Influence of Social Hierarchy on the Expression of Estrus and Subsequent Fertility of Dairy Heifers

摘要

A study was conducted to determine if social status of heifers within a herd influences estrus activity and subsequent fertility. Thirty cyclic dairy heifers were observed over a 14-d period and ranked by social status, based on a displacement index.The estrous cycles of the heifers were synchronized by treatment with two injections of prostaglandin F2a (PGF2a; Lutalyse, 25 mg) given 14 d apart. At the second PGF2a injection, HeatWatch transmitters were placed on the heifers for continuous monitoring of mounting (estrus) activity over the next 45 d. All heifers were artificially inseminated at estrus, using semen from a single sire. Pregnancy status was determined by ultrasonography post-insemination. For analysis, heifers were placed into three groups based on social status. Subordinate heifers, ranked in the bottom one-third of the herd, exhibited a shorter (P = 0.001) estrus than more dominant heifers. The number of mounts recorded during estrus declined with decreasing (P = 0.009) social status. There was a trend for the number of mounts per hour to be greatest for heifers in the top one-third of the social hierarchy (P = 0.074). Of those heifers detected in estrus and inseminated, pregnancy rate was similar among the social groups for the first and subsequent inseminations (P > 0.315). These results indicate that social hierarchy of dairy heifers influences length and expression of estrus. However, social standing within the herd does not influence fertility of heifers detected in estrus.