The Nutritive Value of Range Grasses in Northern Brewster County, Texas
Blue grama, sideoats grama, hairy grama, sprucetop grama, silver bluestem, johnsongrass, sand dropseed, tobosa, and big alkali sacaton were randomly collected from one of five sites in northern Brewster County, Texas. Each grass was analyzed for crude protein (CP) content and amino acid (AA) composition for the determination of biological value (BV) using an amino acid index (AAI). Means were compared by ANOVA and separated using Student Newman Keul’s multiple comparisons test. The quadratic response in CP content throughout the year was determined using regression analysis. The mean CP content for all grasses throughout 1991 was described by the equation y= 0.98 – 0.13x2 + 1.76x (r2 = 27%; P < 0.001). Big alkali sacaton contained the highest CP content in the spring (8.3%) and fall (10%); johnsongrass CP was highest in the summer (13.2%) and sprucetop grama in the winter (5.3%) (P < 0.05). The grasses had BVs in the mid-to upper 70s except silver bluestem (2%) and johnsongrass (80%). The low BV of silver bluestem was suggestive of an AA imbalance in the plant due to stress before sampling. Barring any factors that may have inhibited digestion and absorption, range grasses in northern Brewster County contained adequate CP to meet CP requirements of dry gestating beef cattle (500 kg) during the summer, fall, and spring of 1991.