Long-day photoperiod that enhances puberty does not limit body growth in Holstein heifers

Previous research has demonstrated that extended photoperiod accelerates pubescence in dairy heifers thereby limiting time for mammary development, which could be detrimental to future milk yield. We hypothesized that the potential negative effects of rapid growth and puberty through long-day photoperiod (LDPP) exposure could be overcome with a greater supply of metabolizable protein in dairy heifers fed rumen-undegradable protein (RUP). In an initial slaughter study, we compared deuterium oxide (D₂O) and direct chemical analysis to assess body composition at 5 and 7 mo of age in heifers (n = 20) exposed to LDPP or short-day photoperiod (SDPP). Before slaughter, D₂O dilution was used to estimate body composition and results were compared with actual values determined by direct chemical analysis of body tissue. In 5-mo-old heifers, the correlations between estimates of body protein, water, and mineral contents as determined by D₂O dilution and direct chemical analysis of body tissue were 0.86, 0.85, and 0.76, respectively; however, fat content values were not correlated (r = −0.068). In 7-mo-old heifers, we were unable to accurately estimate body composition using the D₂O dilution method. A second study was conducted to determine if LDPP, which has previously been shown to hasten puberty, could be combined with RUP to promote lean growth without limiting body stature in prepubertal heifers. Thirty-two weaned heifers (86 ± 2 d old; 106.2 ± 17.3 kg of body weight) were assigned to LDPP or SDPP and RUP or control diet in a 2 × 2 factorial arrangement until the onset of puberty. Relative to SDPP, LDPP increased prolactin secretion and promoted lean growth. Exposure to LDPP also enhanced body weight, withers height, and heart girth. Furthermore, RUP supplementation increased withers height and heart girth. There was a significant interaction between LDPP and RUP for hip height. Moreover, LDPP hastened the onset of puberty. In summary, D₂O was a feasible method to estimate lean composition in heifers at younger ages; however, it failed to accurately estimate body composition in heifers around puberty. Long-day photoperiod hastened puberty and accelerated lean growth without limiting skeletal growth in dairy heifers.