Sustainable orchard intensification: Cover crops and management intensity

We initiated a field study in November 2019, which involved planting various cover crop programs in a walnut orchard in Davis, CA. Plots were installed in an orchard designed to mimic standard commercial cultural practices, and plots were about 20’ wide by 140’ long, covering the orchard alley between two tree rows for the length of seven trees. The cover crop programs included a standard low-input cover crop treatment, a ‘boosted’ cover crop with additional fertilizer and starter irrigation, a multi-species cover crop mix, and a grass forage intercrop. Each of these cover crop programs featured cereal rye, and they represented a range of management intensity that could be feasible for California growers to implement in existing tree nut orchards. Additionally, a nontreated check treatment was created by planting a cereal rye cover crop and later spraying it out with glyphosate to mimic typical low-vegetation orchard floor management practices (while minimizing the logistic challenges of planting several cover crop treatments in our experimental orchard).

The standard cover crop treatment was cereal rye drill-seeded at 50 lbs per planted acre into a 13’ alley between tree rows; no other amendments were added to this treatment. In addition to the standard cereal rye planting, the boosted cover crop treatment received microsprinkler irrigation immediately after planting until the first winter rainfall and a top dress of 50 lbs N fertilizer per planted acre in the spring after planting. The multi-species cover crop featured the standard cereal rye planting overseeded with a mix of additional cover crop seeds; the seed mix was 5 lbs per planted acre daikon radish, 5 lbs per planted acre white mustard, 10 lbs per planted acre white clover, and 10 lbs per planted acre hairy vetch. The forage treatment was drill-seeded with 100 lbs per planted acre of cereal rye, which received starter micro-sprinkler irrigation, 45 lbs N and 25 lbs P fertilizer at planting, 50 lbs N topdressed in the spring after planting, and a postemergent herbicide application with carfentrazone. For all treatments, cover crop planting occurred on November 15, 2019. For relevant treatments, fertilizer topdressing occurred on March 12, 2020. For the forage treatment, the starter fertilizer application occurred on the day of planting and the postemergent herbicide was applied on February 25, 2020.

Cover crops were terminated on April 24, 2020. All cover crops were mowed with a flail mower, except for the forage treatment which was terminated with a swather, left to dry, and finally baled and removed. Mowing was used as a commercially-practicable management practice for orchard growers who often do not have access to other cover crop termination equipment (e.g. roller-crimper) and who have concerns about cover crop residue interfering with nut harvest. Immediately before termination, we collected biomass data to determine cover crop and weed biomass in each treatment. In each plot, we removed plant material from two 0.25 m² quadrats, separated cover crops and weeds, and recorded the fresh weight of each type of plant material. We also determined moisture content by drying a subsample of each plot’s crop or weed biomass. We had additionally planned to survey plant cover by using point-intercept transects at termination time. However, we determined that this method would have been inaccurate at that time given the height of the cereal rye relative to low-growing weeds and shorter-statured mustards and legumes. We did perform transect surveys approximately eight weeks after cover crop termination to evaluate any cover crop regrowth and the emergence of summer weeds. One 25-m long transect was laid diagonally across the center of each plot, and plants were identified at one-meter intervals along the transect. This survey occurred on June 17, 2020.

The field study was repeated in the same orchard beginning with cover crop planting on November 9, 2020. The same experimental design, treatments, plots, and management practices were used in this repetition. Supplemental starter irrigation was not used on boosted and forage crops this year because of a rain event followed by frost concerns immediately following cover crop planting. The sprayed (cover crop removed/nontreated) plots were burned down on January 12, 2021. Cover crop termination occurred on April 9, 2021. Post-termination weed and cover crop regrowth surveys took place on May 21, 2021. Local conditions and UC Davis regulations regarding the pandemic conditions in the fall of 2020 caused us to limit the replication of this study to just the on campus location (i.e. no on-farm locations with grower cooperators).

Throughout the duration of the field experiment, we have also been keeping record of the logistics and economic costs associated with implementing each cover crop treatment. Namely, there have been costs associated with cover crop planting (seed cost, tractor and seed drill operation/fuel/amortization, damage to orchard trees) in each treatment, and input costs associated with certain treatments. Such input costs we have been recording include fertilizer application in boosted and forage treatments, microsprinklers and irrigation line in boosted and forage treatments, and herbicide application in the forage treatment. Economic data recording also occurred around the cover crop termination, namely understanding the costs of using a flail mower in relevant treatments and understanding the costs and benefits of baling cover crops in the forage treatment. These factors were evaluated in both the first and second repetitions of the study.