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Marbling continues to be a major factor affecting profitability for beef producers, processors, retailers, and restaurateurs. However, feeding animals to "˜fatten' is quite inefficient, requiring about 2.25 times more energy than is needed for producing lean muscle. For the cattle feeding industry to be sustainable in the future, increases in marbling must be accomplished without increasing days on feed, slaughter age carcass weight, and fatness and without sacrificing feed efficiency and carcass cutability. A 2002 survey of feedlot nutritionists revealed that most recommended supplementation of vitamin A to feedlot cattle at levels exceeding the guidelines of the National Research Council (NRC) by three to five times. Because vitamin A fortification of cattle diets is an inexpensive method used to improve the immune response of receiving cattle, it is likely that few have considered the negative consequences of over-supplementing vitamin A on marbling and carcass quality grades of feedlot cattle. The objective of our research was to evaluate the effects of supplementing vitamin A at either zero (NA) or seven times (HA) the NRCrecommended level in feedlot diets and age at weaning on carcass marbling development and USDA quality grade of crossbred beef steers.

Abstract. Entire and castrated male pigs were actively immunised against the steroid 5α-androst-16-en-3-one (androstenone) conjugated to bovine thyroglobulin as its carrier protein. This immunogen was incorporated with either Freund's complete adjuvant or a water-in-oil emulsion adjuvant; only the latter was found to be acceptable in the pig tissue. Levels of free and antibody-bound androstenone were measured in the plasma of immunised and control boars. In the controls, levels of free androstenone rose from 15 ng/ml at 13 weeks old to 36 ng/ml at 27 weeks. In the immunised boars, levels of free steroid fell from 16 ng/ml at 13 weeks old to zero at 17 weeks old while levels of antibody-bound androstenone rose from zero at 13 weeks old to a mean level of 70 ng/ml at 27 weeks. When immunised and control boars and non-immunised gilts were killed at 135 kg live weight, the levels of androstenone in their carcass fat were measured. Their meat was also subjected to organoleptic testing at the Meat Research Institute, Langford. Meat from animals whose fat contained significant levels of androstenone (0.4–1.6 μg/g) was found to taste of 'boar taint' while meat from control gilts with low fat levels of the steroid (0.1–0.2 μg/g) tasted normal. The uptake of [3H]androstenone was compared in 12 separate body tissues of one immunised and one control boar. No significant differences were found. The presence of specific antibody in the immunised boar had not inhibited movement of steroid between plasma and body tissue. Some hypotheses raised by these findings are discussed and the likelihood of successfully using immunisation against 'boar taint' is examined and questioned.

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