Continuous low animal density grazing of tobosa [Hilaria mutica (Buckl.) Benth.] in the northern Chihuahuan desert results in nonuniform forage utilization. Stocking smaller tobosa rangeland paddocks with high numbers of cattle for short periods of time may facilitate more uniform forage utilization. Two grazing periods in each of 2 consecutive years were monitored to investigate the frequency with which tobosa tillers were defoliated and the intensity of defoliation (change in height) in relation to grazing pressure under high-density seasonal rotational and low-density seasonal continuous grazing. Approximately 40% of tiller height, including leaves, was removed at each defoliation in the rotational treatment, while intensity of defoliation per grazing event remained consistent. In the continuous treatment, amount of tiller removed varied widely and was not consistent among the 4 periods. Percentage of tillers defoliated in the rotational treatment was always greater than 75%, and always less than 30% in the continuous treatment. The probability that a tiller would be grazed at least once in the rotational treatment was more than twice as great as in the continuous treatment. However, within the rotational treatment, the probability of multiple grazing events (greater than or equal to 2) on an individual tiller was less than the probability of a tiller being grazed just once. In general, high-density rotation grazing promoted more uniform. forage utilization of tobosa than low-density continuous grazing.

Research was conducted to quantify the effects of various range improvement treatments on diet quality and summer weight gain of steers grazing semiarid rangeland from 1983 through 1988. Treatments were: no treatment (i.e., control), contour furrowing, intertilling with a prototype range improvement machine and combinations of the range improvement machine, nitrogen fertilization, legume interseeding, or brush control. Diet quality was measured in 1987 and 1988. Data were analyzed using vitrious repeated measures analysis of variance models. Various relationships between the animal performance data and previously published herbage standing crop data were examined using standard correlation procedures. There were no significant treatment (P > 0.17) or year by treatment (P > 0.82) interaction effects relative to average daily gains, total gain steer-1, and gain ha-1. However, all year effects were significant (P < 0.05) for these variables with years accounting for about 67% of the observed variation in weight gains. Percentage crude protein in diet samples was greater in 1987 compared to the severe drought year of 1988 and was greater at the beginning than at the end of each grazing season (P < 0.05). However, in vitro dry matter digestibility of diets was greater in 1988 than 1987 primarily because digestibility of diets increased in 1988 from the beginning to the end of the trial. Significant correlations between gain steer-1 and gain ha-1 and early season total and perennial cool-season grass standing crop estimates indicated some rudimentary information is available at the beginning of each grazing season for predicting season-long weight gains. Because average daily gains during late spring and early summer were several fold greater than late season gains in all years except one, it is hypothesized that intensive early stocking strategies may be appropriate for stocker cattle grazing in the Northern Great Plains.

Seed of Leucaena leucocephala (Lam) Dewit variety K-8 and L. retusa Gray variety Yellow Puff were germinated in growth chambers maintained at 10, 25,30, and 35 degrees C. A fifth treatment of 30 degrees C for 13 hours followed by 25 degrees C for 11 hours was also included. Seed were monitored every 4 days for 16 days for percent germination, testa weight, cotyledon weight, hypocotyl weight and length, and radicle weight and length. Yellow Puff exhibited higher maximum germination and a higher germination rate (expressed as percent of germinable seed germinated 4 days after imbibition) than K-8. There was no temperature effect on maximum germination of K-8, while the 35 degrees C treatment substantially reduced germination in Yellow Puff. Despite faster germination, Yellow Puff seedlings elongated slower compared to K-8. Radicle/hypocotyl ratios were similar for both species across all temperatures. Due to heavier seed weight of K-8 than Yellow Puff, testa and cotyledon weights were also heavier. Rate of nutrient reserve mobilization from the tests and cotyledon increased with increasing temperature in K-8, while rate of mobilization of nutrient reserves in Yellow Puff seed declined at temperatures above 30 degrees C.

Multiparous cows (n = 91, 1986; n = 92, 1987) were selected from 2 populations to obtain cattle with high and low milk production. After March-April calving, high and low producing cows grazed either native range (treatment 1) or seeded range (treatment 2) until weaning in September. Seeded range included paddocks of crested wheatgrass (Agropyron desertorum, Fisch. ex [Link]Schult), contour furrowed native range interseeded with "Ladak" alfalfa (Medicago Sativa L.), and Russian wildrye grass Psathrostachys juncea[Fisch.]Nevski.). In treatment 2, crested wheatgrass was grazed mid-April to 17 June, followed by contour furrowed rangeland 18 June to 5 August, and ended with Russian wildrye 6 August to weaning (mid-September). Data were analyzed as a split plot with treatment and year in the main plot and cow type in the subplot. Number of cows exhibiting estrus before the beginning of the breeding season and fall pregnancy rate were not influenced (P > 0.05) by range treatment. Twelve-hour milk production during May, June, August, and September ranged from 11.3 to 6.8 kg and 7.6 to 3.9 kg for high and low producing cows, respectively. Cows with high milk production lost body condition during and after the breeding season, whereas cows with low milk production maintained body condition during the same period. Live weight gain of calves was greater (P < 0.01) for cows with high production than cows with low production but was not affected (P > 0.05) by range treatment. We concluded that native and seeded ranges were of similar nutritive value for cows with high and low milk production and that cows with high milk production may have greater nutrient requirements during late summer-early fall than what was provided by native or seeded ranges. Protein may be the primary limiting nutrient in forages during the late summer for lactating cows grazing Northern Great Plains range.

Research on fire in tallgrass prairie focuses almost exclusively on dormant season fires, primarily in winter and spring. Relatively little is known about the response of tallgrass prairie vegetation to growing-season fires or follow-up management. We evaluated vegetation response of grazed, high-seral tallgrass prairies to herbicides after late-summer burning. Dry matter yields of little bluestem [Schizachyrium scoparium (Michx.) Nash], other perennial grasses, total perennial grasses, and total herbage were significantly reduced by late-summer burning. Tallgrass and annual grass standing crop did not differ between burn treatments. Forb standing crop on burned plots averaged about twice that of unburned plots. Follow-up treatment with 2,4-D[(2,4-dichlorophenoxy)-acetic acid] significantly reduced forb standing crop compared to atrazine [6-chloro-N-ethyl-N'-(1-methylethyl) 1,3,5-triazine-2,4-diamine]. Standing crop of tallgrasses and total perennial grasses was greater on plots treated with 2,4-D than on plots treated with no herbicide or with atrazine. Because late-summer burning did not severely reduce herbage production nor drastically alter community composition for more than 1 year, late-summer burning may be a viable alternative for brush control and for improving wildlife habitat in high-seral tallgrass prairie. If management objectives require reduced forb production and increased perennial grass production following late-summer burning, application of 2,4-D would be an appropriate management practice.

This study was conducted on Chihuahuan desert range near Las Cruces, in southcentral New Mexico, to determine the relationship of blacktailed jackrabbit (Lepus californicus) densities to good (GC) and fair (FC) range condition. The Soil Conservation Service procedure was used to classify ecological range condition. Line transect procedures were used to estimate jackrabbit populations from July 1988 to December 1990. Concurrently vegetation cover and mean plant height were determined with the line intercept procedure. Jackrabbit densities on the fair condition range were higher (P < 0.10) than those on the good condition range. This difference is attributed to the fair condition range containing more protective cover and preferred forage than good condition range. Jackrabbit abundance showed no season (P < 0.10) or year differences (P < 0.10). Jackrabbits preferred grass-shrub mosaic habitats more than shrubland and grassland habitats. The need for diverse food sources and protective cover were apparently major determinants of habitat selection by jackrabbits. The good condition range contained greater (P < 0.10) grass cover and less (P < 0.10) shrub cover than the fair condition range. Our results indicated that maintaining Chihuahuan desert ranges in good to excellent condition is the best means of achieving lower abundance of jackrabbit populations.

Research was conducted to examine the effect of Rhizobial race and N fertilization on N2 fixation and N partitioning in seedlings of the genus Leucaena. Seedlings of Leucaena leucocephala (Lam) Dewit variety K-8 and L. retusa Gray variety Yellow Puff were grown in slender tubes filled with fritted clay and watered with nutrient solutions containing 0, 2, or 8 mM nitrate N labeled with 0.1% N15. The seedlings were inoculated with Rhizobia loti race 94A3 or R. logi race 9408. After 12 weeks the plants were harvested and the effects of N fertilization and Rhizobial race on nodulation N2 fixation, and N partitioning were examined. Using the N15-dilution method total N fixed by dinitrogen fixation was determined as well as distribution of fixed versus fertilizer N within the plant. Both Rhizobial races infected both Leucaena species resulting in an effective symbiosis. However, the 2 Leucaena species responded quite differently to N fertilization. The addition of 2 mM N to the nutrient solution effectively eliminated nodulation in L. retusa yet increased both nodulation and N2 fixation in L. leucocephala. The 8 mM N rate eliminated nodulation in both species. Due to the elimination of nodulation and N2 fixation by N fertilization in L. retusa, the effect of N fertilization on partitioning of fixed N was only examined in L. leucocephala. Increasing the N content of the nutrient solution increased the proportion of total N in the shoot and reduced the proportion in nodules. A larger proportion of the fixed N was retained in the root and nodules than fertilizer N. Although the 2 mM N treatment increased the amount of N2 fixed in L. leucocephala compared to the 0 mM treatment, the proportion of fixed N2 was reduced to less than one third of the total N in the plant.

Observations suggest spring grazing of riparian areas is a good management strategy because of a reduced tendency for cattle to concentrate along streams during that season. In this study, June cattle distribution was examined within 4 experimental pastures located along Stanley Creek, Sawtooth National Recreation Area, Sawtooth National Forest, in central Idaho. Two pastures were grazed at a light stocking rate and 2 pastures were grazed at a medium stocking rate. Streamside graminoid utilization averaged about 24% under light stocking, while on the adjacent meadow graminoid utilization was 28%. Under medium stocking the average utilization at streamside was 37%, while that on the adjacent meadow was 50%. Residual herbaceous stubble heights under light stocking were 11 to 12 cm for both grazing locations, whereas streamside and meadow stubble heights were 10 cm and 7 cm, respectively, under moderate stocking. Cattle were not disproportionately attracted to the streamside areas during the June period. As stocking rates increased from light to medium, the cattle concentrated most of their additional use on the adjacent drier meadow. Utilization of riparian plant communities during this early summer period had no relationship to the amount of plant moisture content, but was negatively associated with surface soil moisture.

About 40,000 km2 of the caldénal in central semiarid Argentina are rangelands where the most important economic activity is cow-calf operations. Some information on forage species, mainly regarding taxonomy, ecophysiology, and nutritive value, is available, but detailed studies on composition of free-ranging cattle diets are lacking. The objective of this work was to study the botanical composition and seasonal trends of cattle diets in the southern caldénal. Diets were studied through microscopic analysis of cattle feces collected monthly in a typical plant community during a 12-month period. Grasses were the bulk of the diets, except in November when more than 50% of the diet was Medicago minima (L.) Grufberg. Highest consumption among the grasses was of Piptochaetium napostaense (Speg.) Hack., one of the dominants in the grass layer. High consumption of Pappophorum mucronulatum Nees, one of the few warm-season grasses in the region, was found during the summer. Caldén (Prosopis caldenia Burk.) pods, consumed in late summer and fall, were the only woody fragments found in appreciable amounts. Classification and ordination techniques were used to analyze seasonal trends. In spite of a relatively high homogeneity in the diets, 2 well-defined seasonal trends were detected, one in the fall-winter period and another in the spring. Cattle diet diversity was minimum during the coldest months of the year.