Date Published:

JUL

Abstract:

Short-day photoperiod (SDPP; 8 h light:16 h dark) during the dry period increases milk production compared with long-day photoperiod (LDPP; 16 h light:8 h dark). We hypothesized that the impact of photoperiod on lactation is mediated by alterations in the circadian system. Twelve Saanen goats were blocked at dry off into SDPP (n = 6) and LDPP (n = 6) treatments and mammary biopsies were taken in the middle of light and dark phases at 3 wk prepartum and 5 wk postpartum. Total RNA was isolated, and the expression of clock genes was analyzed by qPCR. SDPP goats produced more milk than LDPP goats (3.15 +/- 0.04 vs. 2.7 +/- 0.05 kg/d). In the 24 h period, LDPP goats had a greater body temperature than SDPP goats at 3 wk prepartum (39.6 +/- 0.06 vs. 39.3 +/- 0.1 degrees C) and 5 wk postpartum (40.1 +/- 0.15 vs. 39.7 +/- 0.1 degrees C). Cosinor analysis revealed that physiological state affected body temperature mesor (P< 0.001), peak (P< 0.01), amplitude (P< 0.05), and phase (P< 0.001). Plasma prolactin was 20, 10, and 17-fold higher in LDPP than in SDPP goats at 3 wk prepartum, 3 wk postpartum and 5 wk postpartum, respectively. Cosinor analysis revealed that photoperiod affected prolactin mesor (P< 0.0001), peak (P< 0.0001), trough (P< 0.001), amplitude (P< 0.01), and the peak to trough ratio (P< 0.01). Mammary expression of a core clock gene, PER1, was affected by the light-dark phase and the photoperiod treatment (P< 0.05). It also exhibited a photoperiod-physiological state interaction. Changes in the circadian rhythms with the onset of lactation and photoperiod manipulation support further studies of their role in the regulation of milk yield.