Blood parameters predicted from milk spectra are candidate indicator traits of hyperketonemia - a retrospective study in the Italian Holstein population.

Abstract

At the onset of lactation, high-producing dairy cows commonly face a negative energy balance and consequent metabolic disorders, such as hyperketonemia. Blood concentrations of nonesterified fatty acids (NEFA), BHB, cholesterol, glucose and urea provide valuable information about the metabolic, health, and nutritional status of lactating animals. Milk mid-infrared (MIR) spectroscopy has been successfully used for the prediction of several health traits, including concentration of blood metabolites even though the models' accuracy is moderate. In fact, MIR-predicted blood parameters are useful for population screening and may be used for selective breeding if they are heritable and genetically variable within a population. In the present study we estimated h2 and genetic correlation of MIR-predicted BHB, NEFA, glucose, cholesterol and urea and assessed their genetic correlation with milk yield and composition traits in the Italian Holstein population using phenotypes of 9,943 cows in 460 herds. Two sets were considered: early (8,277 records - 1 per cow - between 5 and 35 d in milk) and whole lactation (105,293 records - at least 5 per cow - between 5 and 305 d in milk). The h2 and genetic variability of blood traits were greater in early than whole lactation, confirming that there is room to manipulate metabolic disease incidence in the transition period through selection. Blood BHB was the most heritable trait, no matter the lactation stage (0.13 and 0.08 in early and whole lactation), while NEFA was the least heritable trait, with h2 not significantly different from zero. Blood BHB was positively genetically correlated with NEFA, whereas glucose was negatively correlated with BHB, NEFA and urea. Blood BHB, NEFA and cholesterol were generally positively correlated with milk fat-to-protein ratio; BHB was negatively correlated with lactose content and positively with SCS. Estimated breeding values of sires with at least 20 daughters with phenotypes available were extrapolated for a-posteriori evaluation of the observed performance. The progeny of the top 5 sires exhibited a lower incidence of hyperketonemia compared with the other cows, with only 2.16% of cows having BHB concentration above the conventional threshold (1.2 mmol/L). Conversely, the prevalence of hyperketonemia was 5 times higher in the offspring of the bottom 5 bulls (10.55% cows with BHB >1.2 mmol/L). These findings suggest that, despite of the low h2 estimates, there is room to identify animals with low or high genetic merit for traits linked to the cow's metabolism. Therefore, the selection toward healthier and metabolically resistant cows is pursuable, with the infrared-predicted blood traits being potential auxiliary traits.