Sustainable orchard intensification: Cover crops and management intensity

Progress report for GW19-194

Project Type: Graduate Student
Funds awarded in 2019: $24,944.00
Projected End Date: 07/31/2021
Grant Recipient: University of California, Davis
Region: Western
State: California
Graduate Student:
Major Professor:
Bradley Hanson
University of California, Davis
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Project Information

Summary:

Sustainable orchard production depends on effective orchard floor management to provide ecosystem services, such as limiting soil erosion, managing water cycling, and improving soil structure. While a growing body of literature has established the general sustainability of cover crops, few studies have investigated the importance of cover crops used specifically for weed management, and fewer still are applicable to perennial cropping systems in the arid west. Therefore, orchard growers have begun to recognize the need for research that will enhance specific ecosystem services from orchard floor vegetation. Weed-suppressing cover crops could contribute to an integrated weed management program by displacing weeds and reducing the need for chemical weed management. We intend to implement small plot cover crop trials in orchards in the Sacramento Valley of California to test a management-intensity gradient, including minimal cover crop management, multi-species mixtures, cut forage crops, and a harvested grain intercrop within the orchard. This research will help determine the level of management that best contributes to a competitive cover crop, including various combinations of agronomic factors like mowing, irrigation, and fertilizer applied to the cover crop. Results from this research will inform economic studies of cover crop intensification, and these data will be disseminated through professional scientific networks and Extension media and demonstrations. We will monitor outreach activities in order to understand the baseline level of awareness of cover crop benefits and identify knowledge gaps about weed-suppressing cover crops. The proposed research will make cover crops a more viable component of integrated weed management programs in western orchards, contributing to the general sustainability of orchard cropping systems and reducing the economic risk of orchard establishment.

Project Objectives:

Objective 1: Design and implement cover cropping systems for weed suppression in nonbearing orchards. Intentional orchard floor cover is relatively uncommon in young fruit and nut orchards. If intensified cover crops are viable in orchards, then winter cover crops will outcompete weeds but not negatively impact trees in a nonbearing orchard. Successful evaluation of this objective will integrate knowledge from existing cover crop research to develop systems that support orchard intensification while reducing negative externalities of orchard production.

 

Objective 2: Evaluate weed response to cover crop competition across a range of management intensities. Weed communities respond and adapt to the artificial selection imposed by agricultural management practices. If cover crops create a competitive environment that suppresses weeds, then orchards with a highly-competitive, highly-managed cover crop will be less weedy than those without. Successful evaluation of this objective will identify the relative advantages and drawbacks of high-intensity cover cropping and similar lower-intensity cover cropping programs.

 

Objective 3: Understand economic factors that motivate cover crop adoption. Realistically, broad adoption of integrated weed management requires an economic improvement over current practices. If cover crops can provide weed management services at a lower net cost to comparable weed management practices, then growers will be eager to adopt cover crops. Successful evaluation of this objective will include quantification of the economic costs and benefits of various cover crop systems in orchards.

Cooperators

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Research

Materials and methods:

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.

 

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.

Research results and discussion:

Results from biomass collection suggest that cover crop programs of various levels of management intensity can reduce weed biomass and variation in weed biomass (table 1). However, we do not see a consistent relationship between management intensity nor between cover crop biomass or cover crop biomass and weed biomass. Based on existing literature, we hypothesized that cover crop biomass would have an inverse relationship with weed biomass, but that is not supported by the data we have to date. Furthermore, we cannot make definite conclusions about what management practices can support a vigorous cover crop. Continuation of this study across additional site-years will help us understand the stability of these relationships between cover crops, agronomic management, and weed populations.

 

Based on qualitative observation of the cover crop, such as time to canopy formation and greenness at termination, we still predict that intensive management leads to a vigorous cover crop and that our null results are primarily related to uncertainty in parameter estimation. However, the general life cycle and plant architecture of the weeds we found growing in our cover crop (i.e. low-growing winter annual and short-lived perennial species) could partially explain a true lack of relationship between cover crop and weed biomass. Preliminary results from summer weed surveys indicate that cover crop residues do have a negative effect on summer weed emergence, and cereal rye regrowth could be a potential management concern, especially in the forage treatment (data not shown).

 

Table 1. Mean crop and weed dry biomass plus or minus standard deviation. Biomass was collected at cover crop termination from two 0.25 m-2 subsamples per plot.

 

Treatment

Crop biomass

(g sample-1)

Weed biomass

(g sample-1)

Nontreated

0±0

242±260

Standard cover crop

268±71

69±24

Boosted cover crop

468±166

82±20

Multi-species cover crop

292±155

50±19

Forage crop

345±38

40±44

 

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

We planned to present preliminary findings from this project and a related cover crop project at an orchard cover crop field day this spring and the University of California Weed Day, but these events were cancelled due to COVID. Naturally, the occurrence of extension and outreach events is uncertain for the near future, but we do have concrete plans to use create outreach activities in the next several months. We have written and are currently editing a post for the UC Weed Science blog to be released in September or October 2020 in order to provide preliminary information to California growers who are transitioning to winter orchard floor management. We are developing scientific presentation to be presented to weed science professionals and researchers at the online 2021 California Weed Science Society meeting in late January 2021. We are planning to develop a fact sheet about the costs of various weed-suppressing cover crop programs compared to herbicides; this fact sheet can ideally be released by sometime winter 2021. We also hope to be able to participate in in-person extension field days related to orchard cover crops in February/March 2021, depending on the feasibility of such events.

Project Outcomes

Did this project contribute to a larger project?:
No
Project outcomes:

Our project is aimed at increasing the sustainability of tree nut production by contributing to integrated weed management programs, combining weed management with broader management goals, and supporting sustainable intensification of orchards. Namely, this research can help demonstrate that cover crops have the potential to strengthen winter vegetation management without the winter herbicide application that is currently common in California orchards. By using integrated pest management principles to consider weed management alongside soil health, water conservation, and secondary farm products, growers can increase the ecological and economic resilience of their farm operations. So far, we think that our research is making advances to untangle the specific management practices that can help California orchard growers successfully add some ecological complexity back into their cropping systems.

Knowledge Gained:

Our previous experiences implementing cover crops in orchards involved newer and older almond orchards, compared to the walnut orchard used here. While very young orchards have a somewhat similar environment to an annual crop field (e.g. similar light availability, similar weed community), very mature orchards are shady enough to make cover crop establishment difficult. The walnut orchard used in this study represents a balanced situation in which the cropping system is very different from an annual system but not so mature that abundant cover crops are not possible. There were some logistic challenges working in an orchard like this; for example, the grain drill that we had previously used for cover crop planting in new orchards was just large enough to make planting in this orchard challenging.

 

We are also having new experiences related to the phenology of cover crop establishment, development, and termination when a very abundant cover crop interferes with the increased management needs of trees that are mature enough to be harvested. These issues were compounded in cover crop treatments that required additional irrigation. Our experiences setting up irrigation in this orchard confirm previous suspicions that orchards with an irrigated cover crop should be designed with a larger irrigation system from the beginning, rather than retrofitted at a later date. This project reinforces the idea that cover cropping is not a one-size-fits-all solution for orchard floor management. While the walnut orchard environment has been fairly similar to our previous experiences in almond orchards, many decisions have to be made based on local management goals and agroecosystems.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.