SEASON AND SEX INFLUENCES ON BOTANICAL COMPOSITION OF CATTLE DIETS INSOUTHERN NEW-MEXICO

We conducted a study in southern New Mexico to determine seasonal vari ation in botanical diet composition of cattle and to compare cow and s teer diets, The climate and vegetation is typical of semidesert grassl and. Fecal samples were obtained from a group of cows and steers durin g spring, summer, fall, 1989; winter and summer, 1990. Results showed that cattle diets were highest in grass content during spring (57%), s ummer (78%), and winter (54%), while forbs comprised the highest propo rtion of cattle diets during the fall (47%). Shrubs were moderately im portant during winter (18%). Dropseeds (Sporobolus spp,), black grama (Bouteloua eriopoda [Torr.] Torr.), threeawn species (Aristida spp.) a nd leatherweed croton (Croton pottsii [Klotzch] Muell, Arg,) were key forage species for cattle, The importance of these species varied with season, availability, physiological stage, and presence of other spec ies, Differences between cow and steer diets varied with season, The r elative similarity ranged from 70% (fall) to 90%, (summer), The lower fall similarity compared to that in the summer might be related to phy siological variation or past differences in grazing experience between cows and steers, For practical purposes, steer diets might generally be used to represent cow diets, but caution should be exercised during periods of low forage quality.