Crop production in the rainfed areas of Montana has been dominated by wheat/fallow for many years. The total number of acres in fallow has steadily declined over the past 20 years but 3 million acres continue to be fallowed in Montana as of 2016. Fallow farming has been successful at reducing the risk of crop failure in winter wheat production, but is considered largely responsible for a decline in soil quality. Cover crops have been suggested as a means to stabilize and rebuild soil quality replacing fallow in the traditional wheat—fallow crop rotation. A lot of information exists on the impact of single species cover crops as they were a common practice early in the previous century prior to the availability and use of modern fertilizers. There is less information on the impact of cover crop species composition and the impact they may have on soil properties such as water infiltration, amelioration of soil compaction, and addition of organic nitrogen to the soil system. Different specie C:N ratios, rooting structures and effective rooting depths may influence soil properties. The use of mixed cover crops has mostly occurred in regions of temperate or continental climate under greater annual precipitation than what is received in the semiarid region of the northern Great Plains. This study was initiated to provide a location where mixed cover crops are included in a small grain cropping system. Soil quality measures are to be assessed over time to determine the impact of adding various species of cover crops in place of fallow. Changes in soil properties are slow to occur, so it is important to continue this study for several years
A two-year rotation of cover crops/small grain crop was established in 2015 at the Southern Agricultural Research Center near Huntley, MT to provide a means for evaluating changes in soil quality and comparison of small grain yield and quality as impacted by cover crop mixtures. There are 2 blocks in Field J dedicated to this study, with the east block in cover crops and the west block in spring wheat treatments for 2021. The impact of four different plant species type, or ‘functional groups’ are compared.
Steady state water infiltration rates were measured in October after cover crops had been terminated using Saturo automated single ring infiltrometer with a 2-ponding head approach (Reynolds and Elrick, 1990). The 14.4-cm diameter ring was inserted 10-cm and measurements were made approximately 3 hours after initiation for both pressure head settings. Readings were taken from the center of the preceeding high N rate for wheat following termination of the cover crops for a subset of treatments.
On Apr 7, 2021, Egan spring wheat was planted at 70 lb/a perpendicular to the cover crop strips of 2020. This strip-plot design used four nitrogen rates with fertilizer placed in bands separated but adjacent to the seed to evaluate the impact of cover crops on nitrogen response of the spring grain. Nitrogen rates for spring wheat were 0, 22, 56, and 91 lb N/a using urea as the N source. Mixed species cover crops and a chemical fallow were established using a randomized complete block with 4 replications on May 5, 2020 in Field J at the Southern Agricultural Research Center. Each group was composed of multiple species (see Table 1). Treatments included legume (L) mix, brassica (B) mix, grass (G) mix, taproot (T) mix, a mixture of all 4 groups as B-G-L-T, and then mixtures of 3 of 4 groups, G-L-T, B-L-T, B-G-T, and B-G-L. A cover crop of pea only, and a chemical fallow check were also included. Cover crops were planted using a Seedmaster no-till drill in plots 15 ft wide by 60 ft in length. Targeted plant populations for mixes (Table 1) were adjusted by dividing the target population of each species by 4 since there were 4 species in each group. Then when mixed across groups, the targeted population for each species mix was divided by a factor of the numbers of groups within a mix. Spring wheat was harvested on 7/22/2021. Cover crops were terminated on July 13, 2021.
The year started out with regular precipitation and growing conditions which allowed for good establishment of both wheat and the cover crops. But after a 0.21-in rain in mid-May very little precipitation was received until August. Consequently, cover crop biomass production was limited (Table 2) to a mean of 1144 lb/a. The taproot mix produced the greatest amount of biomass while the brassica mix produced the least. Treatments are arranged in Table 2 for pairwise comparisons of with and without a functional group. For example taproot mix produced 1614 lb/a biomass while the mix without taproot produced a non-significantly different biomass of 1411 lb/a. Similarly biomass from ‘grass’ and ‘no grass’ treatments were not significantly different, nor were ‘legume’ and ‘no legume’ biomass production. The treatments of ‘brassica’ and ‘no brassica’ showed the greatest separation in biomass production. This result is mostly due to poor stand and poor growth of brassica crops at this location.
Spring wheat grain yield was highest following fallow, which in a dry year was not surprising. Spring wheat yield as a percent of fallow averaged 68% and was Lowest following the ‘all’ treatment. Treatments of ‘pea’, ‘no legume’, and ‘brassica’ yielded around 80% of that following ‘fallow’. The pea and brassica treatments likely had more stored soil moisture than the other cover crop treatments but for different reasons. The brassica cover crops produce very little biomass each year. The pea cover crops mature the soonest of all cover crop species in this trial, which allows for more summer precipitation to recharge the soil profile. There is some indication that water infiltration rates were greater following ‘legume mix’ than following all other treatments. This was a preliminary assessment of infiltration rates and due to dry conditions in the fall and formation of soil cracks in these soils the data is somewhat noisy.
This study will continue in 2022. As part of our long-term plan for this study we will continue to measure physical soil properties in 2022 to evaluate potential soil quality changes. One change will be to perform soil infiltration measurements in spring when soil profiles are less dry.