The intensification experiment was implemented in an established walnut orchard at the Plant Sciences Field Facility in Davis, CA, USA . The orchard was planted in the spring of 2015 with ‘Chandler’ walnuts. The entire orchard was 0.7 ha in area consisting primarily of Yolo silt loam soils . Orchard management included microsprinkler irrigation and weed-free tree strips maintained with preemergent herbicides. Experimental plots included the orchard alley between seven pairs of trees, approximately 6 m by 40 m. Cover crop programs were based on cereal rye, since it is known to be a competitive, weed-suppressing species that has desirable termination characteristics . Furthermore, this species thrives under various cultural management conditions and has cultivars that are well-adapted to grow as a winter cover crop in Central California. We used ‘Merced’ rye, which is a relatively tall cultivar. The whole experiment was conducted in one orchard over two growing seasons. Cover crops were established in the fall of each year, on November 11, 2019 and November 9, 2020, and terminated in the spring of each study year, on April 24, 2020 and April 9, 2021. Each plot received the same cover crop management program in both years of the experiment. Except for the forage treatment described below, rye was direct-planted with a seed drill at 22.5 kg planted ha-1 , drying rack and cover crop termination was performed with a flail mower. Planting and termination operations were planned to minimize equipment traffic in the orchard, and only one tractor pass was made across each orchard alley at each planting and termination date.
Flail mowers are practical for cover crop termination in California, since these implements are more common than other cover crop termination tools and they minimize crop residue ahead of nut harvest. We had five treatments which represented a range of different cover crop management intensities. The ‘sprayed’ treatment was used as our non-treated control, and the rye planted in these plots was terminated with a glyphosate application when rye plants reached 5 to 10 cm in height. These burn down applications occurred on January 13, 2020 and January 12, 2021, and included a broadcast application of Roundup Weather MAX at 1.607 L ha-1 with a carbon dioxide-propelled backpack sprayer. This treatment mimics a relatively intense commercial management system where orchard alleys are kept weed free. The ‘standard’ treatment included rye with no other cover crop management until termination. The ‘multi-species’ treatment included the base planting of rye and several additional cover crop species. The other cover crop species in the mix were common vetch 4.5 kg planted ha-1 , ‘PK’ berseem clover at 4.5 kg planted ha-1 , daikon radish at 2.25 kg planted ha-1 , and ‘Braco’ white mustard at 2.25 kg planted ha-1 . These seeds were broadcast spread immediately before rye was planted. We used these methods to establish the sprayed and multi-species treatments to minimize logistical challenges and orchard traffic, while also relying on the tractor and seed drill to enhance seed-to-soil contact of our additional cover crop species in the multi-species treatment. The multi-species treatment in this experiment has the same species and approximate planting rates as the multi-species mix in the planting date experiment described below.The ‘boosted’ treatment included a 45 kg ha-1 N top dress with granular urea after rye tillering which were made on February 25, 2020 and February 26, 2021.
The ‘forage’ treatment was managed as a rye hay intercrop. This treatment was planted at a rate of 45 kg planted ha-1 . At planting, we fertilized with 40 kg ha-1 N and 28 kg ha -1 P as granular urea and monoammonium phosphate at planting. We also top dressed with 45 kg ha-1 N after rye tillering. On the same day as topdressing, we broadcast-applied carfentrazone at 73 mL ha-1 with a backpack sprayer as a post emergent herbicide application for broadleaf weed control. The top dress and herbicide applications were applied on February 25, 2020 and February 26, 2021. The forage treatment was terminated with a swather, and the crop material was subsequently baled and removed. Immediately before cover crop termination, we destructively sampled cover crop and weed biomass. We collected biomass samples from two 0.25 m2 quadrat subsamples in each plot. Cover crops and weeds were separated before being dried in forced air drying ovens. Finally, we weighed dry plant biomass. Summer weed emergence was assessed after cover crop termination using point intercept transects. One transect was placed diagonally across the alley in each plot. Transects were 25 m long with 25 points spaced evenly along the transect. Plants were identified visually at each point. These summer weed transects were performed on June 17, 2020 and May 21, 2021, when summer weed emergence and potential cover crop regrowth might be scouted by a grower planning summer weed management. Planting date experiment. The planting date experiment was implemented in a non-bearing almond orchard at the Wolfskill Experimental Orchard near Winters, CA, USA . The orchard was established in the fall of 2017 with alternating rows of‘Nonpareil’ and ‘Aldrich’ almonds.
The entire site was about 1.1 ha in area with primarily Yolo loam soils . Orchard management included microsprinkler irrigation and weed-free tree strips treated with preemergent herbicides. The experiment was laid out as a randomized complete block design with five repetitions. Experimental plots were roughly 25 m long and 12 m wide, comprising five trees in length and two orchard alleys in width. We had five treatments, including a non-treated control and two multi-species cover crop mixes each planted at two different planting dates. The non-treated control had commercial standard vegetation management practices, which included several glyphosate applications throughout the winter months. We used cover crop mixes in this experiment because of their existing use by California orchard growers . Orchard growers frequently choose among cover crop mixes that support a variety of ecosystem services aside from vegetation management, such as pollinator health or improved soil structure, and multi-species cover crops can support some of these multifunctionality goals. Additionally, using different cover crop mixes allowed us to evaluate cover crops with different germination timings and a range of emergence phenologies. The two cover crop mixes used in this study were a ‘multi-species’ mix and a ‘brassica’ mix. The multi-species mix used the same species as the multi-species treatment in the intensification study, and it included a common combination of cover crop functional groups including a small grain, legumes, and mustards . The mix consisted of 10% ‘Braco’ white mustard, 10% daikon radish, 30% ‘Merced’ rye, 20% ‘PK’ berseem clover, and 30% common vetch planted at 56 kg planted ha-1 . Each of the cover crop mixes was planted at a relatively early planting date and a late planting date. These dates were chosen to represent a timely cover crop planting soon after nut harvest and coincidental with the onset of winter rains as well as a later cover crop planting coincidental with nut pruning, sanitation, and other winter management activities. This experiment was conducted in one orchard over three growing seasons. The early planting date occurred on October 15, 2018, October 24, 2019, and November 9, 2020. The late planting date occurred on January 31, 2019, February 10, 2020, and January 21, 2021. Cover crops were direct-seeded with a conventional grain drill. Ground preparation occurred before each planting date. Before the early planting date, the whole orchard received light tillage immediately before a glyphosate burndown. Before the late planting date, cannabis curing late planted plots and the non-treated control received an additional glyphosate burndown but no additional soil disturbance. Cover crops were terminated with a flail mower on April 19, 2019, April 27, 2020, and April 22, 2021. Weed emergence was monitored throughout the cover crop growing season using permanent point intercept transects. Each plot had one transect placed diagonally across one orchard alley. Each transect was 10 m long with 10 points along the transect. Plants were identified at each point along the transect, and monitoring took place weekly while cover crops were growing. This experiment did not have different residue management treatments, so summer weeds were not evaluated. Immediately before cover crop termination, we sampled cover crop and weed biomass using the methodology described above, including two 0.25 m2 quadrat subsamples in each plot.Data analysis. Analyses were performed in R 3.0.3 .
For biomass data from both experiments, we used ANOVA and performed multiple comparisons with Fisher’s LSD. ANOVA was performed by specifying a model with lm and entering it into Anova from the car package . The models we used had treatment, replicate, and their interaction as predictors and either weed biomass or cover crop biomass as a response variable. We inspected ANOVA assumptions visually using plot. Subsequently, weed biomass from the intensification experiment was analyzed with one outlier removed and a square root transformed response variable due to leptokurtosis. However, unabridged and non-transformed data are displayed in the figures. Finally, we performed Fischer’s LSD with LSD.test from agricolae using a significance level of P<0.05 . Summer weed emergence data were analyzed in the same manner but using cover crop regrowth and summer weed emergence as response variables. Weekly transect surveys were analyzed with multiple linear regression. We compared the slope of each regression line in to evaluate the relative rates of weed and cover crop emergence after each plant date. Cover crop emergence was represented as the change in ground cover as observed in weekly observations throughout the first ten weeks following the respective planting date of each treatment. There was only one non-treated plot in each repetition, and we evaluated ground cover following both the early and late planting dates in the same non-treated plots. Weed and cover crop emergence were modeled as functions of treatment, weeks after respective planting, and their interaction. These linear models were created using lm. We created additional linear models using other possible combinations of predictor variables and compared these various models using anova. However, we determined the model described above to be the most parsimonious. Parameter estimates for the slope of each line were compared with Tukey’s HSD using lstrends from the emmeans package . All figures were made with ggplot2 .In the intensification experiment, cover crop biomass varied with management treatment . While year was not a significant predictor of cover crop biomass , we detected an interaction between year and treatment . Furthermore, multiple comparison testing led to different conclusions from each year of the intensification experiment. With data pooled across years, the forage and boosted treatments resulted in higher cover crop biomass than multi-species or standard treatments. Within each year, the boosted treatment alone resulted in the highest cover crop biomass in 2020, while the forage treatment did so in 2021. Cover crop treatment and year both predicted weed biomass. The interaction term was also important . In general, the four cover crop programs resulted in less weed biomass compared to the sprayed treatment and similar weed biomass compared to each other. This conclusion was supported in both years of the study, but we observed less weed biomass overall in 2021. Intensified cover crop programs can increase cover crop biomass, but all of the cover crop programs we tested were similarly effective at reducing weed biomass. Less rainfall in 2021 could have contributed to differences between study years, and we attribute some decrease in boosted cover crop biomass to dry conditions after top dress fertilizer application which likely caused a reduction in plant-available nutrients from the applied fertilizer. Some cumulative effect of two years of cover cropping could have also contributed to these results.In the planting date experiment, cover crop biomass varied with cover crop treatment . Year was not significant , but the interaction between treatment and year was . In 2019 and 2020, the early planting treatments resulted in higher cover crop biomass than the late planting treatments. Differences between cover crop treatments were greatest in 2020, and the multi-species mix also resulted in greater cover crop biomass compared to the brassica mix in this year. There were no differences in cover crop biomass between treatments in 2021. Year , treatment , and their interaction all contributed to weed biomass. While we observed a lot of year-to-year variation, the late-planted multi-species treatment was consistently in the lowest statistical group for weed biomass, and the early-planted brassica treatment was consistently in the highest group.