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Screening for forages and foraging managements that reduce N excretion and CH4 emissions while maintaining or increasing animal production
Farmers face complex decisions at the time to feed animals, trying to achieve their production goals while contemplating social and environmental constraints. Our purpose was facilitating such decision-making for pastoral dairy farmers, aiming to reduce urinary N (UN) and methane emissions (CH4), while maintaining or increasing milk production (MP). There is a considerable number of forages the farmers can choose from and combine. First we used three grasses, three legumes and two herbs combined in 72 mixed swards. Then 50 feeds (forages and grains) were combined systematically in different proportions producing 11,526 binary diets. Swards and binary diets were screened, using an a posteriori approach and a Pareto Front (PF) analysis of model (Molly-DairyNZ) outputs. The objective was identifying combinations with the best possible compromise (i.e. frontier) between UN, CH4, and MP. All PF solutions are considered optimal and equally good. Using MP and low UN as objective functions, PF included seven optimal swards; with fescue, alfalfa and plantain as key species. Adding CH4 emissions as objective function increased the number to 23. For binary diets, the MP-UN frontier included 10, 14, 12 and 50, for non-lactating, early- mid- and late-lactation periods, with cereals and beets featuring strongly. Using the same objective functions, but including ryegrass as dietary base PF included 2, 4, 8 and 4 diets for those periods. These results suggest that from a wide range of diverse diets, farmers could choose from a handful of mixed swards and binary diets to reduce UN while maintaining or increasing production. If the criterion is maintaining pasture-based systems, there are fewer suitable options. Reducing UN will simply require dilution of N supplied by pasture by either supplementing low N forages or strategic foraging managements. The results also indicate that reducing UN may imply increments in CH4 emission and vice versa,i.e. pollution swapping. While there is no perfect sward or diet which optimizes all the objectives at once, there are feeding options to offset pollution swapping, if the current diet is not in the frontier. Ultimately, it is up to the farmers to choose the best options, according to their farming context.
Keywords: Forages, ruminant nutrition, environmental footprint