107. Gene Editing the Slick Mutation to Mitigate Fescue Toxicosis Losses

Abstract Tall fescue pastures support 40% of U.S. cow–calf herds, but fescue toxicosis causes $2 billion in annual losses. Management strategies often rely on genetics to reduce heat stress under toxicosis. In this ongoing project, we evaluated a gene editing approach to introduce the slick mutation (mutation in the prolactin receptor that gives cattle a sleek short hair) in Angus cattle grazing toxic fescue. We hypothesized that calves inheriting slick mutation have similar development to wild-type calves up to weaning but greater weight gain when grazing toxic fescue in the stocker phase. Multiparous Angus cows (n = 222) were synchronized and artificially inseminated with the semen of an Angus bull submitted to CRISPR-Cas9 edition to carry the slick mutation mosaic (Aa allele). 75% of the offspring were expected to be slick and 25% expected to be wild-type. Calves born within the first 30 days of the calving season (n = 94) were monitored for behavior. At birth coat color, sex, hair length in the scapula and shoulder to rump length (length from neck–shoulder junction to the end of the tailhead, following the spine) were recorded. In a subgroup (n = 13), the time to stand and time to nurse were measured as indicators of calf vigor, defined respectively as the interval from complete expulsion of the calf to standing on all four limbs for ≥ 5 consecutive seconds and to the first successful suckling. Within 20-30 days after birth thermal drone imaging (EVO II Dual 640T V3 manual) was conducted to assess surface temperature and 4 experience operators watched the behaviors of each calf for 30 minutes from 12 pm to 3:30 pm. The following behaviors in the 30 minutes interval were analyzed: time in the shade, time in the sun, time walking, time laying, and time nursing. Statistical analyses were performed in GraphPad Prism using simple linear regression. Hair length was not associated with birth weight, time to nurse, shoulder-to-rump length, or surface temperature (P > 0.10). However, hair length showed a positive linear association with time to stand (P = 0.024; R²=0.59). For each unit of increase in hair length the time to stand increase in 27.3 times. For behavior variables, there was no association between hair length and time in the shade, time in the sun, time walking, time laying down, or time nursing (P > 0.10). In conclusion, during the first 30 days of life we detected no significant effects of slick mutation in the calf behavior and development in agreement with our hypothesis. By the time this abstract will be presented we will have more information of calf development to share and genotype results to assure which ones are slick and wild-type.