Dietary Concentrate Supplementation Increases Milk Production and Reduces Predicted Greenhouse Gas Emissions Intensity in Pasture-based Commercial Dairy Farms

Abstract

Controlled studies have extensively documented that concentrate supplements typically increase enteric methane (CH₄) emissions and milk yield and reduce emissions per unit of milk produced and dry matter intake. However, there have been no studies conducted to determine the effect of concentrate on predicted greenhouse gas emissions from dairy farms representing the Australian pasture-based farming system. Thus, this study sought to determine how dietary concentrate supplementation affects enteric and manure CH₄, and N₂O of Australian pasture-based dairy farms. The Australian Dairy Carbon Calculator was used, which incorporates emission factors and methodologies used in the National Greenhouse Gas Inventory as reported to the International Panel on Climate Change. Primary data were collected and analyzed from 120 commercial farms in Australia's major dairy regions. Then the farms were divided into 4 groups based on their dietary concentrate supplementation: ≤ 1 (low; 15 farms), 1-2 (moderate;35 farms), 2-3 (high; 35 farms), and ≥ 3 (very high; 35 farms) ton (t) of concentrate dry matter per cow per year. Sources of greenhouse gas emissions were CO₂ from concentrate production, enteric CH₄, and manure CH₄ and N₂O. Total dry matter intake, milk yield, and daily enteric CH₄ production (g/day) quadratically increased with concentrate level, whereas greenhouse gas emission intensity of milk production (kg CO_2eq/kg fat and protein corrected milk) decreased by 14% for farms supplementing with ≥ 3 tDM/cow per year compared with those supplementing with ≤ 1 tDM/cow per year of dietary concentrate. The emissions of N₂O and CH₄ from manure increased quadratically and linearly, respectively, with the increasing supplementation of concentrate. Farms supplementing 2-3 tDM/cow per year showed substantial increases in gross income, gross margin, earnings before interest and tax, and net income ($/cow/year) compared with those supplementing of ≤ 1, 1-2, and ≥ 3 tDM/cow per year. In conclusion, increasing dietary concentrate supplementation for dairy cows resulted in increased milk production per cow, reduced greenhouse gas emissions per unit of milk produced, and increased income and profit. However, a comprehensive life cycle assessment study is needed to account for carbon sequestration by other farm components such as pastures and trees, which were not considered in the present study. In addition, the present study was based on modeling and did not gather ground truth information for DMI, digestibility, crude protein, and urinary and fecal N excretion. Therefore, data should be interpreted with caution, and studies gathering such information are encouraged.

Keywords: Methane; Milk yield; Nitrous oxide.