How Can Overgrazing Leave Grass Vulnerable to Drought?
1. Increased Grazing Pressure = Increased Tiller Defoliation
Developmental Stages of Grass Growth Image: Iowa State University PM1791
Cattle are selective grazers. They select immature tillers over mature tillers and leaves over stems.
Livestock use of each grass species tends to be highest when tillers are vegetative, and predominantly leaves. Livestock may defoliate over 50% of preferred mid- and tallgrasses early in the growing season.
Livestock use becomes progressively lower as tillers age or transition into seed stalks. The opportunity for livestock to select a high quality diet becomes stocking-rate sensitive through the balance of the growing season.
As grazing pressure increase, both the percentage of tillers grazed and percent defoliation of grazed tillers will increase. If most of the tillers are grazed while they are elongating, next year's herbage production is likely to be reduced. Learn how to determine when cool- and warm-season grasses are elongating, or in rapid growth windows.
Stocking rate and grazing system decisions are most likely to affect animal performance in the second half of the grazing season. In contrast, these decisions are most likely to affect plant vigor and herbage production potential during the first half of the summer grazing season. When precipitation is average or above average, preferred grass species generally recover from grazing stress by the end of the current growing season, if pastures are grazed before their respective rapid-growth windows (marked by stem elongation) and then deferred until killing frost. However, when moisture is inadequate, declines in subsequent-year herbage production are likely to be greatest for preferred plant species grazed during rapid growth.
2. "Take Half & Leave Half" May Reduce Next Year's Plant Production, Vigor
The "Take Half & Leave Half" mantra implies that removing up to 50% of the herbage has no ill effect on individual species. However, a recent study shows that 50% defoliation of the warm-season tallgrass, prairie sandreed, during its rapid-growth window, actually reduced its subsequent-year herbage production by about 20% (see figure).
The only option available to grazing managers is to shift grazing in the subsequent-year to a time that precedes or follows rapid-growth windows of preferred species.
3. Overgrazing Leads to Fewer Plant Roots
Plant vigor and plant species composition affect the soil depth from which vegetation uses moisture.
Roots are sensitive to defoliation; heavy defoliation of green plants stops root growth for one to two weeks. Reduction in root length often corresponds to decline in plant vigor.
The percentage of total root length lost increases as root depth increases. Losses of deep roots are measurably greater than loss of shallow roots in tallgrasses. Short grasses often increase as range condition declines.
The figure at right shows the percent of total root length (sand bluestem) in each depth of soil compared to plants not clipped until October, after killing frost. (Modified from Engel et al. 1998) Defoliation in June and August resulted in the biggest losses of root length at all soil depths.
4. Heavy Grazing + Drought = Double Loss of Perennial Plant Cover
Grazing intensity had a dramatic impact on the reduction of perennial plant cover during the 1950’s drought in the Great Plains. Moderate grazing generally caused little change in cover compared to un-grazed sites. Heavy grazing nearly doubled the loss of perennial plant cover caused by drought alone.