File(s) under embargo
Reason: Unpublished work in my thesis
until file(s) become available
An Evaluation of Tissue Mobilization and Chronic Circadian Disruptions in Transition Dairy Cows
A successful dairy cow transition period is paramount to a productive lactation. Termed as the several weeks before and after calving, the transition period is characterized by several metabolic changes that occur in all tissues to support copious amounts of milk production in early lactation. Due to insufficient intake in early lactation, cows undergo tissue mobilization whereby they reduce their insulin sensitivity to liberate adipose and muscle tissue to meet nutrient requirements. However, physiological stresses during this period are responsible for an increased risk of disease, which animal welfare and thus productivity. Furthermore, a cow’s metabolism is regulated by the circadian system, which integrates timing cues from the environment to coordinate the body’s physiological systems to regular occurring daily events. Disruptions to the timing of events can negatively affect animal health and thus potentially lactation performance. Sixteen multiparous Holstein cows were subjected to either a control (CON) consistent timing of the light-dark cycle or a 6 h forward shift light dark cycle phase (Phase Shift; PS) every 3 d in the last thirty-five days before expected calving (BEC). Once animals calved, they were given CON timing of light through 60 days in milk (DIM). Secondly across two phase-shift studies, 48 multiparous Holstein cows had ultrasound measurements collected at the 12th intercostal space and blood metabolite markers of tissue mobilized analyzed at various timepoints between 35 d BEC and 60 DIM. Cattle that had a longissimus dorsi depth (LDD) >4.10 cm at 35 d BEC were determined to be in a high muscle (HM) group whereas those below that threshold were relegated to a low muscle (LM) group.
Cattle that were exposed to the PS light pattern in the late gestational period were determined to have a disrupted, weaker circadian rhythm than CON animals as a result of a decreased internal temperature circadian rhythm amplitude and delayed start of the daily circadian rhythm. These circadian disruptions resulted in PS cows developing greater insulin resistance than CON cows at 14 d BEC (4,303 vs. 2,386 mIU Insulin AUC/180 min) and at 7 DIM (1,053 vs. 697 mIU Insulin AUC/180 min) after the light timing treatment was removed. The PS timing of light was associated with reduced mammary development at 21 d BEC (5.22 vs. 12.44% epithelial proliferation) and those animals produced less milk compared to CON through 60 DIM (40.3 vs. 42.6 kg/d). Thereby demonstrating the importance of maintaining consistency in the timing of events during the dry period to maximize animal health and performance through early lactation.
HM cattle mobilized more muscle tissue than LM cows from 35 d BEC through 60 DIM (1.64 vs. 0.30 cm LDD mobilized) to have no difference in LDD at 60 DIM (3.41 vs. 3.09 cm). While there were no differences in milk production (40.6 vs. 42.0 kg/d), milk components, or intake between HM and LM cows, there was a tendency for HM cows to give birth to heavier calves (44.3 vs. 42.3 kg) and have greater 3-MH:CRE ratio (0.153 vs. 0.135) over the first three weeks of lactation. Overall, cattle mobilized muscle and adipose tissue reserves through 30 DIM relative to the amount of those reserves they had available at 21 d BEC (R2=0.47 and 0.82, respectively). Thereby suggesting that cattle tissue reserves in the last few weeks of gestation affect the nutrient source that cattle primarily use to meet nutrient requirements from late gestation into early lactation. Continued research will need to address how alterations in the timing of events and quantity of tissue reserves through the lactation period affect cattle performance and heath in the postpartum period.