Some Factors Affecting Age at Puberty in Holstein-Friesian Heifers

Age at puberty in the heifer is a reproductive phenomenon of importance in dairy production. Breeding efficiency may be lowered by extreme delays in occurrence of estrus ; possible interrelations with growth rate, mammary development, and fertility also may exist. That age at puberty is influenced by both genetic and environmental factors has been shown by many investigators. Mirskaia and Crew (10) concluded that relatively early and relatively late ages at puberty in mice were hereditary. B!unn (2) found hereditary differences in age at vaginal opening in different strains of rats and also noted that faster growing rats tended to reach puberty at an earlier age than slower growing ones. Engle, Crafts, and Zeithaml (5) determined that female rats from small litters reached puberty at an earlier age than females from large litters, mainly as the result of differences in growth r~te. Asdell and Crowell (1) observed that age of vaginal opening in rats could be delayed by limiting the caloric intake and thus retarding growth. According to Upp and Thompson (18), pullets hatched during the normal hatching season were slower to reach sexual maturity than those hatched during other seasons of the year. Riddle (14) reported a similar seasonal effect in doves and pigeons. Hammond (6) found rabbits born during the spring months to be older at the first fertile mating than those born during other months of the year. MCKenzie (9) concluded that a low plane of nutrition retarded body growth and development of the reproductive organs in gilts. Phillips and Zeller (11) noted that large and small type Poland China gilts differed in age at puberty, averaging 199 and 208 days, respectively. Warnick et al. (19) showed a significant negative correlation between weights at 56 and 154 days and age at puberty in inbred gilts. Robertson et al. (15) observed a similar negative correlation in outbred gilts. Squiers et al. (17) obtained evidence that crossbred