Illini DairyNet Papers
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- High-producing cows had numerically higher feed and water intakes. Body temperature was higher in high- than low-producing cows. Respiration rates were numerically higher and heart rates numerically lower in high-producing cows as environmental temperature increased.
- Feed intake of high-producing cows dropped more steeply than that of low-producing cows with increasing environmental temperature.
As environmental temperatures increase, the dairy cow responds in several ways. Responses to thermal stress include behavioral and physiological adjustments, increased water intake, reduced feed intake, and concomitant reductions in milk production. Less understood is whether high- and low-producing cows are affected by heat stress at the same environmental temperature. Our objective was to compare the responses to increasing temperature of high- and low-producing Jersey cows, and to study whether milk production influenced the point at which responses to heat stress start to occur.
MATERIALS AND METHODS
Environmental, production, and physiological data from four lactating Jersey cows in mid-lactation, assigned to two groups of two animals each based on their milk production (high and low) were collected at 6-hourly intervals over an 11-day period from mid-April 1999. A day before the trial began, all four cows received a rBST injection. Cows were housed in single tie-stalls and fed a TMR based on corn silage for ad libitum intake. Good quality hay was offered at 2 lb per cow per day. Feed and water intakes were individually monitored daily. Cows were milked at 4:00 A.M, P.M and milk production measured.
Body temperature, heart rate, respiratory rate, room temperature, and relative humidity were measured at 4:00, and 10:00 A.M., and 4:00, and 10:00 P.M. Cows were weighed at the beginning and at the end of the study. After a 3-day acclimation period, the room temperature was raised by 5 degrees F every two days from 63 up to 83 degrees F .
RESULTS AND DISCUSSION
Average daily milk production was 57.3 and 42.4 lb per cow for high- and low-producing cow groups. This difference was significant (P <0.0001). There were numerical differences in heart rates at the morning (78.6 and 80.0) and afternoon (77.4 and 79.9) recording times between high- and low-producing cows respectively. The respiration rates were (60.9 and 58.4) in the morning and (69.1 and 61.0) in the afternoon for high- and low-producing cows respectively. Body temperature was different (P < 0.0005) at morning (101.8 and 100.9 degrees F) and also (P<0.0016) at afternoon recording (101.8 and 101.0 degrees F) for the high- and low-producing cows respectively. As expected, the average daily feed intake (59.2 and 55.7 lb) and average daily water consumption (183.9 and 177.7 lb) were numerically higher for high- and low-producing cows respectively.
Heart rate was significantly (P < 0.001) correlated to the respiration rate (r = 0.6; and r =0.4) at the afternoon morning and afternoon recording times respectively. In line with current knowledge, environmental temperature was significantly (P = 0.0001) correlated to the heart and respiration rates (r = 0.4; r = 0.7) in the morning and (r = 0.3; r = 0.7) in the afternoon for both high- and low-producing cows.
These findings suggest that high-producing dairy cows are likely to enter heat stress earlier than their low-producing counterparts. High-producing cows have higher levels of feed intake, and concomitantly produce more metabolic heat when compared to low milk producers. If high-producing cows are to maintain and sustain high levels of milk production, their thermal environment needs to avoid heat stress. Our findings indicate a need for better management of thermal environments, especially of high-producing cows.