Explaining differences in residual energy intake between lactating Holstein cows

Residual feed intake, usually used to estimate individual variation of feed efficiency, requires frequent and accurate measurements of individual feed intake. Developing a breeding scheme based on residual feed intake in dairy cows is complicated, especially because feed intake is not measurable for a large population. Another solution could be to focus on biological determinants of feed efficiency which could potentially be broadband measurable on farm. Several phenotypes have been identified in literature as being associated with differences in feed efficiency. The present study aims to identify which biological mechanisms are associated with residual energy intake (REI) differences among dairy cows. Several candidate phenotypes were recorded frequently and simultaneously throughout the first 238 days in milk for 60 Holstein cows fed on a constant diet based on maize silage. The REI was defined as the residuals of a linear regression of the lactation average of net energy intake predicted from lactation averages for net energy in milk, metabolic body weight, lactation body condition score loss and gain. A partial least square regression (PLSr) was fitted over all recorded traits to explain REI variability. Linear multiple regression explained 93.6% of net energy intake phenotypic variation, with 65.5% associated with lactation requirement, 23.2% to maintenance, 4.9% to body reserves change. Overall measured traits contributed to 58.9% of REI phenotypic variability, with activity (26.5%) and feeding behavior (21.3%) being the major contributors. However, the level of contribution of each trait was dependent on the variables included in PLSr because REI variability was comparably explained by rumen temperature variation, body reserves change and digestibility when all traits associated with activity and behavior were excluded from PLSr. Drawing a conclusion on biological traits which explain feed efficiency differences among dairy cows was not possible due to this apparent confusion between traits. Further investigation is needed to characterize the causal relationship of feed efficiency with feeding behavior, digestibility, body reserves change, activity and rumen temperature.