Effects of early herbage cutting and vine leaves on methane emission, urine nitrogen losses, and the milk fatty acid profile of dairy cows

Methane mitigation in dairy cows is an essential part of combating global warming. Governments and consumers have become increasingly interested in herbage-based feeding, and premium prices are often paid for these types of dairy products. However, this feeding strategy is presumed to produce more methane per unit of feed or milk than corn silage- or concentrate-based diets due to higher fiber intakes. Immature herbage is preferred to maintain dairy cow performance, but the high content of N and digestible fiber may increase methane and urine N emissions compared with more mature herbage. Tannin-containing feeds, such as vine leaves (Vitis vinifera), may help to combat the emissions associated with feeding immature herbage. Our study aimed to evaluate differences between early-stage (ES; 21 d of regrowth) versus late-stage (LS; 42 d) herbage and the effects of vine leaves on methane and nitrogenous emissions and the milk fatty acid profile of dairy cows. Twenty-four mid- to late-lactating dairy cows were randomized to 4 dietary groups (n = 6) in a factorial study design. Each of the 4 diets contained 69% fresh mixed legume-grass herbage, 13% grass hay, and 5% concentrate on a dry matter (DM) basis. Two diets were based on immature fresh mixed legume-grass herbage and grass hay (ES), and 2 contained more mature fresh mixed legume-grass herbage and grass hay (LS). Of these, 1 contained 13% vine leaves (VL+) and the other an additional 13% hay (VL-). No significant differences were observed in DM intake or milk yield across the diets. Methane emission intensity was lowest with ES-VL+ diets compared with LS-VL- diets (-30%; 17.1 vs. 24.5 mg/kg of energy-corrected milk). Methane yield decreased by 17% and 20% when related to the intake of DM and digested organic matter for ES-VL+ compared with LS-VL- diets (16.9 vs. 20.3 g/kg of DM intake; 23.5 vs. 29.3 g/kg of digestible organic matter). Immature grass and vine leaf addition each caused about half of the respective declines. Cows consuming any of the ES diets and the LS-VL+ diet consumed and excreted (urinary N) significantly more N than those consuming LS diets. However, when related to N intake, no differences were recorded. Unexpectedly, vine leaves did not mitigate urine N excretion; however, they lowered the n-6:n-3 ratio and increased concentrations of vaccenic and rumenic acids in both ES and LS diets. Our results demonstrate that feeding immature herbage in combination with vine leaves reduces methane yield; however, the associated high urinary N losses need to be addressed.