Sherbrooke Research and Development Centre, Agriculture and Agri-Food Canada, Sherbrooke, Quebec, Canada J1M 0C8.
Sherbrooke Research and Development Centre, Agriculture and Agri-Food Canada, Sherbrooke, Quebec, Canada J1M 0C8.
J Dairy Sci. 2021 Apr;104(4):4991-4998. doi: 10.3168/jds.2020-19514. Epub 2021 Jan 21.
Holstein heifers (n = 45) were subjected to treatments according to a 2 × 2 factorial design where the main effects were the photoperiod treatments during the second isometric (ISO, 52-61 wk of age) and the second allometric (ALLO, 62 wk of age to 8 wk before calving) periods of mammary gland development. During the ISO period, heifers were subjected to either a short-day photoperiod (SDP; 8 h light, 16 h dark; n = 22) or a long-day photoperiod (LDP; 16 h light, 8 h dark; n = 23). During the ALLO period, the photoperiodic treatments were either maintained (SDP:SDP, n = 11; LDP:LDP, n = 11) or switched (SDP:LDP, n = 11; LDP:SDP, n = 12). The treatments ended 8 wk before calving. All animals were then subjected to about 16 h of light per day. Serum prolactin (PRL) concentration during the ISO period was greater in heifers exposed to LDP than in those exposed to SDP. For the first 20 wk of the ALLO period, heifers exposed to LDP had greater serum concentration of PRL than those exposed to SDP. On the other hand, previous exposure to LDP during the ISO period reduced the concentration of PRL compared with those exposed to SDP during that period. During the second 20 wk of the ALLO period, PRL concentration remained greater in the serum of heifers then exposed to LDP than SDP, but serum PRL was greater in heifers exposed to LDP during the ISO period. During the last weeks before calving, when all animals were exposed to LDP, previous exposure to LDP during the ALLO period reduced serum PRL. Early-lactation milk (wk 1-5) and energy-corrected milk (wk 2-6) production were higher in the heifers exposed to SDP than in those exposed to LDP during ALLO. Photoperiod had no effect on milk production after that period. In conclusion, the results do not support to the hypothesis that photoperiod affects mammary gland development during the second allometric phase. However, they confirm that a short-day photoperiod in late gestation enhances milk production in the following lactation in primiparous heifers. Using serum PRL as an indicator of the photoperiodic response, we can conclude that responsiveness to the photoperiodic signal is still conditioned by a previous photoperiod several months after it ends.
荷斯坦小母牛（n = 45）根据 2 × 2 析因设计接受处理，主要效应是在第二个等长（ISO，52-61 周龄）和第二个异速（ALLO，62 周龄至分娩前 8 周）乳腺发育期间的光周期处理。在 ISO 期间，小母牛接受短日照（SDP；8 小时光照，16 小时黑暗；n = 22）或长日照（LDP；16 小时光照，8 小时黑暗；n = 23）。在 ALLO 期间，光周期处理要么维持（SDP：SDP，n = 11；LDP：LDP，n = 11）要么切换（SDP：LDP，n = 11；LDP：SDP，n = 12）。处理在分娩前 8 周结束。然后，所有动物每天都接受约 16 小时的光照。在 ISO 期间，暴露于 LDP 的小母牛的血清催乳素（PRL）浓度高于暴露于 SDP 的小母牛。在 ALLO 的前 20 周，暴露于 LDP 的小母牛的血清 PRL 浓度高于暴露于 SDP 的小母牛。另一方面，与该期间暴露于 SDP 的小母牛相比，ISO 期间暴露于 LDP 会降低 PRL 浓度。在 ALLO 的第二个 20 周期间，暴露于 LDP 的小母牛的血清 PRL 浓度仍然高于暴露于 SDP 的小母牛，但在 ISO 期间暴露于 LDP 的小母牛的血清 PRL 浓度更高。在分娩前的最后几周，当所有动物都暴露于 LDP 时，在 ALLO 期间暴露于 LDP 会降低血清 PRL。与 ALLO 期间暴露于 LDP 的小母牛相比，暴露于 SDP 的小母牛的早期泌乳（第 1-5 周）和能量校正乳（第 2-6 周）产量更高。在此期间之后，光周期对产奶量没有影响。总之，结果不支持光周期影响第二个异速阶段乳腺发育的假设。然而，它们证实，在初产小母牛中，妊娠后期的短日照会增强随后泌乳期的产奶量。使用血清 PRL 作为光周期反应的指标，我们可以得出结论，对光周期信号的反应仍然受几个月前结束的先前光周期的调节。
