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Benjamin F. Tracy Benjamin F. Tracy
Assistant Professor of Agroecology
Department of Crop Sciences
(217) 265-5313; bftracy@illinois.edu

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Ian J. Renne
Postdoctoral Research Associate
Department of Crop Sciences
(217) 244-3977, irenne@illinois.edu

Pasture Forage Composition, Disturbance History and the Incidence of Weeds

Pastures, which are managed grazing lands dominated by perennial grasses and forbs, are increasingly being valued as self-sustaining crops because of low energy, fertilizer and pesticide inputs, low rates of soil erosion and nutrient leaching, and a temporal accumulation in soil carbon and nitrogen content. A common concern of pastures regards the establishment and growth of low palatability weed species. Weed invasions can markedly lower their value as grazing lands and rank high among current management issues.

In pastures, cattle trampling following heavy rain can damage forage species and facilitate weed establishment by exposing patches of bare ground. An unresolved question is whether a large-scale disturbance event results in a temporary increase in weed abundance or a persistent weed problem. Our previous work at the Orr Agricultural Research and Demonstration Center in Pike County, IL found that a single, intense trampling event became undetectable in less than 1.5 years in aboveground community components (e.g., amount of bare ground, and forage and weed species abundance), but that an augmented weed seed bank resulted. Because of high weed seed bank longevity, disturbances to formerly disturbed pastures would likely result in higher weed abundances than in those without previous disturbances. Pasture health cannot therefore be judged on aboveground characters only, and a site’s disturbance history should be integrated into management planning. We discuss the management implications of our current work from the Orr Center, which focuses on how past disturbances interact with the timing and intensity of subsequent disturbances to affect weed invasion patterns.

Pasture damage

Figure 1: Pasture damage from cattle
trampling following heavy rain
(Orr Center, Pike County, IL).
These events facilitate weed invasions.

The susceptibility of conventional crops and pastures to weed invasions depends in part on the system’s composition and on fluctuating ecological conditions. A strongly supported ecological theory is that a crop’s susceptibility to weed invasion increases as the difference between the crop’s rate of resource availability and resource uptake increases. For example, massive fertilizer applications quickly increase soil fertility but fast-growing weeds often use these abundant resources more rapidly than the developing crop. Data also suggest that species-rich communities generally use “resource pulses” more efficiently than those that are species poor (e.g., monocultures) because diverse rooting and leaf architectures capture a relatively high amount of available resources. In pasture systems, urination by cattle provides a locally intense fertilization event, and the composition of the resident forage species in these locales affects the rapidity in which these resources are consumed, and hence, affects the availability of resources for potential invaders.

During Agronomy Day, we show the second phase of a project that originally investigated the effects of simulated cattle urine and precipitation events, disturbance, and forage species composition on the invasion success of smooth pigweed (Amaranthus hybridus). We discuss how past environmental conditions and current grazing intensity differences affect smooth pigweed invasion in plots containing monocultures and mixtures of legume and nonlegume forage species.

Forage composition

Figure 2: Forage composition and
nitrogen availability interact to affect
the invasion success of smooth pigweed.

Discussion points include:
1. The disturbance/weed invasion history of your pasture is important for guiding future weed management strategies.
2. Large fertilization events increase the susceptibility of weed invasions in any system – the largest increases in weed abundance generally occur when crop resource uptakes rates are low but available resources are high.

Department of Crop Sciences
College of Agriculture, Consumer and Environmental Sciences
University of Illinois Extension
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