Minnesota Post-CRP Information Series: Options to Consider When Conservation Reserve Program Contracts End
Craig C. Sheaffer, Jane Grimsbo Jewett, and Roger L. Becker, University of Minnesota
The information presented in this fact sheet is not intended to take the place of professional legal advice. In developing any written lease agreement, it is highly recommended that all parties seek professional legal advice.
The State of Minnesota categorizes weeds as primary or secondary noxious weeds. Primary noxious weeds are an economic problem wherever they grow, causing harm to public health, public roads, crops, livestock, and other property. Secondary noxious weeds are designated by individual counties for additional control.
Almost all Conservation Reserve Program (CRP) fields contain weeds. According to a University of Minnesota survey of 151 CRP fields, Canada thistle is the most common primary noxious weed. (For a description of this survey see Persistence of Planted Forages in this series.) About 70 percent of CRP fields contain Canada thistle, where it covers 2 to 3 percent of the ground. This may be of serious concern since Canada thistle spreads by rhizomes and produces large amounts of seed. Small patches should be controlled to prevent further spread. Bull thistle and perennial sow thistle are other primary noxious weeds frequently found in CRP fields.
About 90 percent of CRP fields contain secondary noxious weeds that cover 13 percent of the ground. The most common secondary noxious weed is quackgrass. It’s found in about 70 percent of fields and covers about 16 percent of the ground. Common milkweed, wild buckwheat, and common ragweed are other secondary noxious weeds.
Other weeds frequently found in CRP fields include goldenrod, dandelion, horseweed, and Kentucky bluegrass. Trees such as boxelder and Chinese elm also are present in many fields. A large number of CRP fields contain non-noxious weeds. They make up most of the total weed coverage in fields planted to native warm season grasses and fields accepted into CRP with cool season plants. Significant seed banks of annual weeds have probably been established in some fields.
Farmers should choose weed control methods based on their post-CRP land use intentions. Depending on the types and amounts of weeds and the intended land use, little or no control may be necessary. For example, many secondary noxious and non-noxious weeds have high forage quality. Quackgrass, as a forage, has yield and quality comparable to many commonly used cool season grasses.
Mowing or grazing can control weeds by preventing seed production and favoring desired crop growth. Mowing also controls weeds by depleting their underground root carbohydrate reserves. Perennials must be mowed often enough to deplete root reserves (3 or 4 mowings per season). Mowing or grazing height must be low enough to remove most of the herbage and flower buds. Many studies have shown that repeated mowing of a forage such as alfalfa reduces Canada thistle stands, usually after 3 or more years of production. However, inappropriate height, timing, or frequency of cutting might reduce forage vigor and increase competition with perennial weed populations.
Grazing strategies, too, can influence the spreading of weeds. For example, Canada thistle spreads more rapidly in continuously grazed pastures than in rotationally grazed pastures. In continuously grazed pastures, animals avoid eating thistles but tend to overgraze desired species. In rotationally grazed pastures, animals are more likely to eat thistles, and rest periods replenish the root carbohydrate reserves of desired species. (See Controlled Grazing in this series for more information on rotational grazing.)
The key to controlling biennial thistles is to prevent seed production. This is best done by mowing in the bud stage, before flowers open. Biennial thistles such as musk, plumeless, and bull thistle must regenerate from seed to reproduce (unlike Canada thistle, which can spread via rhizomes). Biennial thistle seedlings produce a rosette the first year (basal leaves only, with no stem) and a flower stalk and seeds the second year. If clipped early (in the vegetative stage), they can generate new flower stalks from buds. If this happens, reclipping may be necessary. Clipping after flowers open has the undesired effect of aiding seed dispersion.
Tillage can control weeds by burying plants, destroying roots and foliage, and stimulating seed germination. Studies have shown that tillage depletes root carbohydrate reserves. Tilling to get rid of perennial weeds usually involves primary tillage (deep tillage methods such as moldboard or chisel plowing) followed by repeated secondary tillage (field cultivation or disking). To control perennial weeds, tillage must completely disrupt vegetative reproduction. Fields should be tilled frequently for 2 or more years. The spreading of perennial weeds is reduced but not controlled by less frequent or incomplete disruption of reproduction. Minimum or reduced tillage methods can actually spread perennial weeds. Repeated tillage controls localized patches but isn’t viable economically or environmentally for large, entrenched perennial weed populations, especially on erodible slopes.
Tillage does increase control of most perennial weeds when coordinated with crop rotation, crop management (variety selection, planting date, plant population, etc.) and herbicide selection. Fall herbicide application followed by tillage, then clean-up tillage and herbicide application the following year, often is the most effective at controlling perennial weeds.
Herbicides. For conversion of CRP fields to corn and soybeans, a combination of herbicide application and tillage should adequately control perennial grasses and weeds. Perennial grasses that survive tillage and herbicide applications make it difficult to convert CRP lands to small grain production. Herbicides used to control weeds in forage, small grain, and row crop production systems are described in Other Resources #1.
CRP fields in Minnesota often harbor the plains pocket gopher (Geomys bursarius). Gophers eat mostly plant roots but also some above-ground parts, including alfalfa crown buds. They often build their burrows in the most productive field areas that have the most available food supplies. They destroy vegetation directly over their burrows, creating thin plant stands or bare spots.
Gopher mounding occurred in almost half of the CRP fields in the University of Minnesota survey. Although the degree of mounding is limited in most of these fields, severe mounding occurs in about 10 percent of the fields. Therefore, though not a major problem, pocket gophers could inhibit the future use of some fields if not controlled.
There is a relationship between the amount of gopher mounding and bare ground in CRP fields. In fields planted to cool season grasses or grass-legume mixtures, the survey found common lambsquarters, wild buckwheat, horseweed, and bull thistle in greater amounts when gopher mounds were present than when they were not. In fields accepted into CRP with existing vegetation, ragweed, yellow foxtail, plumeless thistle, and white cockle were associated with gopher mounds. Many CRP fields have steep, erodible slopes. Gopher mounding on these slopes creates bare patches that are vulnerable to erosion.
Gopher activity does have some benefits. It increases water infiltration and reduces soil compaction. For forage production, however, the damage can easily outweigh the benefits. In some studies, gophers reduced alfalfa yields by up to 46 percent. The mounds also can damage mowing and baling equipment. Holes or burrows near the surface can injure grazing animals.
As with weed control, gopher control requires a long-term commitment and attention to timing. Poisoning and trapping are the most common ways to control gophers. Trapping takes time and works well in small areas (and larger ones, if a cheap labor source exists). Poisoning works well over large areas. This should be done in early spring or late fall when gophers are actively seeking and storing food. Baits should be placed in burrows, not left on the soil surface.
Table 1. Weeds Commonly Found on CRP Lands (Based on a Survey of 151 Fields). The survey included 108 CP-1 fields (planted to cool season or grass-legume mixtures), 17 CP-2 fields (planted to native warm season grasses), and 26 CP-10 fields (accepted into CRP with existing vegetation).
Percentage of fields containing weeds
All primary noxious weeds
Perennial sow thistle
All secondary noxious weeds
All non-noxious weeds
Average ground cover(Only the fields that contained a species were used to calculate the average for that species.)
Perennial sow thistle
The first two publications are available from your local Minnesota Extension Service (MES) office or the MES Distribution Center, University of Minnesota, 20 Coffey Hall, 1420 Eckles Avenue, St. Paul, MN 55108-6069. (612) 624-4900 or 1 (800) 876-8636.
Barbara Weisman, Conservation Program Specialistbarbara.email@example.com or 1-800-967-2474Ag Marketing & Development Division