A Survey of CRP Land in Minnesota: II. Weeds on CRP Land

Abstract

The federal Conservation Reserve Program (CRP) funded the conversion of eroding cropland to grass or grass-legume cover that was not to be tilled, hayed, or grazed for 10 yr. It was not known what the species composition of CRP fields would be after years of minimal disturbance. Our objective was to document the presence and percentage groundcover of weeds in 151 CRP fields located in 10 Minnesota counties; including 108 Conservation Practice (CP)-1 (cool-season legumes and grasses) fields, 17 CP-2 (native grasses) fields, and 26 CP-10 (existing vegetation) fields. Groundcover of each species present and of bare ground was scored in six 106-sq-ft sample plots per field. The most prevalent species were the primary noxious weed Canada thistle [Cirsium arvense (L.) Scop.], the secondary noxious weed quackgrass [Efytrgia repens (L.) Dew. ex. Nevski], and the non-noxious weeds dandelion (Tarmacurn oflcinale Weber.) and goldenrod (Solidago spp.). Weed percentage groundcover was higher in CP-10 fields than in CP-1 or CP-2 fields, probably because many CP-IO stands were already thinning at the start of the CRP contract. Volunteer legumes and grasses were common in CP-10 fields. In CP-1 fields, legume and grass percentage groundcover usually was correlated negatively with weed percentage groundcover. Weed percentage groundcover and species richness were correlated positively. Gopher mounding was correlated positively with the amount of bare ground and with the percentage groundcover of annual and biennial weed species. Primary, secondary, and non-noxious weeds were each found in nearly 90% of the fields studied. Widespread presence of noxious weeds on CRP fields is a cause for concern. Weed control issues should be addressed in planning a new CRP. Research Question Most lands in the Conservation Reserve Program (CRP) were to remain in grass or grass-legume cover for 10 yr with no tillage, haying, or grazing. It was not known what changes in vegetation would occur during that time. Establishment of noxious weeds in CRP fields is a concern because they may supply weed seeds to adjacent cropland. This study documented weed presence on CRP fields in Minnesota; and related groundcover by weeds to species diversity, groundcover of grasses and legumes, soil pH, P, and K, and gopher damage. Literature Summary On land retired from haying or grazing, the number of species increases over time. Destruction of vegetation and mounding of soil by small mammals contributes to weed invasion by providing bare ground in which seedlings may become established. Invasion by annual species is common in the first few years after removal of land from cropping. In later years, tall perennials and woody species increase. Species initially present in low numbers may eventually become dominant. A study of pasture adjacent to cropland showed that pasture weeds supplied seed to the field edge. Many of the Minnesota primary noxious weeds have seeds adapted to wind dispersal. Study Description Six 106-sq-ft sample plots were marked in each of 151 fields, selected from 10 Minnesota counties with a high amount of land in CRP contracts. Within each plot all species were identified and their percentage groundcover determined; percentage of bare ground was also determined. Soil samples were taken from each field and amount of gopher mounding was scored. The relationship of weed percentage groundcover with soil pH, P, and K; weed groundcover with number of weed species; weed groundcover with grass-legume groundcover; and weed groundcover with gopher mounding was determined. Applied Questions How many and what kinds of weeds are invading CRP lands, and how does weed invasion relate to soil fertility and groundcover of planted legumes and grasses? Primary noxious weeds were found in 88% of the fields studied. Canada thistle was the most common primary noxious weed. Secondary noxious weeds and non-noxious weeds were found in 89 and 90% of fields, respectively. Quackgrass was the most common secondary noxious weed; dandelion and goldenrod were the most common non-noxious weeds. Fields planted to grasdlegume mixtures at the start of the CRP contract had the lowest ground-cover of weeds for all weed types. Fields accepted into the CRP with existing vegetation had the highest groundcover of primary, secondary, and non-noxious weeds. High groundcovers of planted legumes and grasses were associated with low groundcover for some weed species. Soil pH, P, and K levels had little effect on the groundcover of weed species. Gopher mounding was associated with increased bare ground and presence of annual and biennial weeds.