R.K. Heitschmidt

Drought and grazing. I. Effects on quantity of forage produced.

This research addresses the hypothesis that grazing intensity during and following drought can dramatically alter community level, post-drought recovery patterns. Research was conducted during the 1993 through 1996 growing seasons at the Fort Keogh Livestock and Range Research Laboratory located near Miles City, Mont. Study plots were twelve, 5 × 10-m non-weighing lysimeters constructed in 1992 on a gently sloping (4%) clayey range site. An automated rainout shelter was constructed to control the amount of precipitation received on 6 lysimeters during the 1992 growing season. We conclude from study results that the independent and combined effects of the imposed late spring to early fall drought and associated grazing treatments were minimal relative to soil water dynamics and aboveground net primary production although both grazing treatments reduced herbage standing crops. We attribute the absence of a strong response to the drought to its timing (i.e., late growing season) in that most herbage production in these cool-season dominated grasslands is completed by early summer. Thus, annual production processes in these grasslands avoided the major impacts of the drought. The results do not provide convincing evidence, however, that would lead us to completely reject our original hypothesis. Rather, they simply provide evidence that these grasslands are well adapted to surviving late growing season drought with or without intensive grazing by ungulates.

Effects of mechanical treatments and climatic factors on the productivity of Northern Great Plains rangelands.

Impacts of 7 range treatments and climate on late spring herbage standing crops (SC) were measured in rangelands near Miles City, Mont., from 1983 to 1990. Treatments, established in 8 pastures at 2 sites, were: (1) untreated control + season long grazing (SL); (2) soil tillage (ST) + SL; (3) ST + drill seeding legumes (DS) + SL; (4) brush control (BC) + ST + DS + switchback grazing (utilizing 2 pastures); (5) BC + ST + DS + SL; (6) ST + nitrogen fertilization + SL; and (7) contour furrowing (CF) + aerial seeding legumes + SL. Data were analyzed using years as a repeated measure. Treatments increased (p less than or equal to 0.05) total SC 320 kg/ha over controls, but did not affect species/species group composition. Treated pastures produced similar (p greater than or equal to 0.10) SC of 881 kg/ha. Total SC averaged 490 kg/ha more (p less than or equal to 0.05) in 1983, 1986, 1987, 1989, and 1990 than in 1984, 1985, and 1988. Perennial cool-season grass SC was greatest in 1986 (651 kg/ha). Peak annual grass SC (337-506 kg/ha) occurred in 1983 and 1984, the 2 years following ST or CF, and 1989 and 1990, the 2 years following severe drought. Although regression analyses showed fall, winter, and spring precipitation and temperature were closely related to spring SC, less than 50% of the variation in SC was accounted for when precipitation and temperature were summed on a 1-month, 2-month, or 3-month basis. Above-average fall and spring precipitation (September and April) resulted in the greatest total SC. Species composition varied temporally with changing weather conditions and management strategies.

Drought and grazing. IV. Blue grama and western wheatgrass.

An understanding of the impacts of grazing during and following drought on rangeland ecosystems is critical for developing effective drought management strategies. This study was designed to examine the effects of drought and grazing on blue grama [Bouteloua gracilis (H.B.K) Lag. ex Griffiths] and western wheatgrass [Pascopyrum smithii Rydb. (Love)] tiller growth dynamics. Research was conducted from 1993 to 1996 at the Fort Keogh Livestock and Range Research Laboratory located near Miles City, Mont. An automated rainout shelter was used during 1994 to impose a severe late spring to early fall (May to October) drought on 6 of twelve, 5x 10-m non-weighing lysimeters. Twice replicated grazing treatments were: 1) grazed both the year of (1994) and the year after (1995) drought; 2) grazed the year of and rested the year after drought; and 3) no grazing either year. Drought had minimal impact on tiller relative growth rates of plants grazed twice, although it reduced (P s 0.01) rates of axillary tiller emergence for blue grama (79%) and western wheatgrass (91%), respectively. Defoliation periodically increased relative growth rates (P 0.05) and tiller emergence (P 0.01) of both species. Neither drought nor grazing affected tiller densities or tiller replacement rates of either species nor did they affect productivity of blue grama. Drought, however, reduced (P 0.01) productivity of western wheatgrass 50% in 1994 whereas grazing reduced productivity (P 0.01) by 46% in 1994 and 69% in 1995. Moderate stocking levels (40-50% utilization) during and after drought did not adversely affect the sustainability of these dominant native grasses.

Vegetation Biomass Dynamics and Patterns of Sexual Reproduction in a Northern Mixed-Grass Prairie

Abstract The sustainability of natural grasslands is in large part dependent upon the successful ingress and propagation of alien plant species. We attempted to characterize the propagation potential of the dominant plant species of a southeastern Montana mixed-grass prairie community including two introduced grasses, Bromus japonicus and B. tectorum. Aboveground standing crops and densities of seeds were estimated on 10 dates between September 1991 and August 1993 and seedling densities were estimated on 21 dates between September 1991 and July 1993. Total aboveground standing crop ranged from about 110 to 340 g m−2 and was dominated by three grasses: Bouteloua gracilis, a warm-season, perennial; Pascopyrum smithii, a cool-season, perennial; and B. japonicus, a cool-season, annual grass. Seed production was dominated by B. japonicus (∼10,400 seeds m−2). Bouteloua gracilis and P. smithii seed production was negligible with maximum densities of about 120 and 25 seeds m−2, respectively. Density of seedlings was greatest for B. japonicus (∼2100 seedlings m−2), Festuca octoflora (∼1100 seedlings m−2), and Plantago patagonica (∼350 seedlings m−2). Maximum number of B. gracilis seedlings was 25 m−2. No P. smithii seedlings were observed during the study. Temporal differences in seedling densities were closely tied to late summer-early fall precipitation patterns. Although community-level seed production and seedling establishment processes were dominated by B. japonicus, the overwhelming presence of herbage produced by the two vegetative propagating species, B. gracilis and P. smithii, casts doubt on a trend in ecological succession toward an annual grass-dominated community.

Influence of Japanese brome on western wheatgrass yield

Japanese brome (Bromus japonicus Thunb.) has invaded many northern mixed-prairie plant communities. Understanding interactions of population dynamics of this and associated species is critical for proper management of communities infested with this annual. Objectives of this study were to determine the effect of Japanese brome removal on aboveground forage production and daily plant water relations of western wheatgrass [Pascopyrum smithii Rydb. (Love)] and Japanese brome in a western wheatgrass dominated northern mixed grass prairie community. During early spring of 1991, a wet year, and 1992, a dry year, 2 treatments, undisturbed and complete (total) removal of Japanese brome seedlings, were applied by hand at silty clay loam and clay field sites in circular, 1-m2 quadrats. Vegetation in additional quadrats was left undisturbed or Japanese brome was removed to assess plant water relations at each site in 1992. Total standing crop was reduced 500 kg ha-1 with removal of Japanese brome, while standing crop of western wheatgrass was increased 220 kg ha-2 with Japanese brome removal. Increased standing crop of western wheatgrass appeared to result from increased tiller density of 153 tillers m-2 rather than increased weight per tiller. Western wheatgrass water relations were essentially unaffected by Japanese brome removal in 1992. Removal of Japanese brome from Northern Great Plains plant communities may increase production of associated perennial grasses, but managers should also expect a short-term decrease in total standing crop.

Suppression of annual bromes impacts rangeland: vegetation responses.

Presence of invading annual bromes (Bromus spp.) can alter seasonal patterns of forage production and quality and require management changes for efficient use of infested rangelands in the Northern Great Plains. We studied biological impacts of the presence of brome by comparing brome infested rangeland to similar sites in which brome had been suppressed with autumn applications of atrazine [6-chloro-N-ethyl-N’-(1-methylethyl)1,3,5-triazine-2,4-diamine] at 0.56 kg ha in 1992 and 1993. Each treatment was randomly assigned to three, 12-ha pastures. Vegetation was measured for 5 months (May to September) each year from 1993 to 1995. Each pasture was stocked with 8 crossbred steers (Bos taurus) from mid-May to mid-September 1993 and 1995 and to mid-August 1994. The forage base varied temporally by date and year, but generally was not less than 800 kg ha. Brome suppression increased (P ≤ 0.05) crude protein concentration for western wheatgrass (Pascopyrum smithii Rydb. [Love]) in July (7.1 vs. 9.1%) and August (6.0 vs. 7.1%). With the variation in annual brome stands among years, as influenced by growing conditions, this experiment demonstrated that improvement in forage nutritional quality can be expected from suppression of annual bromes on semiarid rangelands.

Clipping and Japanese brome reduce western wheatgrass standing crop

Japanese brome (Bromus japonicus Thunb.) and downy brome (Bromus tectorum L.), introduced annuals, have invaded many northern mixed-prairie plant communities. This study determined the effect of removing Japanese brome and clipping western wheatgrass [Pascopyrum smithii Rydb. (Love)] on aboveground forage production of a western wheatgrass dominated northern mixed-prairie community. During early spring 1993, a wet year, and 1995, a drier year, western wheatgrass tillers were clipped to ground level in May or June and Japanese brome seedlings were left undisturbed or removed in circular, 1-m2 plots on a clay-pan field site. Western wheatgrass standing crop and tiller densities were estimated by clipping and counting in May and June, and these plus community standing crops were estimated in all plots after Japanese brome matured in mid July. Year effects were significant for standing crop and tiller density due to annual variation in amount and distribution of fall, spring, and early summer precipitation. Conditions were most favorable for tiller initiation of western wheatgrass and germination of annual brome seed in fall 1994 and for herbage production in 1993. Clipping western wheatgrass tillers reduced accumulated standing crop 230 to 350 kg ha-1 and reduced tiller weight by 17 to 58%. Standing crop of western wheatgrass was increased 102 kg ha-1 with removal of Japanese brome, while total standing crop was reduced 284 kg ha-1 with brome removal. Increased standing crop of western wheatgrass appeared to result from increased tiller density rather than increased tiller weight. Removal of Japanese brome from northern mixed prairie plant communities may increase production of associated perennial grasses, but managers should also expect a short-term decrease in total standing crop.

Steer performance on native and modified Northern Great Plains rangeland.

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.

Late summer protein supplementation for yearling cattle

Abstract Three studies were conducted to evaluate late summer protein supplementation for growing steers on Northern Great Plains rangeland. In Experiment 1, crossbred yearling steers (N = 80 per year, mean initial live-weight = 275 kg) were allotted to 1 of 2 treatments replicated in 3 pastures in each of 3 years. Treatments were summer-long grazing with or without protein supplementation in late summer. Protein supplement (26% crude protein) was fed at a rate of 1.68 kg (dry matter basis) every third day. In 1995, a third treatment was added to additional pastures consisting of 1.62 kg (dry matter basis) of a 40% crude protein supplement fed every third day. There was no weight gain response to protein supplementation. In Experiment 2, yearling steers grazing rangeland from May to September were fed either no supplement, 1.5 kg of a 22% crude protein safflower meal-based supplement, 1.2 kg of 26% soybean meal-based supplement or 1.2 kg of a 26% safflower and soybean meal-based supplement every third day in late summer. Live-weight gain, forage intake, and digestibility were not affected by supplementation. A third experiment using ruminally cannulated steers fed grass hay and the 3 protein supplements based on safflower and soybean meals showed an increase in ruminal ammonia concentrations but no other appreciable change in ruminal fermentation with protein supplementation. Supplementation with as much as 648 grams of protein every third day was not a viable means to increase gains of steers grazing Northern Great Plains rangelands during late summer under the conditions of this experiment.