Browsing Journal of Range Management, Volume 38, Number 1 (January 1985) by Subjects
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Cattle Grazing Blue Grama Rangeland. I. Seasonal Diets and Rumen FermentationFour field trials were conducted from early August to late October, 1982, on blue grama (Bouteloua gracilis) rangeland in south-central New Mexico to examine relationships among grazing season, diet botanical and chemical composition, and rumen fermentation in beef steers (Bos taurus). Diets contained an average of 83% grasses and 17% forbs from early August through late September and 77% forbs in late October. Cell wall content of the diet decreased from the early growing season (74.9%) through the onset of dormancy (64.9%) while acid detergent fiber and lignin increased (41.9 to 52.9% and 5.2 to 12.7%, respectively) and crude protein content declined from 18.4 to 11.7%. Soluble and insoluble nitrogen (N) fractions of the diet reflected crude protein; from 13 to 36% of N was in unavailable forms. The extent of in vitro organic matter digestion declined from the early growing season (66.5%) through onset of dormancy (47.9%). Ruminal ammonia concentrations declined as season progressed: 6.0 mg/100ml was the lowest concentration observed. Declining diet quality was accompanied by an upward shift in digesta pH and altered proportions of volatile fatty acids (VFA) in rumen contents. Total VFA concentration was highest in late August (106.3 mmoles/liter). Ruminal measures generally reflected changes in dietary protein and digestibility but concentrations could also reflect changes in digesta flow rates. Finally, data suggest that crude protein may not be a good measure of protein supply to livestock grazing on ranges with diverse forage types.
Cattle Grazing Blue Grama Rangeland. II. Seasonal Forage Intake and Digesta KineticsFour field trials were conducted from early August to late October, 1982, on blue grama (Bouteloua gracilis) rangeland in south-central New Mexico, to examine relationships among grazing season, forage intake and digestive function in beef steers (Bos taurus). Organic matter intake declined from 24.0 g/kg body weight (BW) in early August to 17.4 g/kg BW in late September and then increased to 20.9 g/kg BW in the early dormant season (late October). From early growing season to early dormancy, diet digestibility and passage rates decreased while retention time of digesta in the rumen increased. Rate and extent of in vitro organic matter disappearance were 66.5, 6.7; 63.1, 6.7; 51.6, 4.8; and 47.9%, 5.6%/hour in early August, late August, late September and late October, respectively. Fluid and particulate passage rates (%/hour) were 24.9, 4.6; 12.7, 3.9; 11.1, 3.7; and 10.5, 3.5, respectively, for the same periods. Retention of particulate digesta varied from 26.1 hours in the early growing season to 34.3 hours in early dormancy. Gastrointestinal fill gradually increased as season progressed. Results suggest that maintenance of a diverse plant community, containing not only desirable grasses but also palatable forbs, may allow cattle to maintain a higher level of nutrient intake during periods of grass dormancy.
Effects of Clipping on Burned and Unburned Creeping BluestemCreeping bluestem (Schizachyrium stoloniferum), a major decreaser on Florida range, is adversely affected by grazing during the growing season after a winter burn. To compare the effect of defoliation of burned and unburned bluestem range, creeping bluestem was burned (or not burned) on 2 similar sites in February 1978 and 1979 and cut at 2, 4, 6, 8, and 10 months after burning. Once forage was initially cut, it was recut every 2 months. Dry matter (DM) yield, tiller density, total nonstructural carbohydrate (TNC), crude protein (CP), and in vitro organic matter digestibility (IVOMD) were determined. When forage was cut in April and recut every 2 months, average yield of creeping bluestem from unburned areas was 3,000 kg/ha while that of burned was 2,350 kg/ha. Creeping bluestem yield from unburned areas declined linearly within both years as initial harvest was delayed, but delaying initial harvest date had no effect on forage yield from burned areas. Final tiller density was usually a cubic response in burned and unburned plants. Tiller density generally increased in plants cut 1 month after treatment, decreased in plants cut 2, 4, 6 months after treatment, and increased in plants cut 10 and 12 months after treatment. Rhizomes of plants burned in February 1979, cut initially in April, and reharvested 4 times had 9.0% TNC in March 1980, while unburned plants contained 10.2% TNC. The response of CP and IVOMD in initial growth was quadratic or cubic with time of initial harvest because percentages were raised initially due to burning, then they dropped steadily to a low in August, after which they began to rise slightly. Protein content in unburned forage had either a negative linear response or had no significant regression with time of initial harvest. IVOMD in unburned forage exhibited a quadratic (1978) or a polynomial (1979) response with time of initial harvest where IVOMD increased or fluctuated in the growing season, usually decreasing in fall and winter. Analysis of regrowth data indicated that the response of crude protein and IVOMD was similar regardless of burn treatment. Quality of regrowth declined from June to August and steadily increased from August to December. Creeping bluestem would be weakened by grazing on 60-day intervals after a February burn. When prescribed burning of creeping bluestem range is carried out every 3 to 4 years, deferment of grazing until June after a February burn will allow grazing of relatively high quality forage and still maintain creeping bluestem stand vigor.
Precipitation, Soils and Herbage Production on Southeast Wyoming Range SitesHerbage production and precipitation were determined at 13 locations in 483 ha of mixed grass range 1975-1979; production was determined at all locations 1982-1983, but precipitation was measured only at the main weather station. Vegetation and herbage production were more uniform on sites with similar subsoil than on sites with similar surface soil, the usual basis for site classification. Within any year, herbage production on similar sites was not correlated with spatial distribution of precipitation. Across years 1975-1979 and 1982-1983, herbage production on sites with sandy subsoil was correlated with March-April weather station precipitation (r2=0.866**) and March-April plus May-August precipitation (R2=0.95**). Herbage production on sites with loamy subsoil was not significantly correlated with precipitation in March-April (r2=0.32) or any other period.