Methane emissions from dairy heifers as affected by residual feed intake and breed : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Animal Science at Massey University, Manawatū, New Zealand

Abstract

Reducing methane (CH4) emissions without reducing milk production requires an improvement in feed conversion efficiency: that is an animal’s efficiency in utilising feed for maintenance and production. Residual feed intake (RFI) is one measure of feed conversion efficiency; it can be defined as the difference between an animal’s actual intake and its predicted intake based on its metabolic size and productivity. More efficient animals eat less than predicted (low RFI); inefficient animals eat more (high RFI). Enteric CH4 is an important source of digestible energy loss in ruminants, and research in beef cattle has reported a positive relationship between RFI and daily CH4 production. Jersey (Jer) cows have also been reported to be more feed efficient than Holstein-Friesian (HF) cows. Thus, I hypothesized that high feed efficient (low RFI) animals would emit less CH4 than the lower efficiency (high RFI) animals, and that Jer heifers would have lower CH4 yield than HF heifers. I measured the CH4 emissions of 56 growing dairy heifers (20-22 mo old) in a 2 x 2 factorial arrangement: factors included two breeds (HF and Jer; n=28/breed) and two previously determined RFI categories (low RFI; -2.1 kg DM and high RFI; +2.0 kg DM; n=28/RFI category). All heifers were co-mingled and offered the same diet of dried lucerne cubes. Between RFI categories, heifers did not differ in body weight (BW) or BW gain (BWg); but low RFI heifers had 9.3% and 10.6% lower dry matter intake (DMI) and DMI/kg BW, respectively, than high RFI heifers. Similarly, RFI category did not affect CH4/d or CH4/kg BWg; but, CH4/kg DMI was greater in low RFI heifers because of their lower DMI. These results might reflect more complete digestion of ingested feed in more efficient, low RFI heifers, consistent with previous reports of greater apparent digestibility of organic matter. Breed did not affect DMI/kg BW or BWg; Jersey heifers produced less CH4/d, but not CH4/kg DMI or CH4/kg BWg. In conclusion, selecting dairy heifers for low RFI is unlikely to affect daily CH4 production (g/d), but may increase CH4 yield (g/kg DMI).