Genetic variation in bovine mononuclear leukocyte responses to dexamethasone.

The objective of this study was to determine whether bovine mononuclear leukocytes exhibit genetic variability prior to and after a glucocorticoid hormone challenge in vivo. Test animals included 60 pedigreed Holstein bulls treated on 3 consecutive days with dexamethasone and 5 untreated control bulls. Eight indicator traits of leukocyte responsiveness to dexamethasone included the percentages of circulating B cells, T cells (CD4, CD8, and workshop cluster 1 molecule expressed by bovine gammadelta T cell), major histocompatibility complex (MHC) I and II expressing cells, and mean expressions of surface MHC I and MHC II on circulating cells. Blood for this work was collected from each test bull 10 times before, during, and after dexamethasone administration, with corresponding samples taken for control bulls. Random regression models with treatment-specific serial correlation were applied to the leukocyte data sets to estimate genetic and nongenetic sources of variation in baseline and recovery aspects of the traits. All traits responded predictably to glucocorticoid challenge. Genetic variation was observed in baseline measurements of all traits, with heritability estimates ranging from 0.21 +/- 0.03 to 0.60 +/- 0.06. Genetic variation in linear recovery from nadir values following dexamethasone administration was significant only for percentage CD4, percentage CD8, and for surface expression of MHC II. The genetic covariance between basal and linear recovery was positive and significant for percentage CD4, percentage CD8, and MHC II expression. The bovine lymphocyte antigen DRB3.2 locus accounted for significant proportions of total variation in percentage MHC II cells and MHC I expression. These results suggest that genetic variability exists for important basal and glucocorticoid-modified phenotypes of bovine mononuclear leukocytes, implying that immunocompetence traits impacted by this stress hormone may be enhanced by genetic selection.

Energy intake and gross efficiency comparisons from calorimetric and field data on the same lactating cows.

Field estimates on gross efficiency were obtained from intake and production data on 30 pluriparous Holsteins cows during wk 5, 7, 9, 11, 13, and 15 postpartum. Energy intake and efficiency from the energy chamber on the same cows were measured during wk 6, 10, and 14 postpartum. Measures of gross efficiency were expressed in terms of the utilization of metabolizable energy or net energy for lactation and maintenance. Within corresponding postpartum periods, chamber measures and field estimates were compared by canonical correlation analysis. All estimates from field data of energy and gross efficiencies closely approximated measures of the same traits from energy chamber data. Variation among cows in traits of energy partition and gross efficiency was similar for field estimates and energy chamber measures. Correlations were > .66 between field estimates and chamber measures on observations of maintenance energy and milk energy. Field estimates and chamber measurements of metabolizable energy and net energy had correlations of .76 and .70, respectively.

Sources of variation in partitioning of intake energy for lactating Holstein cows.

Variation in energy partitioning was quantified. Data were from 34 multiparous Holstein cows using indirect respiration calorimetry during the 6th, 10th, and 14th wk postpartum. For each period, cows were placed in digestion stalls for a 6-d excreta collection, followed by two consecutive 11-h determinations of methane and heat production. Sources of variation among cows were analyzed for gross energy consumed; for fecal, urinary, CH4, heat, milk, and maintenance energies; for tissue energy balance; and for tissue partitioning. Ratios of heat production, energy balance, milk energy, and maintenance energy to intake energy were also analyzed for sources of variation. A within-period model was used that contained fixed effects of treatment, parity, season, and covariates for metabolic BW and SCM. None of the diet or parity effects were a significant source of variation in any of the variables. Season effects were only significant for milk energy and water intake. As a covariate, SCM was highly significant for all variables except energy for heat production and energy for maintenance. However, metabolic BW as a covariate was a significant effect for heat production at wk 10 and 14 postpartum. The significance of metabolic BW differed between periods for most energy partition and efficiency measures. The most significant proportion of the variation in energy utilization was attributable to the energy in the milk.