Relationship between pre-weaning gain, age at puberty, and reproductive tract development in Angus heifers

Abstract Text: Beef heifers should initiate reproductive cycles by 12 mos of age to insure multiple estrous cycles before the start of the breeding season to maximize fertility.  Previous research indicated that pre-weaning gain positively influenced the onset of puberty and antral follicle numbers.  Therefore, the objective of the current study was to examine differences in ADG and reproductive tract development between heifers that attained puberty before or after 12 mos of age.  Angus heifers born in the spring of 2012 (n = 84) and 2013 (n = 115) were weaned and developed in a feedlot to attain 55% of estimated mature weight by the start of the breeding season.  From March (322.3 ± 1.3 d) until May, heifers were examined monthly (age) by ultrasonography to determine pubertal status, antral follicle count, ovarian size, and uterine horn diameter.  Heifers were classified as those that reached puberty prior to 12 mos of age (n = 65) and those that reached puberty after 12 mos of age (n = 134).  Average daily gain and BW were analyzed using the MIXED procedure of SAS with pubertal group as a fixed effect and year as a random effect.  Antral follicle count and development of the reproductive tract were analyzed using the MIXED procedure of SAS with age, pubertal group and the interaction as fixed effects and heifer and year as random effects.  Body weights, post-weaning ADG, and age were not different between pubertal groups (P > 0.05); however, pre-weaning ADG was greater for heifers that achieved puberty by 12 mos than those that did not (P = 0.03; 0.91 ± 0.02 vs. 0.87 ± 0.03 kg/d).  There was a significant effect of age on antral follicle count and ovarian size (P < 0.01), but these traits did not differ between heifers that attained puberty before 12 mos and those that did not (P > 0.05).  There was a tendency for an interaction of age and puberty group on uterine horn diameter, such that heifers that attained puberty before 12 mos had a greater uterine horn diameter at a younger age (P= 0.06).  These data indicate that pre-weaning ADG and uterine horn diameter are better predictors of puberty than age or BW.  Since selection on BW increases cow size, replacement heifers should be selected using assessment of reproductive tract development rather than BW.

Keywords:

Beef Heifer, Puberty, Reproductive Tract Development