The interactions between maturity type and pre-feedlot plane of nutrition on the growth and performance of steers.
The growth of tissues (bone, muscle and fat), along their natural growth curve, is controlled by a complex array of interactions. Growth gradients exist between the tissues and the body as a whole, bone being earlier maturing than muscle, and muscle being earlier maturing than fat. Growth waves within each tissue express its rate of deposition within in each area of the body. Differences between maturity types with respect to tissue growth, is that the earlier maturing animal is further along its normal growth curve. Comparison between maturity types must therefore be performed at an equal physiological age. Restriction of nutrients to the growing animal results in an alteration of the body composition. The most affected tissue being the tissue with the highest growth impetus at the time. Severe restriction (loss of liveweight), can result in the tissues following a reverse order of their deposition. Re-alimentation results in tissue growth at a rate superior to animals of an equal chronological age. Those tissues that were most restricted in their growth, are the ones that show the most compensation. A trial was carried out, to examine, the question of animals of differing pre-feedlot planes of nutrition, and maturity type, performances within the feedlot at an equal physiological age. Four treatments were used comprising earlier maturing fat and thin animals and later maturing fat and thin animals. Earlier maturing comprised Hereford and Sussex breeds, while later maturing comprised Charolias and Simmentaler breeds. The pre-feedlot planes of nutrition were imposed for 103 days, with the fat animals gaining at 0.40 kg per day and the thin animals at 0.10 kg per day. Both treatment groups lost condition but started the feedlot period significantly different with respect to liveweight and condition. Physiological age was to be determined, by the weight of the animals body protein, as a proportion of the maturity types, mature body protein weight. The urea dilution technique was used to determine the body composition of the treatments at any one point of time. Due to complications with the application of the technique, the body compositions of the animals were not determined with any degree of certainty. Thus it was impossible to compare the feedlot performances of the treatments at equal physiological ages, or compare their changes in tissue weights over time. Differences in tissue deposition rates were measured. Compensating animals had significantly higher growth rates in terms of height. This equates to a higher rate of bone tissue growth. As height measurements were not taken during the pre-feedlot period this could not be attributed to compensatory growth. Ultrasonic measurements of eye-muscle diameters, showed that the compensating animals and the later maturing fat animals to be growing at a non-significantly different rate. This could, possibly, be due to the animals depositing at the maximum allowable rate. Subcutaneous fat deposition was measured as change in condition score. Compensating animals deposited fat at a significantly faster rate, with no significant differences between maturity types being apparent. All animals were slaughtered at a set condition. This resulted in the early maturing fat animals, spending a significantly shorter period of time in the feedlot. Analysis of subcutaneous fat depths found no significant differences between treatments, indicating that the animals had been slaughtered at equal condition scores. Fat distribution over the measured sites however showed that there were significant differences between the compensating animals. The earlier maturing being to fat and the later maturing being to thin. Thus the two groups could have spent a shorter or longer period of time in the feedlot respectively. liveweight changes over time were as predicted. The late maturing treatments had significantly higher growth rates than their respective earlier maturing treatments. The compensating treatments also had significantly higher growth rates than respective non-compensating treatments. Only the early maturing thin animals managed to make up the deficit sixty kilograms. Feed and net energy available for growth (NEg) intakes, were complicated, with anomalies being found in the data distribution. Animals feed and NEg intakes increased linearly, before peaking at the same point of time irrespective of maturity type, and then followed a linear trend of significantly different slope. The quadratic model failed to follow the data trend accurately enough so the broken stick model was used. Compensating animals ate significantly more food and NEg per kilogram of metabolic weight (W (0.75)). There utilisation of the food and NEg was however significantly more efficient than the non-compensating animals. No significant differences were found between maturity types within pre-feedlot treatments. Further investigation is required into the anomalies surrounding the analysis of the feed and NEg intake data. A biological justification must be found for the use of the broken stick model. The change in the linear trend after peak feed intake appears to be due to a restriction. This restriction should be determined as it affects animals irrespective of nutritional requirements.