The effects of the ‘SLICK’ allele of the prolactin receptor gene on the cold stress response of dairy heifers : A dissertation submitted in partial fulfilment of the requirements for the Degree of Bachelor of Agricultural Science with Honours at Lincoln University

Abstract

Animal welfare concerns such as heat stress is becoming a more pressing issue under the increasing variability in climatic events in the temperate New Zealand dairy industry. Assessment of heat stress and management strategies are key areas of research, for which the SLICK allele is a potentital genetic management strategy for heat stress in New Zealand. However the interaction between this allele and cold stress is largely unknown. This research was undertaken to investigate the effects of the SLICK allele on the cold stress response of dairy heifers. Three groups of heifers representing the national herd genetics (Control) or, SLICK, and Non-SLICK, were investigated over 29 days between 13th June and 11th July in an outside grazing trial to identify any behavioural, or physiological responses to periods of natural cold stress. Control and SLICK calves (n= 6 and 7 respectively) wore SensOor accelerometers to monitor behaviour, and rectal temperatures were recorded on potential cold stress days. The frequency of cold stress days, which were determined as wind chill temperature falling below 0°C (dry) or 15.5°C (wet), were 7 days out of 29. SLICK heifers had 5% lower liveweight than control or non SLICK (221.9 vs 233.9kg, P = 0.62). However, there was no difference between treatments for liveweight gain, rectal temperature, rumination, active, or high active time. SLICK calves spent more time inactive compared with control calves (466 vs 426 minutes/day, p < 0.008). An interaction between treatment and date for eating time (minutes/day, p <0.05) showed that at high pasture allocation SLICK spent less time eating than control calves, but similar eating time under restricted pasture allocation. Differences in coat characteristics suggested SLICK heifers had an improved insulative effect compared with Non-SLICK heifers. SLICK heifers had a greater mean fibre diameter ( 36 vs 30, p < 0.005), medulation percentage (28.18 vs 22.3 %, p = 0.047), and greater percentage of coarse edge fibres (21.8 ± 1.79% vs 14.0 ± 0.90%, P = 0.005). Overall, the results suggest that under mild levels of cold stress conditions there are small differences in response to cold stress between heifers carrying the SLICK allele compared to heifers which do not carry the SLICK allele. Control heifers appeared to respond to cold stress through increased activity and increased eating time, whereas SLICK heifers appeared to favour inactivity and increasing insulative coat to conserve energy. However without strong temperature, or growth differences it suggests that the cold challenege is not strong enough to observe any different welfare concerns in SLICK carrier heifers.