ABSTRACT
Summary Low levels of nitrogen (N) fertilization can substantially increase forage production on native rangeland if adequate precipitation is available. The objective of this study was to compare nutritive value of diets selected by Hereford steers grazing unfertilized and N-fertilized (45 kg N/ha) rangeland in south central New Mexico. The study area consisted of two adjacent pastures of approximately 60 ha each, containing comparable areas of bottomland, upland, and hillside sites. Stocking rates were 4.8 and 2.4 ha/yearling steer for control and N-fertilized pastures, respectively. Dietary samples were collected from esophageally fistulated steers (average weight 308 kg) during early, middle, and late periods of two growing seasons and one dormant season. Sampling periods consisted of 4 consecutive days using four steers/pasture. Grazed samples were analyzed for organic matter (OM), crude protein, fiber, and in-vitro OM digestibility (IVOMD). Nutritive value of grazed forage from both pastures followed seasonal trends—i.e., protein and IVOMD decreased while fiber increased with advancing plant maturity. During the growing seasons protein levels were higher (P < 0.05) in diets from the fertilized pasture than in those from the control pasture, with differences ranging from about 4 to 12 percentage units. Fiber values were usually about 4 percentage units higher in diets from the control pasture than in those from the fertilized pasture. IVOMD values favored the fertilized pasture during the latter part of the first growing season by 7 to 12 percentage units (P < 0.05), but during mid- to late dormancy, digestibility was higher (P < 0.05) on unfertilized range. Increased nutritive value of rangeland forage selected by grazing cattle during the growing season, which was attributed to N fertilization, did not continue through dormancy. In order to be of benefit in terms of increased beef production on N-fertilized native ranges, such areas could be utilized during the growing season, thus relieving grazing pressure on unfertilized range until dormancy. Seasonal cattle gains recorded over a period of years on these pastures would substantiate such a management practice.
