Effects of concentrate supplementation on enteric methane emissions and milk production of grazing dairy cows

•Two levels of concentrate supplementation of grazing dairy cows were studied.•Five kg of concentrate increased milk and methane production compared to 1 kg.•Methane per unit of milk or feed intake was not affected by concentrate level.•At levels offered, concentrate supplementation does not reduce methane emissions.

Although concentrate supplements in ruminant diets have been recognised as an effective enteric methane mitigation strategy, very few studies have examined the effects of concentrate supplementation on enteric methane emissions under grazing conditions. Twenty four multiparous Holstein Friesian cows were used in a crossover design study to investigate the effects of two concentrate feeding levels across two periods on enteric methane emissions and milk production of grazing dairy cows. Each period had a duration of four weeks (three weeks for diet adaptation and one week for measurements) and no interval in between them. Dietary treatments consisted of two concentrate feeding levels per cow (1 vs. 5 kg; as-fed basis) offered daily in equal meals during milking. Enteric methane emissions from cows grazing perennial ryegrass pasture were measured during the final week of each period using the sulphur hexafluoride tracer technique. Milk yield and liveweight were determined daily during each methane measurement period, whereas milk composition and body condition score (BCS) were determined weekly. Daily herbage intake by individual cows during methane measurement weeks was estimated using an energy requirement model and animal records and diet composition. In period 1, cows receiving 5 kg concentrate supplement were estimated to reduce herbage intake by 1.8 kg DM/d compared to cows receiving 1 kg of concentrate, whereas in period 2 cows receiving the 5 kg concentrate supplementation were estimated to reduce herbage intake by 4.4 kg DM/d, compared to cows receiving 1 kg of concentrate. In both periods, milk yield increased with increasing concentrate level, with an average milk response to concentrate supplementation of 0.68 kg milk DM/kg concentrate DM over the two periods. Concentrate feeding level had no effect on milk fat, protein or total solids contents. In period 2, lactose content increased in cows offered 5 kg/d concentrate. Increasing concentrate feeding level increased liveweight and BCS in period 1, but not in period 2. Feeding 5 kg of concentrate supplement increased enteric methane emission by 34 g/d in period 1 (323 vs. 357 g/d) and 41 g/d in period 2 (349 vs. 390 g/d) compared to 1 kg of concentrate supplement. However, enteric methane emission per unit of estimated feed intake (dry matter or gross energy) or milk output (gross or energy corrected) was not affected by level of concentrate supplementation. It was concluded that under generous grazing conditions (high allowance of good quality herbage) a moderate increase in concentrate supplementation resulted in a simultaneous increase in milk yield and enteric methane emission, so that enteric methane emission per unit of milk yield was unaffected. Thus, a moderate level of concentrate supplementation of dairy cows grazing pastures of high digestibility would not be an effective enteric methane mitigation strategy.