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Evaluating the effects of heat stress on rumen pH and temperature
As the climate changes, heat stress is becoming an increasing concern for heavily utilized agriculture assets like dairy cattle. The goal of this study is to evaluate the effects of ambient temperature change on ruminal pH and temperature. The study was conducted using 8 Holstein heifers (250 kg) housed in climate-controlled rooms (4 heifers per room). Heifers were housed at a thermoneutral (20 oC) temperature for 2 weeks after which room temperature was raised to 30 oC. Measurements were taken with a ruminal pH and temperature bolus that was manually inserted into the reticulum through a rumen cannula. Each bolus wirelessly transmitted ruminal pH and temperature every 10 minutes for the duration of the study. Response variables included: hourly mean, minimum, and maximum pH; proportion of hourly time spent below 5.5 or 5.75; and mean, minimum, and maximum rumen temperature. Data was used from 2 days prior and 2 days after changing the ambient temperature to specifically assess this temperature transition. A mixed-effect model was used to evaluate the data with fixed effects for group (heat stress; HS; or thermoneutral; TN), period (thermoneutral, P1; stressed, P2), and group by period interaction and a random effect for cow. The results are summarized in Table 1. The significance of the group by period interaction for mean (P=0.053) and maximum (P=0.005) pH demonstrates that that heat stress lowers average rumen pH by reducing the height of pH peaks occurring in between digestion events. However, this pH shift is not associated with decreases in minimum pH or a change in time spent below cutoff pH values (P>0.05). Further work must be conducted to evaluate what effects this pH shift has on rumen fermentation kinetics during heat stress.
Keywords: heat stress; rumen pH; rumen temperature