Methane emissions of grazing dairy cows fed graded levels of concentrates : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Agricultural science at Massey University, Manawatū, New Zealand
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Date
2024
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Massey University
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Abstract
The GHG inventory of New Zealand currently assumes that all dairy cattle emit 21.6 grams of methane (CH4) for every kilogram of dry matter (kg DM) of pasture eaten. However, supplement feeding has increased in New Zealand pastoral systems in recent decades to comprise ̴18% of the total feed offered to New Zealand’s dairy herd. Previous studies have shown different diets can alter the CH4 emissions of dairy cattle and therefore the objective of this study was to determine the effect of increasing concentrate intakes on pastoral dairy cows’ CH4 emissions and milk production. Early lactation dairy cows (n = 72) were allocated (n = 18 per treatment group) to receive 0, 2, 4 or 6 kg DM of concentrates per day while grazing pasture ad-libitum over 63 days. Methane emissions were measured in the field for individual animals using the ‘GreenFeed’ automated emissions monitoring system. Changes to liveweight and body condition score, daily milk production and weekly milk composition were recorded and used to estimate individual animals’ dry matter intakes. Liveweight change, milk production and estimated dry matter intakes were not found to significantly change with increased concentrate feeding rates. Methane production (g CH4 / day) was not affected by concentrate feeding and was similar across all treatment groups, however CH4 yield (g CH4 / kg DM) and CH4 intensity (g CH4 / kg fat and protein corrected milk) linearly decreased with increasing concentrate inclusion in the diet (P = 0.041; P = 0.022, respectively). This was also confirmed by a significant and linear decrease of the methane to carbon dioxide ratio (CO2 : CH4) emitted by animals with increased concentrate feeding (P = 0.011). These results have demonstrated that CH4 yields change when feeding increasing levels of concentrate feed to pasture-based dairy cattle in New Zealand, which differs from the current assumption for calculating the national GHG inventory. Responses in CH4 emissions and milk production parameters were however relatively small in this study however, which was likely due to generous pasture offers that resulted in a large substitution of pasture as concentrate feeding rates increased.