Muscle histology changes in first-lactation dairy cows from pre- to postpartum

To support the nutritional demands of fetal growth and the onset of milk production, dairy cattle mobilize body tissues; and though this has been established in multiparous animals, the extent of this mobilization is less understood in primigravid dairy cattle. The objectives of this study were to characterize the longissimus dorsi muscle depth (LDD) and extent of LDD and backfat mobilization, as well as to characterize the muscle histological differences in pre- and postpartum primigravid dairy animals with varying muscle reserves as they transition into their first lactation. This prospective cohort study enrolled 21 heifers that were post hoc divided into 2 groups: high muscle (HM; LDD > 5.28 cm, n = 10) and low muscle (LM; LDD ≤ 5.28 cm, n = 11), based on the day of enrollment of the study, 5 wk before calving. Weekly measurements of LDD, BW, BCS, and insulin were recorded from enrollment until 28 DIM. Milk weights and DMI were recorded daily, and milk components were measured weekly. Biopsies of the longissimus dorsi muscle were collected ∼3 wk before and after parturition. Biopsy samples were sectioned and analyzed for myofiber type profile and fiber type-specific cross-sectional area. The statistical model for repeated measures included the fixed effects of muscle group, time, their interaction, and the random effect of cow within group. The statistical model for single measurements per cow included the fixed effect of muscle group. Animals in the LM group produced 2.3 kg/d more milk compared with animals in the HM group, with no differences in component yields or ECM yield between groups across the first 4 wk in lactation. From 1 to 4 wk in lactation, HM cows lost 20 kg less BW than LM cows during the same time. The mean cross-sectional area of type I, IIA, or IIX myofibers was not different between groups in either the pre- or postpartum stage. Animals in the HM group had a greater proportion of type IIX fibers than LM cows, which had more type IIA fibers than HM cows. Animals with lower LDD before calving lost more BW through 4 wk in lactation but produced more milk compared with animals having higher LDD. Prepartum muscle depth affected BW loss, milk yield, and muscle fiber type in primigravid animals. These results indicate that greater abundance of glycolytic myofibers may limit milk yield at the onset of lactation due to greater glucose demands by muscle, suggesting that muscle amount and myofiber profile may be associated with differences in milk yield.