The impact of late gestation circadian disruption, by two weeks of exposure to chronic light dark phase from 5 weeks before expected calving to 3 weeks expected calving, on dairy cattle hepatic and mammary transcriptomes.

Our previous studies found circadian disruption increased insulin resistance and decreased mammary development in late pregnant cows. The effect of circadian disruption on liver and mammary transcriptomes was measured to understand how it impacted hepatic function and mammary development. At 35 d before expected calving (BEC) multiparous Holstein cows were assigned to either control (CON) or phase-shifted treatments (PS). CON was exposed to 16 h light to 8 h of dark, and PS treatment was exposed to 16 h light to 8 h dark, but the phase of the light-dark cycle was shifted 6 h every 3 d. At 21 d BEC, liver and mammary were biopsied. RNA was isolated (n=6 CON and n=6 PS per tissue), libraries prepared and sequenced using paired end reads. Reads mapping to bovine genome averaged 27 M ± 2 M, and aligned to 14,222 protein coding genes in liver and 15,480 in mammary analysis. Milking was ceased in multiparous Holstein at 60 d before expected calving (BEC) to begin their non-lactating dry period. At 35 d BEC, cows were moved to a tie-stall barn and assigned to one of two study treatments, control (CON; n=6) or phase shift (PS; n=6), by blocking for lactation number and previous lactation yield. All cows were housed in the same experimental barn. To separate treatments, the barn was partitioned with floor to ceiling double draping of fire-retardant tarps. To block natural light, windows were covered with black plastic, and caulking was applied around doors. Bright white light-emitting diode (LED) lights were positioned for 150 lx of light at the eye level of cows. CON cows were exposed to consistent light-dark cycles of 16 h light to 8 h of dark, with the light phase exposure from 0500 to 2100. The PS treatment was exposed to 16 h light to 8 h dark, but the phase of the light-dark cycle was shifted forward by 6 h every 3 d. Tissue used for this study was obtained by biopsy of mammary and liver at 21 d BEC. The PS cows had been exposed to 2 weeks of chronic light-dark phase shifts, and at least 5 shifts in the light-dark cycle at the time of tissue biopsies.