Effect of growth and development during the rearing period on the subsequent fertility of nulliparous Holstein-Friesian heifers.

This study investigated the effect of growth parameters and metabolic indices during the rearing period on the fertility of nulliparous Holstein-Friesian heifers managed on 17 UK dairy farms. Growth parameters (body weight [BW], heart girth, height, and crown-rump length) and metabolic indices (insulin-like growth factor-I [IGF-I], insulin, glucose, and urea) were measured at approximately 30, 180, and 450 d of age. Fertility data collected included age at first breeding (AFB), number of services per conception, pregnancy rate to first artificial insemination (AI), and age at first calving (AFC). Of the heifers initially bred (n=428), 4% failed to conceive. The mean pregnancy rate to first AI for heifers that conceived and calved without suffering reproductive loss (n=392) was 67%, and 6% required >2 inseminations. The mean AFB and AFC was 473+/-5 d (range, 357 to 936 d) and 791+/-6 d (range, 636 to 1529 d), respectively. Increased BW, girth, and IGF-I concentration (at 30, 180, and 450 d) and increased skeletal growth (at 180 and 450 d) was associated with a reduced AFB and AFC (P<0.05 to 0.001). Heifers calving at <775 d had a mean BW gain of 0.82+/-0.01kg from 30 to 180 d. Increased glucose concentration at 180 d was associated with a reduced AFB (P<0.01), but no associations were found between insulin and urea concentrations and any of the fertility traits recorded (P>0.1). Suboptimal growth associated with an increased AFC could be alleviated by improved monitoring of replacement heifers during the rearing period.