<p dir="ltr">The periparturient period involves coordinated physiological adaptations as the dairy cow transitions from a non-lactating to lactating state. The ability of dairy cattle to adapt to the onset of lactation is impacted both by physiological and nutritional factors, and a poor transition can result in reduced productivity and welfare for the animal. Additionally, disease and disorder development are heightened in the transition period, with increased risk for involuntary culling occurring in early lactation. This study aimed to evaluate if the amount of late gestational muscle reserves and prepartum supplementation of branched-chain volatile fatty acids (BCVFA) impacts health and production parameters in multiparous, periparturient dairy cattle. Forty-eight multiparous Holstein dairy cattle were assigned to either a high or low muscle group (HM or LM, respectively) based on their <i>longissimus dorsi</i><i> </i>muscle depth 42 days before expected (BEC). After assignment to group, cattle were then randomly assigned to a control (CON; 80 g/d soyhull pellets as-fed basis) or BCVFA (40 g/d isobutyrate product, 20 g/d isovalerate product, 20 g/d 2-methybutyrate product, fed as calcium salt products on an as-fed basis) treatment, which was top-dressed in the prepartum period only. After parturition, treatment was no longer provided and cattle were fed a common lactating diet. Blood samples, ultrasound images, and feed intake were collected and recorded from 42 BEC through 28 days in milk (DIM), milk yield and composition data was collected from parturition until 28 DIM.</p><p dir="ltr">HM cattle began mobilizing muscle reserves prior to parturition, while LM cattle began to accrete muscle reserves prior to parturition. This difference in prepartum muscle utilization did not impact other body measurements (i.e. body weight or body condition score) between the groups of cows but did result in increased blood glucose concentrations prepartum for HM cows compared with LM. This increase in glucose concentrations is likely due to the increased supply of gluconeogenic precursors as a result of the degradation of muscle tissue. The difference in glucose concentration was not observed postpartum, neither was there a difference in tissue mobilization between the groups postpartum. HM cattle had greater DMI both pre- and postpartum, and produced greater yields of milk, milk fat, milk protein, and milk lactose postpartum compared to the LM group. Despite the increased milk yield, there was no difference in feed efficiency between the groups, as the HM cows consumed more feed. Prepartum supplementation of BCVFA did not impact body measurement changes throughout the entire transition period, but did increase pre- and postpartum DMI, likely due to increased fiber digestibility. The BCVFA treatment increased blood glucose concentrations both pre- and postpartum and reduced milk urea nitrogen concentrations postpartum, likely due to improved nitrogen efficiency. Results show that prepartum supplementation of BCVFA has an improved ruminal carryover effect into early lactation.</p><p dir="ltr">At 14 days BEC and 7 DIM, an intravenous glucose tolerance test (IVGTT) was performed on a sub-set of cows, to evaluate if insulin response could be a mechanism impacting the efficiency and production differences observed between muscle groups and BCVFA supplementation. BCVFA supplementation increased glucose area under the curve in the prepartum period only. No other differences were observed between muscle group or treatment in either the pre- or postpartum period. Because there were no major differences between the cows in response to an IVGTT, we cannot conclude that glucose metabolism is a mechanism to explain differences in production responses observed. IVGTT cannot measure peripheral tissue insulin sensitivity, which is a limitation of this assessment, so our conclusions cannot assess if muscle reserves or BCVFA treatment impact peripheral tissue insulin sensitivity response. These results highlight that the amount of muscle plays a key role in the production responses observed in early lactation and that providing a BCVFA supplement could increase DMI during a period of negative nutrient balance and improve rumen efficiency.</p>

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