Changes in the respiration rate pattern of cattle exposed to prolonged heat load

Exposure of non-acclimated cattle to prolonged periods of high heat load has a negative impact on their ability to cope, especially when relief is limited. This study was undertaken, in part, to investigate the effect of prolonged exposure (11 days) to high heat load on respiration rate in cattle. Six black Angus steers (12 months of age) (451.4 ± 10.2 kg) with moderate coats were used. Cattle were housed in a feedlot and exposed to cool conditions (temperature range of -1 to 21 oC) for 40 days, after which they were moved to a climate controlled facility and allowed a 4 day adaptation period before being subjected to high heat load for 11 days.

The steers were housed in 3 x 1 m (3 m2) stalls in the climate facility and restrained via a head halter tied to the front of the stall. Each stall was fitted with an individual water bowl and feed bin. The front half of the stall was a solid floor covered by rubber matting and the back half of the stall floor was wooded slates. The steers were fed a moderate energy grain based diet (8.0 MJ ME; 12 % Crude Protein, 25 % roughage ~ as fed), at 0800 h and 1400 h in equal proportions. Daily feed allocation (kg/head/day) was done on the basis of 2.5% of live weight. Cattle had access to drinking water at all times.

Respiration rate (RR) was measured hourly while cattle were exposed to hot conditions. This was done by counting 10 flank movements and recording the time (seconds) for this to occur. Also, panting scores (PS; 0 = no panting to 4.5 severe panting) were obtained as an indicator of heat stress. Generally cattle are considered to be under considerable stress when PS >3.

Limited relief via short-term (less than 1 min) water application occurred when drooling and/or open mouth panting (panting score >3.0) was observed. Cattle were wetted using a 25 mm diameter hose attached to a pressure pump. Water was applied to the back and head of cattle in sufficient quantity so that water was just beginning to run off the animals. The water applied was approximately 30 oC and contained 3% NaCL. There was little (0.5 – 1.2 m/s) direct air movement over the cattle.

Dry bulb temperature (TA) and relative humidity (RH) were recorded at 5-minute intervals (to a data logger). Wet bulb temperature (WBT) was measured hourly via a wet bulb thermometer.

The climatic conditions were designed to move from a 4-day thermoneutral period (TA; 16.2 – 21.5 oC) to a hot period (HOT) that ran for 11 days. The HOT period was designed to increase dry bulb and wet bulb temperature each day over the 11 days. Dry bulb and WBT (WBT in bracket) increased from a mean of 23.4 oC (22.5 oC) on day 1 of hot to a mean of 31.5 oC (25.0 oC) on day 6 and to a mean 36.9 oC (31.5 oC) on day 11. Relative humidity was maintained between 75 and 85%.

Mean daily RR’s were lower (P<0.05) under the cool conditions (76.5 ± 18.3 breaths per minute; bpm) than during exposure to HOT (109.4 ± 10.3 bpm). On day 1 of HOT the mean RR was 99.3 ± 14.0 bpm. The RR continued to increase (P>0.05) on days 2 and 3 of HOT (119.1 ± 15.6 bpm; 128.8 ± 19.8 bpm). In line with previous studies RR decreased from the 4th day of exposure to HOT (121.3 ± 16.0 bpm) and were then maintained within a fairly narrow range over days 5, 6 and 7 (115.9 ± 15.3, 118.9 ± 14.1 and 117.3 ± 10.3 bpm respectively). Respiration rates were lower (P<0.05) on day 8. The mean RR was 109.1 ± 9.4 bpm, which was a 7% reduction from the previous 3 days. Mean RR remained between 96 and 110 bpm over days 8 – 11. Lower RR may be interrupted as cattle coping with the adverse conditions. However, when other factors are assessed, e.g. feed intake (less than 2 kg per head per day was consumed over the last 4 days), animal behaviour (agitated appearance) and a panting score (>3) it was evidence that the cattle were not coping.

For all steers mean panting scores where greater than 2 (fast panting, drool or foam present) after the 3rd day of exposure to HOT. Three steers showed a PS >3.5 (open mouth, tongue out slightly). Steer 1 showed a PS 4 (open mouth, tongue extended, head held up) on two occasions on day 10. Steer 2 also showed PS 4 on day 10, and progressed to PS 4.5 on at least two occasions on day 10. Steer 5 showed a PS 4 on 3 occasions, once on day 9 and twice on day 10. Panting Scores were used as a basis to commence wetting. If any one steer had a PS > 2.5 wetting was initiated. In all cases PS fell to a PS 2 or lower following wetting. The time between wettings decreased from once per day on day 2 of HOT to every 3 – 4 hours on the final day of the study.

Panting scores may be a more reliable indicator of the heat load status than respiration rate. It is common for cattle with PS above 3 to show a reduction in respiration rate, because they have moved from rapid shallow breathing to slow deep breathing.