Feeding strategies to alleviate heat load in feedlot cattle

Abstract

High heat load conditions can adversely affect feedlot cattle well-being and performance. This is particularly evident when animals are exposed to heat wave conditions. Heat stress related annual economic losses of over $16 million have been reported for the Australian feedlot industry, and over $300 million has been reported for the U.S beef sectors. Forecasted climate changes predict increases in the number, duration and intensity of high heat load events, which have the potential to greatly increase the economic costs of heat stress for the feedlot industry into the future. Therefore, there is a need to develop effective management strategies to help alleviate high heat load to support feedlot cattle well-being and performance.Determining the consequential impacts of the thermal environment on cattle has led to the development of many effective climatic indices. The heat load index (HLI) model was developed to account for the effect of numerous environmental, animal and management factors on the responses of feedlot cattle to high heat load. In conjunction with the HLI, the accumulated heat load (AHL) model was developed to account for the effects of the intensity × duration on cattle exposed to high heat load. However, the efficacy of these models in determining cattle responses to high heat load need to be further investigated.Feeding strategies that can be integrated into current feedlot industry practices during high heat load periods may be a method to help ameliorate the negative effects of heat load on cattle well-being and performance. Such feeding strategies include the proposed use of a heat load diet (HLD) during high heat load periods. The HLD utilised in experiments discussed within this thesis had an increased roughage content compared to that of a standard commercially available finisher diet. Transitioning feedlot cattle to a HLD during high heat load periods may help better regulate ruminal conditions and avoid incidence of acidosis, which may improve animal well-being and performance. Dietary supplementation with a yeast and complex-B vitamins supplement during exposure to high heat load is another feeding strategy utilised within the cattle industry. Despite the positive effects of a yeast and complex-B vitamins supplement on animal thermoregulatory and production parameters in dairy cattle, investigation into its use for grain-fed beef cattle is limited.The experiments within this thesis were focussed on;\xa0\xa0\xa0\xa0\xa0 i)\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0 Reviewing and evaluating the underlying aspects of the HLI and AHL models under commercial feedlot conditions to accurately predict the panting score (PS), dry matter intake (DMI) and metabolizable energy intake (MEI) of feedlot cattle with variations for breed type, days on feed (DOF) Class, sex, pens and feedlot.\xa0\xa0\xa0 ii)\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0 Determining the effect of different feeding strategies, and time of feeding on the physiological, rumen conditions and performance responses of grain-fed cattle when exposed to simulated high and acute heat load conditions.\xa0\xa0 iii)\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0 Evaluating the effects simulated acute heat load conditions has on the regulation of rumen temperature (TRUM) and rumen pH, and occurrence of acidosis in grain-fed beef cattle.From the experiments conducted the key findings were;O\xa0 The panting score, DMI and MEI responses to HLI and AHL are highly breed type, sex, DOF Class and feedlot locality specific. O\xa0 Despite disrupted heat loss mechanisms of cattle due to exposure to high HLI conditions, changes in DMI and MEI are more negatively influenced by AHL compared with HLI.O\xa0 Supplementation with a yeast and vitamin B-complex product appeared to negatively influence the thermoregulatory capabilities and performance of grain-fed beef steers when exposed to a simulated heat wave. O\xa0 Cattle that responded adversely to the simulated acute heat wave conditions, warranting removal from the climate control rooms, typically displayed greater respiration rates and TRUM responses during both thermoneutral and heat wave conditions. Despite the removed steers having lighter live weight (LW) and similar body condition scores, it is assumed that individual animal inherent and transient factors may have increased the heat susceptibility and heat stress levels of these individuals, compromising their thermal exchange capabilities. O\xa0 Generally, TRUM surpassing a threshold of 42 oC may result in the homeostatic systems within the body reaching their upper critical limits for normal function. The response of TRUM to the climatic conditions is strongly individual animal driven with no one animal response to conditions being identical. The TRUM appears to steadily increase with increasing heat load conditions before surpassing this 42 oC threshold resulting in a sudden crash in homeostasis. This may be due to an imbalance in the thermoregulatory ability of the animal to maintain homeostasis, hence bodily functions begin to fail due to a severe lack of heat dissipation and a resultant accumulation of body heat.O\xa0 Cattle experience greater adverse alterations in rumen pH during post heat wave periods, resulting in lower mean rumen pH and greater duration of time spent below sub-acute and acute ruminal acidosis thresholds.O\xa0 The increase in the duration of time spent below SARA and ARA acidosis thresholds occurred 4-days after the onset of acute heat wave conditions, which coincided with an increase in DMI as a percentage of LW (DMILW) and decreases in the rumen pH, TRUM and faecal scores of cattle. This may indicate that as a result of exposure to the simulated acute heat wave, the ruminal conditions appear to become compromised and occurrence of acidosis increases.O\xa0 In comparison to cattle fed a HLD, cattle fed a finisher diet spent a greater duration of time below the acute acidosis threshold during the heat wave and post heat wave conditions, possibly indicating that the lower proportion of roughage, and hence fibre, in the finisher diet predisposed these cattle to a greater risk of developing acute acidosis.O\xa0 Current findings support the concept that transitioning to a HLD to alleviate the negative rumen pH responses of grain-fed beef cattle may be more important to implement post heat wave periods rather than during heat wave conditions. Therefore, it is suggested that feeding a HLD during post heat wave periods may help to better regulate rumen pH to decrease the occurrence of acidosis and hence support rumen health to improve animal well-being and performance.