Robert L. Gillen

Patch burning effects on grazing distribution

Abstract Post-fire forage growth is known to be a strong attractant for large herbivores. However, fire has generally been avoided as a grazing distribution tool for fear of localized over utilization of forage resources. Our objectives were to examine whether forage utilization was affected by season of burn, determine cattle grazing preference for burned sites relative to non-burned sites, determine forb response to patch burning, and describe the relationship between end-of-season standing crop and distance from burned sites. Sixteen, 4-ha plots were burned in mid-November or mid-April and left exposed to cattle grazing for the duration of the growing season. Burn treatments were blocked within pastures to allow individual herds access to fall-burned, spring-burned, and non-burned sites. Standing crop estimates for grasses, forbs, and total herbage were made in September by clipping on burned sites and at 50, 100, 200, 400, and 800 m distant from the plots edge. Standing crop was also sampled in exclosures on burned and non-burned sites. Cattle showed no preference for one burn season over the other. Cattle were strongly attracted to burned sites, reducing grass standing crop 78% within burns compared to 19% outside the influence of burns. Grass standing crop decreased in a predictable manner with proximity to burned plots. Forbs increased 60% to 1,095 kg ha−1 on grazed burned plots, but were unaffected by distance from burns. Patch burning can be employed as an effective, inexpensive grazing distribution tool.

Nitrogen fertilization of a native grass planting in western Oklahoma

Native warm-season grass mixtures have been established on the Southern Plains under the USDA Conservation Reserve Program. We studied responses to N fertilizer on such pastures in western Oklahoma over a 4-year period. Experimental pastures were previously cultivated fields with loamy soils seeded to a mixture of native warm-season grasses. Fertilizer treatments were 0 and 35 kg N ha-1 year-1 as ammonium nitrate. Pastures were intensively grazed from early June to early August over 4 years. Stocking rates averaged 52 and 104 AUD ha-1 for the 0 and 35 kg N ha-1 treatments, respectively. These stocking rates are heavy for seasonal grazing in this region. Responses measured included forage mass and nutritive value before and after grazing, plant basal area, and livestock performance. Precipitation was variable but generally favorable over the study period. Peak forage mass was increased by N fertilization (2,480 versus 4,030 kg ha-1; P < 0.01), producing 45 kg forage per kg N applied. Nitrogen fertilization increased crude protein concentration in June (8.2 versus 10.3%; P < 0.05) and August (4.1 versus 4.6%; P < 0.05), but had inconsistent effects on in vitro dry matter digestibility. Total vegetative cover and basal cover of blue grama (Bouteloua gracilis (H.B.K.) Lag. ex Griffiths) increased in the fertilized pastures. Average daily steer gain was not different between treatments (0.96 versus 1.02 kg hd-1 day-1) even though stocking rates were substantially higher on fertilized pastures. Steer gain ha-1 was increased by fertilization (83 versus 176 kg ha-1, P < 0.01). This resulted in a fertilizer N use efficiency of 2.7 kg steer gain per kg N applied. Nitrogen fertilization combined with intensive summer grazing provided a net return of

Stocking rate, precipitation, and herbage production on sand sagebrush-grassland

0.65 to

Leaf development of native bluestem grasses in relation to degree-day accumulation.

0.94 per kg N applied.

SEQUENCE GRAZING SYSTEMS ON THE SOUTHERN PLAINS

Abstract Knowledge of the relationship between stocking rate or grazing intensity and plant production is fundamental to the sustainable management of rangelands. The general management paradigm is that plant production declines as stocking rate increases. Our objective was to determine the impact of stocking rate on herbaceous production of a sand sagebrush (Artemisia filifolia Torr.)-grassland. Stocking rates averaging 43, 57, and 85 animal-unit-days ha−1 (AUD ha−1) for year-round grazing were applied from 1941 to 1961. Herbaceous plant production was determined by sampling standing crop in temporary exclosures at the end of the growing season from 1958 to 1961, the last 4 years of the grazing study. Total herbaceous production averaged over stocking rates and years was 1,490 kg ha−1. Grasses contributed 89% of the total while forbs contributed 11%. Total production averaged 1,540, 1,470, and 1,450 kg ha−1 for stocking rates of 43, 57, and 85 AUD ha−1 y−1, respectively. There were no differences among stocking rates for total production or for the production of any individual grass species (P > 0.05). Forb production was also not affected by stocking rate, averaging 200, 140, and 120 kg ha−1 for stocking rates of 43, 57, and 85 AUD ha−1 y−1 (P > 0.05). Differences in production among years were much greater than differences among stocking rates for all vegetation components. Little bluestem [Schizachyrium scoparium (Michx.) Nash] and sand lovegrass [Eragrostis trichodes (Nutt.) Wood] showed the greatest responses to favorable precipitation. Herbaceous production of this sand sagebrush-grassland was little affected by 20 years of differential stocking rates.

Western ragweed effects on herbaceous standing crop in Great Plains grasslands.

Degree-day accumulation is commonly used to predict crop development and harvest dates. Relationships between degree-day accumulation and phenological development of range forage grasses have received less attention. This research tested the hypotheses that leaf development by big bluestem [Andropogon gerardii Vitman] and little bluestem [Schizachyrium scoparium (Michx.) Nash] is related to degree-day accumulation and that these relationships are stable over environments and years within environments. Study environments included native prairie, a space-planted garden, and a growth chamber. Individual tillers of big and little bluestem were permanently marked and fully developed leaves were counted once or twice weekly over 3 growing seasons and 1 growth chamber trial. Quadratic regression models accounted for 94 to 99% of the observed variation in leaf development for all species-environment-year combinations. Regression models were significantly different (p = 0.05) among environments and between years within environments. Lack of model stability over years was a result of high variation in total leaves produced per tiller relative to annual variation in degree-day accumulation. The simple independent variable, day of year, predicted leaf development equally as well as degree-day accumulation.

Rangeland and steer responses to grazing in the Southern Plains.

Eastern gamagrass (Tripsacum dactyloides (L.) L.) is a perennial warm-season bunchgrass that starts growth earlier in the spring than most other warm-season grasses. This suggests that combining eastern gamagrass with other warm-season grasses in a sequence grazing system could lengthen the period of rapid livestock gain. We studied sequence grazing systems consisting of eastern gamagrass and Old World bluestem (Bothriochloa ischaemum L.) (EG-OWB) as compared to native mixed prairie and Old World bluestem (Native-OWB) from 1993 to 1997. Crossbred beef steers averaging 239 kg grazed eastern gamagrass or native pasture from early May to early June and again from late July through August. Old World bluestem was grazed in mid season. We measured forage yield and nutritive value and steer gain. Standing forage of eastern gamagrass above a 15-cm stubble height averaged 895 kg ha-1 at the start of the first grazing period and 2,430 kg ha-1 at the start of the second grazing period. Dry, cold winter and spring weather reduced this amount to 80 kg ha-1 in May 1996 and precluded grazing the eastern gamagrass that season. Crude protein content of eastern gamagrass was greater than 14% and in vitro dry matter disappearance (IVDMD) was greater than 65% in May. By August, crude protein content had dropped to 5-8% and IVDMD was 45-50%. Peak standing crop of Old World bluestem averaged 4,580 kg ha-1 over years. Steer gain over the entire grazing season, 103 days, did not differ between forage systems, averaging 1.02 kg head-1 day-1 in both systems. Steer gain was higher on native pasture than eastern gamagrass in the late grazing season (0.91 versus 0.60 kg head-1 d-1, p=0.02). As a result of higher stocking rates, steer gain was 257 kg ha-1 for the EG-OWB system and 103 kg ha-1 for the Native-OWB system (p<0.01).

Cattle diet quality under short duration grazing on tallgrass prairie.

Western ragweed [Ambrosia psilostachya DC. ], a major forb species in mixed and tallgrass prairies, is considered to have little value for cattle grazing but is an important food item for bobwhite quail [Colinus virgini anus ]. While often thought to be a strong increaser with grazing pressure, information on the actual relationship between western ragweed and grasses is contradictory. Our objectives were to 1) determine the effect of western ragweed on grass standing crop, and 2) determine the effect of vegetation type and grazing on survival and shoot morphology of western ragweed. Western ragweed did not appear to reduce grass standing crop. Instead, standing crop (40 to 620 kg ha - 1 ) and density (6 to 41 shoots m - 2 ) of western ragweed were positively related to grass and grassforb standing crop in mixed prairie. Standing crop of western ragweed was not related to grass standing crop in tallgrass prairie. Competitive thresholds for western ragweed in mixed and tallgrass prairies appear to be above the levels observed in this study. Density of western ragweed shoots decreased over the growing season under both grazed and ungrazed treatments. Survival of western ragweed shoots from June to September was greater in mixed prairie (81%) than in tallgrass prairie (63%) and was greater in ungrazed (76%) than grazed plots (68%). Western ragweed shoots weighed less per unit of height in tallgrass prairie. Western ragweed shoots in ungrazed plots were taller than shoots in grazed plots but weighed less per unit of height. These differences in shoot morphology are consistent with increased competition for light in tallgrass prairie and in ungrazed sites. Western ragweed may not directly reduce grass standing crop but, rather, increase only when grasses are reduced by other stresses such as improper grazing.

Estimating herbage standing crop with visual obstruction in tallgrass prairie.

This investigation was to determine the carrying capacity of the Southern Plains mixed-grass prairie by measuring vegetation and yearling steer gain responses to 2 replicates of 3 different grazing intensity treatments between 1941 and 1951. The light, moderate, and heavy grazing treatments, set at 41, 53, and 82 animal-unit-days ha-1 (AUD ha-1), were grazed with straight-bred Hereford steers with an initial weight of 213 +/- 11 (SD) kg from about 13 November to 29 September each year. Basal cover of the individual herbaceous species and the canopy cover of the shrubs were measured along 1,289, 10-m line-transects in the 6 pastures (about 215 per pasture). All treatments showed recovery from a long history of severe grazing and the drought of the 1930s. Vegetation change was largely attributed to favorable precipitation during the study. The basal cover of all perennial grasses was about 5% in 1941 and increased to between 8 and 15% by 1951. The increases were greater in the heavily stocked pastures compared with the light and moderate grazing intensity treatments. Steer gains averaged 168 kg per head. Of this total, 134 kg or 80% occurred in the summer period (Apri1-September). Total live weight gain head-1 decreased as stocking rate increased. Stocking rate affected gain head-1 in both the winter and summer grazing periods. Live weight gain hectare-1 increased as stocking rate increased. Apparently, the maximum gain hectare-1 was not reached within the bounds of the experimental treatments. Net return hectare-1 increased as stocking rate increased. Based on this initial study, carrying capacity of this prairie was greater than 53 AUD ha-1. During extended periods of good rainfall, the carrying capacity of Southern Plains mixed-prairie may reach 82 AUD ha-1.

A Comparison of Perennial Polycultures and Monocultures for Producing Biomass for Biorefinery Feedstock

Paddocks of tallgrass prairie were grazed at intervals similar to 8-paddock short duration grazing. Two replicates of a 2 X 3 factorial treatment design were evaluated to determine the influence of stocking rate and grazing schedule on crude protein and digestible organic matter content of cattle diets. Stocking rates were 1.3 or 1.8 multiples of the rates recommended by the Soil Conservation Service for the study site. Grazing schedules were 2, 3, or 4 complete cycles during a 152-day grazing season. Grazing and rest periods were lengthened as the season progressed and forage accumulation rate slowed. Masticate samples were collected from the experimental paddocks on alternate days during the grazing periods in 2 consecutive years. No stocking rate by grazing schedule interactions were observed (P > 0.10). Diet crude protein was depressed (P 0.20). In vitro digestibility was not influenced by stocking rate but tended (P < 0.13) to be higher for the 3- and 4-cycle grazing schedules. The balance of crude protein and digestible organic matter was most favorable (P < 0.05) for the 3-cycle diets. Based on diet composition, more frequent grazing periods appeared to maintain a higher, more stable plane of nutrition than the slower rotation schedules.