The Role of Dietary Protein as a Regulator of the expression of marbling in feedlot cattle (WA)

Summary

The primary aim of this project was to test the effects of dietary protein on the expression of marbling. Two different dietary protein based hypotheses were tested: Feeding steers a diet low in rumen degradable protein will increase total body fatness and so the marbling response. Feeding steers a diet containing extra rumen degradable and extra rumen undegradable protein will increase the expression of marbling. The first hypothesis was aimed at constraining muscle growth and therefore encouraging fat deposition both in the carcass and in the muscle. The second hypothesis was based on the proposal that extra amino acids available for degradation within the animal will primarily pass through as glucose and so increase glucose availability this would in turn lead to increased marbling because the intramuscular adipocytes show a preference for glucose as the main lipogenic substrate. An additional aim was to understand the development of the intramuscular fat depot in relation to the growth of muscle and other fat depots within the carcass.

The hypothesis tested was: The intramusuclar fat depot develops at the same rate as total body fat (intermuscular + subcutaneous). All of the protein supplements were highly degraded in the rumen (78-86% degradable) however each protein source had a significantly different degradability with: lupin grain > commercial expella canola > formaldehyde treated expella canola. The formaldehyde treated expella canola was significantly less rumen degradable by 5 percentage units when compared to commercial expella canola. This degree of protection is substantially less than attained in other studies where solvent extraction canola meal was the base ingredient. Conclusions on the effect of diet on marbling and intramuscular fat are: Marbling score was either unchanged or increased and intramuscular fat was unchanged in the m. longissimus thoracis (LT, eye of the cube role) of steers fed the barley urea rations Extra protein in the form of commercial expella canola meal or formaldehyde treated expella canola meal did not influence marbling or intramuscular fat level in the LT. The inculsion of formaldehyde treated expella canola meal (10%) reduced muscle growth but did not increase marbling score or intramuscular fat. The reason for this is unknown.

There is some evidence that during periods of more rapid growth that the addition of expella canola meal to the diet increased growth rate even at high body weights (i.e.>540kg). However during the final 50 days of feeding (690-736kg liveweight) this effect was reversed (i.e. barley based diets performed better) so that the final liveweight and carcass weight was similar for all diets. A very simple diet based on barley and good quality hay (10.7% crude protein) and fed for the last 100-150 days would appear to be the most economic diet for allowing adequate growth and marbling to meet market specifications for Japan. Even the inclusion of urea seemed unnecessary, although its relatively low cost and lack of negative effects on marbling would suggest that it should remain part of the ration.

Conclusions about the general growth pattern based on carcass composition at 304 & 427kg HSCW are: The expression of the marbling score (increased % intramuscular fat) relates to reduced muscle growth as the animal reaches maturity. Fat accretion in the carcass depots (subcutaneous and intermuscular) and within muscle (at least the LT) occur at the same rate through the 304 - 417kg carcass range. Since fat accretion within muscle and at the subcutaneous and intermuscular depots occurred at the same rate, the increase in the expression of intermuscular fat (marbling) relied upon declining muscle growth. That is, the concentration of fat increased as muscle growth slowed. This means that biologically intramuscular fat is not late maturing BUT that commercially the expression of intramuscular fat (i.e. the % fat or marbling score) is late maturing. The above conclusions are supported for the 304 - 417 kg HSCW range and the genotype used in this trial, but the overall pattern should apply to all genotypes. However the appropriate carcass weight ranges are likely to be different. The above conclusions also do not exclude other control factors affecting marbling i.e. genetics and starch digestion in the small intestine.