Relationship between Growth Pattern, Age at First Calving and Next Reproduction in Holstein Heifers

Growth rate during rearing, which varies depending on provided nutrition, has been related with age at $1^{st}$ calving (AFC). This study investigated the effect of upgrowth parameters during the rearing period on the reproduction of nulliparous Holstein heifers. The study comprised 77 successively born heifers from the same herd. Growth rate and fertility traits were measured during rearing and fertility parameters were recorded in lactations 1. Growth parameters (body weight, height, heart girth and body length) were measured at the approximate birth time, 270 and 450 d of age. Reproduction data collected included age at $1^{st}$ breeding, number of services per conception (S/C), pregnancy rate to $1^{st}$ artificial insemination, AFC. Animals were subsequently divided into 4 AFC groups for analysis: <23 mo, 23~25 mo, 26~30 mo and >30 mo. The AFC reflected both upgrowth rate and heifer reproduction, with later calving heifers smaller. Increased skeletal growth (at 270 and 450 d) was related with a reduced AFC (p<0.05). Early calving animals (<23 mo) had the best reproduction as nulliparous heifers, with most conceiving at first service (87.5%). Fertility in the first lactation was the worst in the oldest AFC group (>30 mo). In the 1st lactation period, a number of services per conception ($3.1{\pm}0.3$) increased with increasing AFC (>30 mo). Sub-optimal upgrowth related with an increased AFC could be mitigated by improved monitoring of replacement heifers during the rearing period.