While the milking cow is the single largest source of on-farm dairy greenhouse gas emissions, non-productive cows are one of the key drivers of emissions intensity. The longer heifers take to reach first calving, the greater the emissions intensity of milk production, as they are emitting greenhouse gases without contributing to farm productivity.
Researchers examined the relative time it takes for dairy heifers in subtropical northern Australia to meet the target liveweight for first mating, estimating the greenhouse gas emissions associated with that period. When optimal liveweight gain is not achieved, mating is delayed and the animal spends a greater proportion of their life not producing milk.
Maintaining heifers on a high quality diets, via supplementary feeding during periods of low quality and/or quantity of pastures, should result in increased daily liveweight gain, lower enteric methane emissions to mating liveweight, and earlier calving. Together these factors reduce their lifetime emissions intensity of milk production.
This clearly has financial implications and has to be weighed up against the greenhouse gas emissions incurred with delaying mating. With increasing pressure for the Australian dairy industry to reduce its carbon footprint, a strategy of increasing growth rates through energy-dense feedstuffs may be a win-win for both the industry and environment for tropical regions of Australia, where heifers are struggling to calve for the first time at two years of age.
Christie KM, Harrison MT, Trevaskis LM, Rawnsley RP, Eckard RJ (2016). Modelling enteric methane abatement from earlier mating of dairy heifers in subtropical Australia by improving diet quality. Animal Production Science, 56, 565–573, http://dx.doi.org/10.1071/AN15296.