PS6-8. High Fat Milk Replacers Promote Early Marbling Development in Angus X Holstein Calf Ribeyes.

Abstract Beef-on-dairy calves raised at calf ranches are fed milk replacer during the pre-weaning stage, creating opportunities to promote enhanced muscle and marbling development through early-life nutritional interventions. The objective of this study was to measure how dietary energy source affects skeletal muscle development in beef-on-dairy calves. 16 same-sire Holstein x Angus calves were assigned to either a high-fat (HF, 34% lactose and 30% crude fat) or high-lactose (HL, 50% lactose and 18% crude fat) diet on day 1 of life, and calves were fed their respective milk replacer on a step-up step-down feeding schedule for 50 days. Diets were isonitrogenous and fed at 150g/L (HL) or 132g/L to ensure an isocaloric intake; chopped wheat straw and water were offered ab libitum. Fecal and respiratory scores were taken daily, all feed refusals were measured, and weight was recorded weekly. Calves were slaughtered at 50 ± 3 days of age and the right longissimus thoracis (ribeye) was frozen at -20 °C for later meat quality assessment. Samples from the left ribeye and brisket were flash frozen in liquid nitrogen and stored at -80 °C until cryosectioning. For meat quality assessment, collected ribeyes were thawed in a low cooler until 3 °C and measured using a colorimeter. Warner-Bratzler shear force was performed parallel to muscle fibers on triplicate cooked ribeye cores using a 1000N load cell. For ex vivo histology, flash frozen muscle was cryosectioned transversely and immunohistochemically stained for perilipin-1 to stain mature lipid membranes, PDGFRa to stain fibro-adipogenic progenitor cells (FAP) which predate marbling, and Ki67 to stain proliferating cells. Statistical analysis was performed using an unpaired t-test, and significance was set at p < 0.05 and trends at 0.05< p < 0.1. Both fecal and respiratory scores were not significantly different between treatment groups, and HF and HL calves had similar feed conversion ratios and average daily gains. HF ribeyes were significantly greener than HL, and HF trended to be more yellow in objective color measurement; shear force was not significantly impacted. HF ribeyes had a significantly higher fat:muscle ratio in the ribeye, but in the brisket HL samples has a higher fat:muscle ratio. Despite this finding, we found no significant difference between HL and HF FAP cell counts or their proliferation rates. This preliminary data shows that while health and performance is maintained between isocaloric diets, a dietary energy source of primarily fat promotes early marbling development in a muscle-specific manner within the first 50 days of life. However, the mechanism behind this remains unclear. Future work will further assess pro-meat quality muscle development and the systemic effects of dietary energy source.