PS1-1. Introducing Gene-Editing Semen for the Slick Mutation to Mitigate Fescue Toxicosis in Angus Cattle.

Abstract Tall fescue pastures support 40% of U.S. cow-calf herds, but fescue toxicosis causes heat stress and reduces animal performance and welfare. A mutation in the prolactin receptor (Slick mutation) gives cattle a sleek hair and reduces body temperature. This mutation does not occur naturally in Angus cattle, but can be introduced by gene editing. We hypothesized that Angus calves carrying the slick mutation have similar development as wild-type (WT) calves during fall and winter but improved productivity and thermoregulatory responses during spring when grazing toxic fescue. Angus cows (n = 222) were submitted to a synchronization protocol followed by timed artificial insemination using gene-edited semen. The slick mutation was introduced by CRISPR-Cas9. At birth, ear cartilage sample was collected for genotype and hair length, body weight, and shoulder-to-rump length were recorded (slick, n = 62; WT, n = 34). In a subgroup (n = 13), calving was monitored and time to stand and to nurse were measured as indicators of calf vigor. At ∼30 and ∼150 days (d) of age (fall and winter), calf behavior was monitored for 30 min at 7am 12pm and 5pm. Behaviors recorded included: panting score and respiratory rate, time spent in the shade, sun, walking, lying, and eating (nursing or grazing). At 60 and ∼150 d (fall and winter), body weight, heart girth, shoulder-to-rump length, hair length and weight, rectal temperature, surface temperatures at the eye and nostril were collected. Data were analyzed with SAS 9.4. Slick calves had shorter hair at birth (1.34 ± 0.05vs.1.76 ± 0.06; P < 0.0001) with no other variable affected by genotype. There was an effect of time on body weight. Calves gained weight over time regardless of the genotype. An interaction time × genotype was observed for hair length, hair weight, and shoulder-to-rump length (P < 0.05). WT calves had greater hair length at birth and fall whereas slick calves had greater hair length in the winter. Hair weight was similar for both genotypes in the fall, but slick calves had greater hair weight in the winter by 0.24 g. Slick calves had greater shoulder-to-rump length in fall only. For the behavioral analysis, in the fall, slick calves had lower panting scores (0.31 ± 0.06vs.1.10 ± 0.20;P=0.0006) and respiratory rate (53.16 ± 2.32vs.77.44 ± 7.35 breaths/min; P = 0.0030) than WT calves, while no behavioral differences were observed during winter (P ≥ 0.1). In conclusion slick calves have similar development and behavior as WT during fall and winter, indicating an absence of cold stress, the reduced panting score and respiratory rate of slick calves during fall are indications of increased thermoregulatory capacity. We expect to have greater growth of slick calves during spring, these variables will be collected before the event and included in the presentation.