Pacific Northwest Cover Crop Decision Aid System

Research Objectives 

The project addresses two key grower-identified questions through two objectives.

1. Compare cover crop mixtures and termination dates for effects on cover crop performance, water infiltration rates, bulk density, nitrogen availability, weed and insect biodiversity and performance of the subsequent cash crop
2. Compare the effects of treatments in Objective 1 on soil moisture profiles during the season and after different termination dates

Design and Treatments

Treatments will be applied in replicated trials on the eight working farms of the Producer Co-PIs and Producer Cooperators for this project. Four farms will be in the region’s lower precipitation Annual Crop-Fallow (AC-F) agroecological zone where business as usual (BAU) cropping consists of a three-year fallow-winter wheat-spring wheat rotation.  Four farms will be in the higher precipitation annual cropping (AC) zone where BAU consists of a three-year rotation, predominantly spring legume-winter wheat-spring cereal. On each farm, nine strips (~ 0.5 ac. each) will be established within BAU fields, replacing fallow in AC-F and spring wheat in AC with experimental cover crop mixtures. A schematic of the plot design on a participating farm over three years is provided in Figure 1.

Cover Crop Mixtures. On each farm, three cover crop mixtures will be established, each with three replicate strips. Configuration will vary among farms to accommodate geometry and equipment. Common treatments will include a low-diversity cover crop mixture (LD) and a high-diversity cover crop mixture (HD). The LD mixture will include one grass, one legume and one brassica species in these proportions: 60% oats, 30% pea, and 10% turnip. The HD mixture will include three grasses (20% oats, 20% millet, 20% triticale), three legumes (10% pea, 10% sweet clover, 10% lentil), and three brassicas (3.3% turnip, 3.3% radish, 3.3% mustard). The third mixture on each farm will be a ‘producer’s choice’ (PC). We recognize that growers in different parts of the region, in addition to their common interests and questions about cover crops, have different challenges that influence their selection of cover crop mixes. The PC treatment will enable producers to test performance of a mixture designed based on their experience and interest. The findings about these mixtures will also inform our cover crop selection tool (see Objective 3). The PC mixtures and rationales for including them (Table 1) were developed as an outcome of a discussion involving all the cooperators and producer co-PIs for this project. Rationales common to several producers include nitrogen fertilization (4/8), weed suppression (4/8), bio-drilling (4/8, all in AC-F subregion), increasing organic matter (3/8), grazing income (2/8, both in the AC subregion). For this project, all three treatments will be established each year in the spring to improve comparability. There is also interest in fall planted cover crops in the region but evaluating those practices will be beyond the scope of this project.

Table 1 Producer-choice cover crops which will comprise the third treatment on each farm.

Table 1

Termination Dates. Each of the standard treatments (HD and LD) and most of the PC treatments (see Table 1) will be terminated using herbicide in a split plot design on three dates based on cover crop vegetative growth profiles. Termination date 1 (TD1) will be on or before June 1, when cover crops are fully established and in accordance with USDA-NRCS guidelines and crop insurance program requirements. Termination date 2 (TD2) will be at first pod formation in any cover crop legume, and TD3 will be when all cover crops have reached maturity. The dates selected will maximize biomass production and related benefits to soil health while minimizing excessive moisture loss through transpiration and seed production leading to volunteer plants in the following.  Producer-choice mixtures will also be terminated on these dates.

Subsequent Crop: Each year, strips will be relocated on each farm, marking their locations. In the subsequent cropping year, the ground with strips will be planted to winter wheat following standard grower procedures. The subsequent winter wheat crop yield and quality and soil nitrogen availability in near-surface soil will be measured in this follow-on wheat crop in the previous year’s cover crop and BAU strips to assess the effects of cover crops on the principal cash crop.  Measurements to address Objective 1 and 2 on all farms will be as follows:

Objective 1.  Cover Crop and Subsequent Cash Crop Performance: Cover crops will be evaluated for percent emergence and established stand diversity and cover crop coverage and biomass by species at each TD. In the second and third year of the project, the subsequent winter wheat will be assessed following cover crop and in the BAU system for emergence, stand, mature biomass, yield, grain moisture and protein.

Soil Physical Properties: Penetration resistance (Dane et al. 2002), bulk density (Grossman and Reinsch 2002), and water infiltration rates (van Es and Schindelbeck 2001) in cover crops at TD3 and in the subsequent winter wheat crops will be assessed to determine the impact of cover crops on soil physical properties.  Penetration resistance measurements and infiltration rates will be measured once a year in cover crops and the following winter wheat crop.  Bulk density measurements will be made prior to planting of the cover crops and at the end of the experiment.

Soil N and Organic Matter: Plant available nitrogen (Mulvaney 1996) in the first 1.5 m of soil will be measured in one TD in LD, HD and PC cover crop treatments at each site both in spring and at harvest.  Three replicate, 1.5-m deep soil cores will be taken within each plot and split into 30-cm increments in the field.  Subsamples from each depth increment will be combined to form one representative sample per treatment that will be analyzed.  Changes in plant available nitrogen with depth, along with measured values of total nitrogen in above ground biomass, will be used to evaluate the impact of each cover crop treatment on nitrogen uptake and availability to the following crop.  Sampling in the winter wheat crop will be limited to spring and results will be utilized to determine nitrogen fertilization application rates. Soil organic matter, estimated by loss on ignition (Nelson and Sommers 1996) will be assessed on the same samples collected to determine plant available nitrogen.

Weed and Insect Pressure and Biodiversity: Aphids (Aphididae; e.g., cereal aphids, pea aphid), wireworms (Elateridae), hessian fly (Mayetiola destructor), pea weevil (Bruchus pisorum), and pea leaf weevil (Sitona lineata) are some of the key insect pests in the iPNW. Additionally, several beneficial insects such as bees, wasps, flies, ladybug beetles, green lacewings are present in the region that need to be documented on cover crops. Similarly, key weeds are Mayweed chamomile (Anthemis cotula), cheat grass (Bromus tectorum), rattail fescue (Vulpia myuros), prickly lettuce (Lactuca serriola), common lambsquarters (Chenopodium album), and catchweed (Galium aparine). Baseline information of weeds, insect pests and beneficial insects within BAU and alternative cropping systems is important to understand the sustainability of future agricultural landscapes. Substantial changes in crop species and diversity could promote ecosystem services such as biological control and pollination, but also promote disservices by exacerbating existing pest populations or introducing new pests. To anticipate these potential effects, pests and beneficial organisms will be assessed in all treatments in the on-farm strip trials and in adjacent BAU crop fields. Briefly, weed abundance, coverage and diversity will be assessed before each termination dates in five 1-m² quadrats in each cover crop strip and adjoining BAU fields during first two years,  and in winter wheat following cover crops in the second and third years. Aboveground weed biomass will be assessed by clipping all plants rooted within quadrats, sorting to species, drying, and weighing. Following a similar schedule, insect pests and beneficials will be assessed on each strip using a D-Vac vacuum sampling along a W-shaped 30 m transect. Insect samples will be sorted to species for key pest species and beneficial insects to genus or family.

Objective 2. Sensors linked to data loggers (Acclima, Meridian ID) will continuously monitor volumetric soil moisture and temperature at two depths in each cover crop treatment at TD2 and TD3 in one replicate block per farm. There will be 14 probes per site, which include 2 probes in each LD, HD, and PC at both TD2 and TD3 as well as 2 probes in the BAU.  As a complement, soil moisture will be measured gravimetrically based on soil samples at the beginning and end of each growing season in all plots on each farm, utilizing the same cores collected under Objective 1. Volumetric water contents will be calculated utilizing measured bulk density values. These measurements will allow assessment of water use and tradeoffs between soil moisture conservation and biomass production. Replicated gravimetric measurements will allow comparisons of treatments within and across farms. Continuous monitoring will provide new information on soil moisture dynamics to understand the origin of point measurement values and guide future efforts to identify optimal cover crop termination dates to balance benefits of biomass production and potential costs of cover cropping to the yield of subsequent cash crops in our region. Termination dates currently are determined by producers based on intuition, scantly replicated experience on individual farms and generally follow some aspect of cover crop phenology as an approximate guide. Some are incentivized to terminate early (before June 1) for crop insurance reasons. Data will be widely disseminated to stakeholders through our decision support system (see Objective 3). Results on termination after June 1 and before end of season will have relevance for those involved in advising on crop insurance policy requirements. Water infiltration rates will be measured in all treatments each spring and after the final TD. This measurement has been encouraged by producers who have seen evidence of water infiltration improvement after cover crops and are greatly interested in this output from the project. Under low rainfall, this can be critical for maximizing harvesting of precipitation for crop growth.

Analysis for Objectives 1 and 2

The effects of cover-crop treatment, nested within farm, on each response variable (soil biological, chemical and physical properties) will be analyzed with analysis of variance (ANOVA), multivariate analysis of variance (MANOVA), or mixed models with data transformed as required for each variable. For common cover crop treatments, the effects of farm site and production zone and their interaction on response variables will also be assessed. Specifically, differences in weed and insect abundance, species richness, diversity, and community composition will be analyzed using appropriate mix-effects ANOVA models and multivariate analysis such as Non-Metric Multidimensional Scaling (NMDS). We will test differences in weed and insect community structures between cover crop treatment and termination timing using Permutational Multivariate Analysis of Variance (PERMANOVA) of the dissimilarity matrix using the vegan package in R statistical software (R Core Team 2019, Oksanen et al. 2020). This will enable detection of effects that are broadly relevant in the iPNW across its precipitation regimes and those that are site or production class specific, critical information for guiding cover crop implementation and management. Summaries of all these findings will be distilled for presentation on the project’s PaNDAS website into thematic segments such as: “Terminating your cover crop to optimize water use”, “Best cover crop species for water conservation”, and “iPNW cover crops and weed suppression”. The findings will also be unique and publishable in refereed journals including Journal of Agronomy, Journal of Soil and Water Conservation, Applied Soil Ecology.

Farm sites in the 2023 field season

Between the project start date and April 15, 2023, the specific plot locations were identified on each of the 8 farmer cooperator/co-PI farms (Fig. 2) and flagged for planting. In this reporting year, plots were established, instrumented, planted and monitored on all 8 farms.

Operations conducted on each farm  - Objective 1

• Pre-plant (spring) and end of season soil samples were collected in the designated second termination date treatment strip of all plots and a BAU area adjacent to plots, in 30 cm increments from 0-150 cm for plant available soil N and gravimetric water content.
• Seed for the common cover crop mixtures was procured and delivered to each farm in preparation for planting and all plots were seeded with cover crop mixes in April- May.
• Germination counts were completed within 2 weeks after seeding.
• Bulk density was measured only in the end of season soil samples in the termination date strip only of our cover crop plots, while collecting samples for soil N and gravimetric water content. This was only measured once due to time limitation in the spring, which is a deviation from what is stated in the last report under ‘Design and Treatments: Soil physical properties’.
• Infiltration was  measured at two farms, one in each rainfall zone, after the final cover crop termination. This is a very time consuming measurement to collect. We used a dual infiltrometer in attempt to get a more accurate reading than the simpler single ring infiltrometer. This original plan is stated in the last report under ‘Design and Treatments: Soil physical properties’ subheading and also mentioned under Objective 2.
• Soil penetration resistance was not measured because by the time we were able to start collecting this measurement, and the cover crops were growing, the soil was too dry to get accurate (or any) readings. This is also a deviation from last year’s report.
• Producers were not requested to adjust  N fertilizer applications based on soil N tests as planned, but we will record rates used for the winter wheat planted on each farm. 
• Cover crop and weed biomass were collected using three 0.5 m² frames per each of nine plots across fields before each of the three crop termination dates.
• Insect survey was performed using sweep nets (40 sweeps on a 40 m W-shaped transect from each plot) per each of nine treatment and three "business as usual"  plots across fields before second and third cover crop terminations.

Operations conducted on each of these sites  - Objective 2

• Soil moisture and temperature sensors were installed at all 8 farm trials prior to planting
• At each termination date, surface 0-10 cm soil moisture was measured with a hand-held TDR sensor. At termination dates 2 and 3, in addition to the surface soil moisture measurement, gravimetric soil water was measured in samples collected from 0-30, 30-60 and 60-90 cm to verify and provide more replication than what is measured by the continuous sensors. Biomass samples were collected at each termination date from 3, 0.5 m² quadrats in each subplot. Plants were separated by family level classification and weeds, dried and weighed.
• Soil moisture was measured in 0-10 cm soil using the handheld TDR probe at winter wheat seeding following the cover crops (all 27 subplots) in Sept-October 2023.