Wetting and the physiological responses of grain-fed cattle in a heated environment

Abstract

A controlled crossover experimental design was used to determine the effect of altered water sprinkling duration on heifers subjected to heat stress conditions. Heifers were subjected to 3 days of thermoneutral conditions followed by 3 days of hot conditions accompanied by water sprinkling between 1300 and 1500 h (HOT_1–3). Then on the following 2 days (HOT_4–5), environmental conditions remained similar, but 3 heifers were sprinkled between 1200 and 1600 h (WET) and 3 were not sprinkled (NONWET). This was followed by a 1-day period (HOT₆) in which environmental conditions and sprinkling regimen were similar to HOT_1–3. Rectal temperature (RT) was collected hourly, and respiration rate (RR) was monitored every 2 h on HOT Days 2, 4, 5, and 6. Dry matter intake and rate of eating were also determined. Sprinkling reduced RR and RT (P < 0.01) of all heifers during HOT_1–3. During HOT_4–5, WET heifers had lower (P < 0.05) RT than NONWET from 1300 to 700 h and lower RR from 1400 to 2000 h. Dry matter intake of NONWET heifers was reduced by 30.6% (P < 0.05) during HOT_4–5 and by 51.2% on HOT₆. On HOT_4–5 the dry matter intakes of WET heifers were similar to intakes under thermoneutral conditions. During HOT₆, RT was again reduced following sprinkling in all heifers. Comparison of RT and RR of NONWET and WET heifers on HOT_{1–3 v.} HOT₆ revealed that under similar environmental conditions, NONWET heifers had increased RT, partially due to carry-over from HOT_4–5. However, NONWET heifers had 40% lower feed intake but tended to have lower RR on HOT_{6 v.} HOT_1–3. Only RR of WET heifers was greater on HOT₆, possibly a result of switching from a 4-h back to a 2-h sprinkling period, while maintaining a 62% greater intake (5.80 v. 3.58 kg/day) than NONWET heifers during this time. Results suggest that inconsistent cooling regimens may increase the susceptibility of cattle to heat stress and elicit different physiological and metabolic responses.