| Dietary vitamin A restriction has been indicated as a method of beneficially altering the site of adipose accretion in beef cattle. A series of feedlot studies were conducted to evaluate the effects of dietary vitamin A concentration, in combination with other management practices that alter adipose accretion, on carcass composition. Corn based diets contained either no supplemental vitamin A or 2,200 IU/kg dietary DM vitamin A as recommended by the NRC. In all studies, liver and adipose retinol stores were reduced by feeding low vitamin A diets. Intramuscular adipose had lower concentrations of retinol than subcutaneous adipose, which may provide for earlier reduction and release of vitamin A inhibition of adipocyte differentiation. Interestingly, serum vitamin A concentrations increased approximately 30% upon feedlot entry regardless of dietary vitamin A concentration. In Chapter 3, the effect of timing and duration of feeding low vitamin A finishing diets did not have any affect on performance, health, or site of fat accretion. Morbidity incidence was high in Chapter 3 and Chapter 4, Exp. 1 regardless of dietary vitamin A, and may have prevented adipose accretion in all steers. Therefore, the ability to detect dietary treatment differences may have been suppressed. Restricting energy intake early in the finishing period improved feed efficiency and Yield Grade; however, no additional benefit of enhanced marbling occurred due to feeding low vitamin A diets (Chapter 4, Exp. 2). In Chapter 5, early weaned steers fed low vitamin A diets had less backfat but equal marbling scores, perhaps if cattle were harvested at a common backfat a positive effect of low vitamin A on marbling score would have been observed. Feeding low vitamin A and D diets increased total adipose accretion (Chapter 6). In agreement with previous work, steers fed low vitamin A diets had increased marbling scores and Quality Grades.;Plant based feedstuffs do not contain preformed vitamin A, however they contain carotenoids that can be converted by animals into vitamin A. Eighteen feedstuffs were surveyed from five locations for quantification of pro-vitamin A carotenoids and vitamin A equivalent determination (Chapter 7). There was considerable variation in the provitamin A content of each feedstuff based on location and storage conditions. A more extensive and comprehensive analysis of feedstuffs would be needed to accurately estimate the vitamin A potential of feedlot diets.;The final study, presented in Chapter 8, quantified adipose differentiation state specific genes to characterize the intramuscular and subcutaneous adipose tissue of Angus crossbred steers with differing carcass characteristics. No differences in gene expression were detected within a depot of this relatively uniform group of cattle. Intramuscular fat had greater preadipogenic DLK-1 mRNA than subcutaneous fat. In contrast, subcutaneous adipose tissue had greater differentiation and functional adipocyte specific mRNA (LPL, PPARgamma, FASN, and FABP4). Adipogenic genes, DLK-1 and FABP4, were identified as possible markers of bovine adiposity. It was concluded that adipogenesis of the intramuscular and subcutaneous depots were independent and that the proportion of adipocytes in each stage of maturation varied in these two depots. |
