Based on Trivers' (1974) theory of parent/offspring conflict, we predicted that weaning would be accelerated under poor dietary conditions in an aseasonally breeding, monomorphic ungulate, the collared peccary (Tayassu tajacu). Suckling behavior was observed in 18 captive litters under a dietary treatment of high or low protein formulated to mimic forage under ecological conditions of normal rainfall vs. drought. Litters on the low protein diet weaned later (mean of 27 weeks) than litters on the high protein diet (mean of 21 weeks). Other indices of cumulative suckling effort (weaning weight, cumulative minutes suckling, cumulative bouts) did not differ between dietary treatments. No differences in suckling effort were found relative to offspring sex or litter size. Milk did not completely compensate for the low protein diet affecting growth of young in the second and third phases of lactation. Some nursers did not conserve bodily resources as would be expected if the benefits of short interbirth intervals outweighed the costs of lactation during unpredictable drought conditions.