gas emissions, is strongly influenced by diet formulation. With growing concerns about climate change, it is important to evaluate whether NG can also help reduce enteric methane emissions. To address this, we developed an open-source linear optimization model to assess the impact of NG on both feed cost and enteric methane emissions. We conducted a case study with 675 cows from the University of Wisconsin's Arlington Agricultural Research Station. Diets were formulated using the farm's actual feed ingredients and prices and were constrained according to current dairy cattle feeding guidelines. Without NG, optimization alone reduced feed cost by $2.52/cow per day (32%) under the cost-minimization objective, and reduced methane emissions by 57 g/cow per day (12%) under the methane-minimization objective, relative to the on-farm diet used during the study period. Under a dual-objective approach that simultaneously considered feed cost and enteric methane emissions, the model identified a pragmatic compromise solution. By placing greater emphasis on economic performance to reflect the current US production context, methane emissions decreased by 55 g/cow per day compared with the cost-minimization scenario while maintaining similarly low feed costs. Nutritional grouping did not consistently outperform the optimized single-group diet, but it modestly improved the nontarget outcome (lower methane when minimizing cost and lower cost when minimizing methane). These findings indicate that well-optimized rations using existing feeds can deliver meaningful economic and environmental benefits, and NG offers an additional layer of management refinement that may not only reduce the feed cost as suggested by literature, but also offer complementary benefits when methane is considered as a secondary performance dimension.