Puberty attainment is affected by growth performance before 4 mo of age in dairy heifers

In seasonal calving systems where heifers first calved at a young age, age at 1st insemination may be delayed for those born at the end of the calving period. A possible alternative is to increase growth intensity during the 1st year of rearing, and then, to reduce age at 1st calving (21 mo of age or less). Puberty should then occur at an early age for optimal fertility. A long-term experiment aiming at studying the effect of controlled growth intensity throughout feeding procedure is being conducted since 2009 including 60 to 70 Holstein heifers / year. In 2012, the effect of rearing intensity on puberty attainment was closely studied on 65 heifers born between August 26 and December 27, 2011. Animals born before December 1 were either fed a standard (SD) or an intensive (ID) diet from 0 to 6 mo of age, to reach 190 to 200 kg or 220 to 230 kg at 6 mo of age, respectively. Animals born after received ID until artificial insemination (AI). From 5.5 mo of age, blood samples were obtained from jugular vein every 10 d to determine plasma progesterone concentration. The plasma progesterone analyses were performed with AIA 360 robot. The intra- and inter-assay variabilities were 12.3 and 7.5%, respectively. Blood samples were collected until synchronization or AI. Single Anova analyses were performed through R Software. Puberty was detected on 56 animals, at an average age of 10.2 (±2.0) mo and 296 (±37) kg body weight (BW). Age varied between 6.2 and 14.4 mo, whereas BW was comprised between 224 and 369 kg. Because of huge ADG variations within feeding regimen and limited number of observations, no effect of SD or ID was noted on puberty onset. Puberty occurred earlier when average daily gain (ADG) before weaning (77 to 80 d of age on average) increased (9.7 vs. 10.8 mo when ADG varied from less than 700 g/d to 700 g/d or more respectively; P < 0.05). To a lesser extent, ADG from weaning to 4 mo of age also affected puberty onset. In agreement with previous published studies, this study indicates that enhanced growth program during lactating phase and/ or early stages of rearing may affect puberty attainment