Feed efficiency dynamics in relation to lactation and methane emissions in dairy cattle
With continued global population growth, there is a need to develop more efficient, environmentally friendly food production to meet increasing nutritional demands. Dairy production provides an opportunity to address these nutritional needs by generating a high-quality protein, fat and energy source (milk) from plant matter that is indigestible by humans. However, low feed efficiency and greenhouse gas production are challenges that need to be addressed. The objectives of this thesis were to 1) critically review methods of determining feed intake in dairy cattle, 2) develop and evaluate different measures of feed efficiency, and examine the associations of feed efficiency with aspects of 3) the lactation curve and 4) methane emissions, with the end goal of the downstream incorporation of these traits into the Canadian dairy cattle breeding program. All experimental analyses were conducted on data collected from a sample of 40 primiparous Holstein heifers over the first 150 days of lactation. It was found that feed efficiency fluctuates on a daily basis regardless of measure used, though different measures generally followed the same trends of increasing or decreasing efficiency. Persistency of lactation was found to be positively associated with increased feed efficiency and decreased methane production and intensity. No significant associations were observed between feed efficiency and methane production, though feed efficiency was associated with lower methane intensity. These collective findings suggest feed efficiency and methane emissions can be improved by selecting for dairy cattle that are smaller and have increased persistency of lactation. Efficiency and methane emissions can be further improved by improved management of body condition score and by extending lactations beyond the conventional 305-day length. Future work should focus on reformulating the equations used in net energy models to reflect the genetic progress made in cattle over recent decades, as well as longitudinal studies to characterize the lifetime efficiency of dairy cattle.