Fertility Parameters in Heifers

This retrospective study was conducted to elucidate reproductive parameters of heifers. The age of insemination resulted with pregnancy

(AIRP), the number of inseminations per pregnancy (NIPP), length of the gestation period (LGP), age at first calving (AFC), maternal body

weight at the first calving (BWFC), and bodyweight of calves at parturition (BWCP) data were compiled from 179 heifers (Holstein, n = 32;

Simmental, n = 147), which were born between June 22, 2013 and October 31, 2019 and raised under the same managerial program in a private

farm. The data were subjected to 2-way ANOVA and regression analysis. The AIRP was longer in Simmental heifers than Holstein heifers

(515 ± 5 d vs. 482 ± 7 d; P<0.01) while NIPP values were similar (1.99 ± 0.10 vs. 2.06 ± 0.10, P>0.05). Likewise, the AFC was 39 d shorter

for Holstein heifers than that for Simmental heifers (753 ± 7 d vs. 796 ± 5 d, P<0.001). The LGP was shorter in Holstein heifers than Simmental

heifers (271 ± 1 d vs. 280 ± 1 d; P<0.0001). However, the BWFC (576 ± 10 kg vs. 585 ± 5 kg) and BWCP (39.0 ± 0.8 kg vs. 40.7 ± 0.5) were

not different. The sire, calf sex, and maternal weight accounted for 83.1% (P<0.01), 9.3% (P<0.05), and 7.6% (P>0.05) of variations in the

BWCP. The linear models describing relationships were 1) LPG (d) = 0.46∙BWCP (kg) + 260.4, R2 = 0.12 (P<0.0001), 2) AFC (d) = 28.05∙NIPP

+ 731.36, R2 = 0.35, (P<0.0001), and 3) BWFC (kg) = 0.32∙AFC (d) + 328.47, R2 = 0.11 (P<0.0001). In conclusion, reproductive management

covering accurate observation for estrus sign, inseminated by experienced inseminator, checked for pregnancy status as early as possible is

crucial to improve life–time productivity. Sire selection should be considered for minimizing dystocia incidence.