Farm Table says:
What is the Problem?
One way to alleviate heat stress in feedlot cattle may be to change the processing method or grain type being fed such that the peak evolution of heat from metabolic processes in the animal does not coincide with maximum environmental heat load.
This study aim was to determine if cattle fed a diet containing slowly-fermentable grain would exhibit less heat stress than when fed a rapidly-fermentable grain under conditions of a cyclical EHL.
In order to select a feeding time that would achieve the desired synchrony between EHL and metabolic heat evolved from the readily-fermentable diet, it was deemed desirable to:
- determine the likely digestion patterns of diets in the rumen
- characterise the patterns of aerobic energy production in (non-rumen) body tissues
- establish an experimental protocol to achieve synchrony between EHL and peak metabolic load for one diet
- to measure the animal response to EHL for cattle fed 2 diets, using behavioural and physiological indices
What did the research involve?
3.1 Trial location and oversight
Trials were conducted in the animal house and two climate rooms at the J M Rendel Laboratory, CSIRO Rockhampton, in Queensland.
3.2 Animals, diets and heat exposure conditions.
3.2.1. Diets The oversight committee advised that the diets be fed at a rate of 2.7% (as fed) of body weight, that the diets should differ only by grain type, specified as wheat or sorghum, both grains processed by dry-rolling, and diets should not contain growth promotants.
3.2.2. Animals. Twenty Bos taurus (Hereford) steers, mean weight 307 kg (range 261 to 370 kg), were trucked from Roma, Queensland in July 2007, treated for external parasites with Cydectin© and fed lucerne hay for 4 weeks while being trained to experimental conditions 3.3 Experimental protocol
3.3 Experimental protocol
Twelve weeks after steers had arrived in the animal house, the 4 animals in group 1, which had been feeding on the mixed grain ration plus 1.5 kg wheat straw, were randomly allocated to the diets and fed at a daily rate of 2.7% (as fed) of live weight, plus 1 kg/d of wheat straw.
– Cattle in groups were exposed to EHL in climate rooms in which ambient temperature and relative humidity could be controlled, and feed intake of the grain based diet (but not straw) was automatically logged every 10 minutes.
What were the key findings?
4.1 Extent and pattern of fermentation of diets.
Diets were further characterised by gas exchange (in vivo) measurement using animals allowed to eat for 1 hour in pens before being moved into respiration chambers.
4.2 Response of animals to cyclical THI
4.2.1. Patterns of feed intake. Steers in groups 1-4 fed the wheat diet tended to eat more slowly than steers on sorghum; 25% and 50% of the wheat ration was consumed by 1.8 and 5.2 hours respectively, compared to 1.2 and 4.5 hours for the sorghum ration.
4.2.2. Behavioural and physiological indices of thermal stress. The heart rate monitors were unreliable, yielding data sets with many missing values. Results were not analysed.
126.96.36.199. First EHL exposure
– Maximum values of PS, RR and RT were obtained at 1400 h (Figure 6), and means of all indices were significantly higher on the third day of EHL compared to previous days
188.8.131.52. Second EHL exposure
For data taken between 0800 and 1700, for EHL days, three quarters of wheat-fed steers, and all sorghum steers were observed to have a RR of above 120 bpm.
4.2.4. Blood stress indicators
Biochemical indicators of stress, including general haematological measures and lymphocyte heat shock proteins, were analysed by Dr Linda Agnew at the CSIRO Armidale laboratory as part of the MLA-funded project “Interactions between the neurophysiological and immune systems as objective measures of animal welfare”.
4.2.5. General comments
When interpreting results, the confounding of diet type with intake of digestible starch (and energy) needs to recognised.
To this end, an appropriate experiment might include different times of feeding of two diets, one readily-fermentable and one slowly fermentable, using an EHL protocol more severe than used in the present study.