Nutritive value of range forage can be inadequate for grazing livestock when forage is mature, during periods of drought or during critical livestock physiological stages. Supplementation has been a common practice to help correct nutritional deficiencies and improve livestock performance. Better productivity associated with supplementation of various classes of range livestock is well documented. Various studies concerning range livestock response to supplementation are reviewed by Allden (1981) and Holechek et al. (1989). Mechanisms by which supplementation improves livestock performance have been reviewed by Horn and McCollum (1987) and Petersen (1987). Diet selectivity can affect livestock nutritional status (Heady 1964). Holechek et al. (1989) reviewed various studies showing livestock vary their preference as the season progresses. Research evaluating the effects of protein supplementation on sheep diet, seasonal botanical composition and quality is unavailable for New Mexico rangelands.