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    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. Copyright © 2021 American Dairy Science Association. Published by Elsevier Inc. All rights reserved.

    Citation

    P Lacasse, D Petitclerc. Effect of photoperiod before and during first gestation on milk production and prolactin concentration in dairy heifers. Journal of dairy science. 2021 Apr;104(4):4991-4998

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    PMID: 33485686

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