How overgrazing can affect wheat
Some fields of heavily grazed wheat are either dead or appear to be dead in southwest Kansas this year. Wheat in other parts of the state may also be affected, but the reports so far have been from the southwest.
Figure 1. Field of wheat in southwest Kansas on February 12, 2015. This field has apparently been damaged by overgrazing. Photos by John Holman, agronomist, Southwest Research and Extension Center, Garden City.
Figure 2. Closeup of wheat showing damage from overgrazing, as of Feb. 12.
Wheat is like any other forage in terms of grazing tolerance. When too much of the topgrowth is removed by grazing, the plants can be weakened and become vulnerable to stresses such as winterkill or drought. The excessive removal of topgrowth deprives the plants of the photosynthates they need for food reserves. This will put a temporary stop to any new root growth.
Perhaps an even more important factor is that excessive grazing of wheat in fall and winter leaves the crowns more directly exposed to cold temperatures, despite the fact that the crown is (or should be) below the soil surface. This could be especially important on plants that have lost some of their winterhardiness during periods of unusually warm temperatures at times this winter.
The following is a discussion of the effects of overgrazing from Stocking Rate and Grazing Management, K-State publication MF-1118: http://www.ksre.ksu.edu/bookstore/pubs/mf1118.pdf/ Although the discussion in this publication is referring to perennial forages, the same principles apply to winter annual forages such as wheat.
“How much of the herbage should remain when the animals are removed? As a general rule, no more than 50 percent of the current season’s growth should be removed during the growing season. By leaving sufficient leaf area, the plants can produce enough foodstuffs for current growth and to rebuild stored food reserves. To maintain 50 percent of the leaf area, about of the current
season’s leaf length can be removed at any one time (Figure 3). Season of use, length of the grazing period, time available for regrowth after grazing, condition of the grazed plants, and current weather conditions influence this decision.”
A convenient way to measure grazing and leaf area reduction is to set up exclusion cages such as the one in the photo below.
Figure 4. Yearling heifers grazing wheat with exclusion cage to monitor grazing and leaf area reduction.
Wheat that has been grazed too severely is more susceptible to sudden and sharp drops in temperature experienced at times this fall and winter. This year, some of the heavily grazed wheat appears to have been impacted by the rapid cold temperature drop in November, and is suffering winterkill. This damage has been observed on both dryland and irrigated fields.
Not all wheat that appears to be dead has been completely killed, however. If the crown has remained alive, the plants are capable of sending out new tillers as the weather warms up. Some new growth has already occurred.
Figure 5. New growth has begun in some cases where the crowns of the wheat remained alive -- although the plants looked dead.
The only way to know for sure at this point whether the wheat is still alive is to dig up a few plants, including the roots, and put them in a pot inside the house with some water. The plants should start producing some new green growth within 7-10 days at the most if they are still alive.
Figure 6. Closeup of wheat crowns that were grazed. The crown on the left when split open is brown and soft (dead), and the crown on the right is white and alive.
John Holman, Agronomist, Southwest Research and Extension Center, Garden City
Jim Shroyer, Crop Production Specialist Emeritus
Curtis Thompson, Extension Agronomy State Leader and Weed Management Specialist