To what extent do variabilities in hormones, metabolites and energy intake explain variability in milk yield?

The objective of the present paper is to describe the extent to which variability in milk yield can be explained by variability in plasma hormones, metabolites and DE intake. Results from a study including 317 cows and 634 lactations were used. Detailed registration of performance was carried out on these cows and 10,809 plasma samples analyzed for selected hormones and metabolites. Univariate analysis was carried out on energy-corrected milk yield and concentrations of selected plasma hormones (insulin, growth hormone (GH) and triiodothyronine (T3)), metabolites (non-esterified fatty acids (NEFA)), glucose, β-hydroxybutyrate (BOHB) and urea nitrogen (BUN), and digestible energy intake (DE intake) to estimate between-cow variation through lactations. Partial least square (PLS) models were subsequently run to estimate the extent to which between-cow differences in energy-corrected milk yield could be explained by between-cow differences in hormone concentrations, metabolite concentration or DE intake. The between-cow variability in energy-corrected milk yield and the hormones and metabolites were generally found to be considerable and total variance changed through lactation, particularly for GH, T3, NEFA and BOHB. In this study, the total variance was highest in third lactation cows. When analyzed separately using partial least square models, hormones, metabolites and DE intake accounted for 24, 25 and 26% of the variability in ECM, respectively. Insulin and glucose were the single most important predictors among the selected hormones and metabolites. When including both the hormones and metabolites, the model explained 36% of the between-cow variability in ECM and this figure was increased to 53% if DE intake was also included. The lack of additivity in the variability explained shows that hormones, metabolites and DE intake were correlated illustrating the integration and orchestration of metabolism and intake. Perspectives of the analysis for use in prevention of diseases and reproduction are briefly discussed.