Behavioural effects of yearling grain-finished heifers exposed to differing environmental conditions and growth-promoting agents

Two groups of 108 Angus cross yearling heifers were utilised to determine the effects of growth-promoting agents on behaviour, when utilised under thermoneutral, hot and cold environmental conditions. Pens of heifers were observed throughout the day for feed intake pattern via bunk score, panting (hot), degree of bunching and shivering (cold). For cattle that were exposed to cold stress, feed intake was greater earlier in the day, with the majority of the feed consumed by 1500 hours and little or no feed consumed at night, while the opposite trend occurred under heat stress. Nearly 46% of the pens containing heat-stressed heifers had greater than 50% of their feed remaining in the bunk at 1900 hours. Pens of heifers exposed to thermoneutral conditions had a tendency to show elevated panting scores at 0700 hours, while heifers exposed to hot conditions did not, indicating some acclimation to heat stress had already taken place for the heifers exposed to hot conditions. Panting score did not appear to be affected by growth-promoting treatment. Under cold stress, 100% of the cattle displayed bunching behaviour throughout the day, while under hot and thermoneutral conditions, maximum bunching (25 to 30%) occurred at 1500 hours. Within an environmental condition, trenbolone acetate cattle tended to bunch more under thermoneutral and hot conditions, particularly in the morning, when compared with other treatment groups; control and oestrogen-treated cattle tended to bunch less under the same conditions, regardless of the time of day. Although data were inconclusive as to overall effects of growth promotants on mitigating cold stress, shivering scores were increased with a more aggressive growth-promoting treatment (P < 0.05). These findings suggest that if growth promotants, which are used in feedlot cattle, impact cattle exposed to adverse environmental conditions, then they tend to be more effective at mitigating heat stress than cold stress