Long-Term and Carryover Effects of Supplementation with Whole Oilseeds on Methane Emission, Milk Production and Milk Fatty Acid Profile of Grazing Dairy Cows

SIMPLE SUMMARY: Dairy cow diets that include oils have shown potential to decrease methane emissions, which contribute to climate change. However, there is limited information on long-term interventions for animals in grazing systems. The aim of this study was to evaluate the effects of feeding oilseeds on the persistency of methane mitigation effects and milk production of dairy cows during a spring and summer grazing season. Carryover effects into autumn were studied. Eight weeks after beginning the feeding trial, cottonseed was more effective than linseed or rapeseed in decreasing methane per kilogram of ingested feed, but the mitigation effect did not persist when evaluated 11 weeks later. All oilseeds maintained milk production in spring, but in summer, milk yield was lower with cottonseed. There were no carryover effects of feeding oilseeds, once supplementation ended. Thus, adding oils to dairy cow diets through cottonseed supplementation had only a temporary effect on methane mitigation. This long-term study, conducted under grazing conditions, can help to assess how proposed interventions to mitigate methane can affect production and sustainability aspects of grazing dairy systems. ABSTRACT: Research is ongoing to find nutritional methane (CH(4)) mitigation strategies with persistent effects that can be applied to grazing ruminants. Lipid addition to dairy cow diets has shown potential as means to decrease CH(4) emissions. This study evaluated the effects of oilseeds on CH(4) emission and production performance of grazing lactating dairy cows. Sixty Holstein Friesian cows grazing pasture were randomly allocated to 1 of 4 treatments (n = 15): supplemented with concentrate without oilseeds (CON), with whole cottonseed (CTS), rapeseed (RPS) or linseed (LNS). Oilseeds were supplemented during weeks 1–16 (spring period) and 17–22 (summer period), and the autumn period (wk 23–27) was used to evaluate treatment carryover effects. Cows fed CTS decreased CH(4) yield by 14% compared to CON in spring, but these effects did not persist after 19 weeks of supplementation (summer). Compared to CON, RPS decreased milk yield and CTS increased milk fat concentration in both spring and summer. In summer, CTS also increased milk protein concentration but decreased milk yield, compared to CON. In spring, compared to CON, CTS decreased most milk medium-chain fatty acids (FA; 8:0, 12:0, 14:0 and 15:0) and increased stearic, linoleic and rumenic FA, and LNS increased CLA FA. There were no carry-over effects into the autumn period. In conclusion, supplementation of grazing dairy cows with whole oilseeds resulted in mild effects on methane emissions and animal performance. In particular, supplementing with CTS can decrease CH(4) yield without affecting milk production, albeit with a mild and transient CH(4) decrease effect. Long term studies conducted under grazing conditions are important to provide a comprehensive overview of how proposed nutritional CH(4) mitigation strategies affect productivity, sustainability and consumer health aspects.