Enhanced early-life nutrition promotes hormone production and reproductive development in Holstein bulls

摘要

Holstein bull calves often reach artificial insemination centers in suboptimal body condition. Early-life nutrition is reported to increase reproductive performance in beef bulls. The objective was to determine whether early-life nutrition in Holstein bulls had effects similar to those reported in beef bulls. Twenty-six Holstein bull calves were randomly allocated into 3 groups at approximately 1 wk of age to receive a low-, medium-, or high-nutrition diet, based on levels of energy and protein, from 2 to 31 wk of age. Calves were on their respective diets until 31 wk of age, after which they were all fed a medium-nutrition diet. To evaluate secretion profiles and concentrations of blood hormones, a subset of bulls was subjected to intensive blood sampling every 4 wk from 11 to 31 wk of age. Testes of all bulls were measured once a month; once scrotal circumference reached 26 cm, semen collection was attempted (by electroejaculation) every 2 wk to confirm puberty. Bulls were maintained until approximately 72 wk of age and then slaughtered at a local abattoir. Testes were recovered and weighed. Bulls fed the high-nutrition diet were younger at puberty (high = 324.3 d, low = 369.3 d) and had larger testes for the entire experimental period than bulls fed the low-nutrition diet. Bulls fed the high-nutrition diet also had an earlier and more substantial early rise in LH than those fed the low-nutrition diet and had increased concentrations of insulin-like growth factor-Ⅰ (IGF-Ⅰ) earlier than the bulls fed the low-nutrition diet. Furthermore, we detected a temporal association between increased IGF-Ⅰ concentrations and an early LH rise in bulls fed the high-nutrition diet. Therefore, we inferred that IGF-Ⅰ had a role in regulating the early gonado-tropin rise (in particular, LH) and thus reproductive development of Holstein bulls. Overall, these results support our hypothesis that Holstein bull calves fed a high-nutrition diet reach puberty earlier and have larger testes than those fed a low-nutrition diet, and they provide clear evidence that nutritional modulation of Holstein bull calves during early life has profound effects on reproductive development.