PSIII-4 Does the intensity of deliberate late-gestation exercise impact body composition and milk components in dairy heifers?

Abstract The initiation of lactation in dairy cattle often results in adipose and skeletal muscle mobilization. Our objective was to determine if the effect of exercise on nulliparous pregnant heifers affects body composition, total milk production, and components. We hypothesized that exercise may mitigate some of these difficulties by mildly and regularly altering homeostasis and improving physiological responses. Nulliparous, pregnant Holstein heifers (n=34) were enrolled in 2 cohorts based on the expected calving date (90.27 ± 6.35 d prepartum) for two concomitant 8-week exercise trials. Heifers in each cohort were stratified into one of three treatment groups based on pre-trial subcutaneous (SQ fat over the proximal gluteobiceps (thurl). Treatments were administered Mon, Wed and Fri with an 8-panel motorized free-walker, and included sedentary (SED; and penned adjacent to free-walker), low intensity (LI; pace estimated to achieve 70% lactate threshold), and high intensity (HI; pace estimated to achieve 130% of lactate threshold in short intervals. Both LI and HI groups walked the same distance for each exercise bout, ranging from 1km to 3.04km. Treatment ceased at 33.96 ± 6.92 d prepartum. Weekly during treatment, heifers were weighed, and ultrasound was used to quantify SQ adipose thickness, and semitendinosus depth, width, and area. Postpartum carryover effects on milk yield (daily) and components (1 d/wk) were assessed for 10 wk. Cohort data was combined; data were analyzed using a one-way repeated measures SPSS (Version 29.0.2.0) with significance determined at P < 0.05. The trial showed significant treatment-by-time interactions exhibiting reduced SQ adipose thickness over time during the TRT period compared to SED. Treatment affected semitendinosus dimensions (P = XX); HI increased (Tukey separated means; P < 0.05) muscle area (.042 vs.058 and.987), a numerical increase in depth (.053 vs.334 and.548), and width (.003 vs..104 and..242cm) compared with SED and LI. During the first 10 weeks of lactation, HI maintained greater exhibited less rapid SQ fat loss as exhibited in significant treatment by time interactions. HI heifers retained greater semitendinosus area (.004;.02 vs..137 and.135 cm). Both HI and LI retained greater semitendinosus depth (<.001 vs..009 and.055 mm) and semitendinosus width (<.001 vs. <.001 and.094 mm) compared to SED. Body weight and BCS saw a significant TRT effect with HI losing both weight and condition less rapidly, along with gaining it back quicker. There was no evidence of treatment effect on milk yield, composition, or component yield. In summary, high-intensity late-gestation exercise increased overall muscle size prepartum and prevented loss of fat thickness and muscle size postpartum without affecting total production, and milk components. Indicating that strategic exercise regimens could improve adaptation to the catabolic demands of lactation in nulliparous heifers.