Cover Crop And Weed Management In An Inter-seeded Cover Crop Plus Reduced-Rate Herbicide System In Wide-Row Vegetables
Two field trials were conducted in 2015 at the Homer C. Thompson Vegetable Research Farm, Freeville NY. Experimental set up and methodology were as described in the ‘Approach and Methods’ section. Treatments however, were changed after greenhouse tests in April 2015. Herbicide combinations consisting of two applications, each of a different kind of herbicide were decided to be more effective than one, or two, applications of the same herbicide. Herbicides were still applied at rates much lower than recommended rates. A more detailed list of the treatments and times of application is provided in Table 1. Cover crops were planted on 28th May. Tomato was seeded during the last week of April, transplanted during second week of June and harvested by mid-September. Possibly as a consequence of the heavy rains during this season, the smaller-seeded sesbania did not emerge in either trial. Owing to patchy emergence in the Sole Cover Crop Trial, sunnhemp was replanted on 6th July. Herbicide effects on cover crops were similar to observations in the greenhouse. Results from the collected data was both expected and interesting. The highlight was a strong positive correlation between tomato yield and the amount of cover crop biomass produced. Sunnhemp produced up to 31 tons ha-1 of fresh biomass.
- Extent of cover crop injury from the different herbicides and application rates were as expected. Cover crops recovered from all herbicide applications. Herbicides in order of extent of cover crop injury were fomesafen > rimsulfuron > imazethapyr > metribuzin > halosulfuron > s-metolachlor. Even at the low rates applied, at both trials, fomesafen caused almost total defoliation. Each herbicide combination consisted of two kinds of herbicides with respect to effect on cover crops. One kind of herbicide (hereafter referred to as Type-1) caused comparatively more severe injury while the other (Type-2) caused less severe injuries, but had good residual activity. In the inter-seeded tomato trial, cover crop height was, on average, greater when Type-2 was applied first (81 cm vs 64 cm when Type 1 was applied first). Cover crop height in the untreated cover crop checks averaged 82 cm. Again, ground cover was greater when Type-2 was applied first (62% vs 43%; 56% for the untreated check). It is likely that when Type-1 herbicides were applied on larger cover crop plants, lateral branching was induced. This could explain similar cover between the first set of treatments (62%) and the untreated check (56%).
In the treatment including fomesafen, cover crop cover was only 33%. In the sole cover crop trial, treatments including fomesafen had the lowest ground cover (66%). Other herbicide treatments were similar with an average cover of 83%; untreated cover crop check had 88% cover. Cover crop height ranged from 78 cm to 88 cm and was similar between herbicide treatments. Cover crop plants in the untreated check were much taller (104 cm). In this trial, the order of application of Type-1 and Type-2 herbicides was not relevant.
- Treatments had a significant effect on tomato yield. The hand weeded control plots had the highest tomato yield (33 tons ha-1) and this was higher than the weedy check (12 tons ha-1) and the untreated cover crop check (12 tons ha-1). This yield from the control plots was also significantly higher than the yield from Treatment 4 (16 tons ha-1). Tomato yields were higher in treatment plots where Type-1 herbicides were applied first (24 tons ha-1 vs 18 tons ha-1).
There was a strong positive correlation between the amount of cover crop biomass and tomato yield. Tomato yield showed no significant response to cover crop height or cover. Treatments had no significant effect on tomato leaf N or P. However, leaf N and P were negatively correlated with cover crop biomass. Leaf K was lowest in the hand weeded control, and K was significantly higher in tomato plants from Treatments 5 and 7, than from the control. Potassium content in tomato leaves was also positively correlated to cover crop biomass.
- In the tomato trial, cover crop biomass, cover, height or density did not differ between treatments and, in these parameters cover crops in herbicide treatment plots did not differ from the untreated cover crop check. In the sole cover crop trial, treatments did not differ in terms of biomass produced or stand density, but cover crop height and cover were significantly different between treatments. Cover crops were tallest in the untreated check (104 cm); among the herbicide treatments, tallest cover crop plants were in Treatments 9 (88cm) and 4 (86 cm).
Fluctuation in cover crop height with no corresponding decrease in biomass or density might be a desirable outcome with respect to the overarching goal of this project. In both trials, cover crop height was positively correlated with cover crop biomass and cover. This might indicate that, while providing temporary growth retardation, cover crop recovery was good. Herbicide applications reduced cover crop vigor adequately, albeit temporarily. Fomesafen and rimsulfuron caused defoliation and growth arrest, respectively.
- In the inter-seeded tomato trial, weed biomass (fresh) in the untreated cover crop check was 5.5 tons ha-1 while that in herbicide treated plots ranged from 1.5 tons ha-1 to 4.6 tons ha-1. Weed biomass in the weedy checks recorded 8 tons ha-1. Weed cover was strongly negatively correlated with density of cover crop stand.
In the sole cover crop trial, fresh weed biomass in the herbicide treatments and the untreated cover crop check did not differ, and ranged from 0.35 tons ha-1 to 2.5 tons ha-1. Average weed biomass in the weedy checks was 12 tons ha-1 and was significantly higher than all other treatments. Percent weed ground cover was negatively correlated with cover crop cover and cover crop density.
Grassy weeds were not a problem in the experimental fields until early August. Herbicides for their control were applied early and mid-season. Predominantly large crab grass, the grasses were prominent during and after tomato harvest. Due to their late appearance, their effect on tomato was assumed to be negligible and therefore, even though their biomass were recorded, the weed biomass values discussed above do not include the grassy weeds.
- Cover crop parameters in the various treatments were similar in the inter-seeded tomato trial. Cover crop cover in Treatment 13 plots was 33%; the same in other treatment plots ranged from 53% to 63%. In the untreated cover crop check, 56% was recorded. Cover crop height ranged from 64 cm to 85 cm; untreated cover crop check- 82 cm. Density of cover crop stand ranged from 94 plants m-2 to 130 plants m-2; untreated check- 118 plants m-2. Both cover crop height and cover were highest in Treatment 12, and lowest in Treatment 12. Fresh biomass accumulation in the treatments was 7 tons ha-1 to 9 tons ha-1; untreated check- 8.3 tons ha-1. Even though in terms of all other cover crop parameters, Treatment 13 recorded lowest data, fresh biomass accumulation was 8 tons ha-1. Corresponding dry matter ranged from 2.1 tons ha-1 to 2.8 tons ha-1.
In the sole cover crop trial, cover crop cover ranged from 88% to 54%. Treatments 13 (88%), 16 (88%) and 23 (86%) had higher ground cover than Treatment 24 (54%). In terms of cover crop height, plants in the untreated check (104 cm) were significantly taller than cover crop plants in Treatments 22 (80 cm), 18 (79 cm), 23 (79 cm) and 20 (78 cm). Cover crop height in herbicide treatment plots ranged from 78 cm to 88 cm. Density of cover crop stand was between 85 and 112 plants m-2. Average biomass accumulation ranged from 17 tons ha-1 to 26 tons ha-1. Highest fresh biomass recorded was 30 tons ha-1. Dry matter recorded from the untreated check (7 tons ha-1) was higher than that from Treatment 17 (3.8 tons ha-1). Photos of the trials and some herbicide treatment effects on sunnhemp are attached along with this section.
- Table 1
- Fomesafen, Sole cover crop trial
- Rimsulfuron, Sole cover crop trial
- Sole cover crop trial
- Tomato trial (2)
- Tomato trial, September
- Tomato trial
- Untreated check, Sole cover crop trial
- Rimsulfuron, Tomato trial
Greenhouse tests were conducted in April to understand effects of different herbicides and rates on the cover crop species and this was used in conjunction with the information from 2014 field trials to establish treatment rates for field trials during this season. Changes were made in the treatments since the original proposal was written. A list of the final treatments are provided as an attachment (Table 1). Based on greenhouse and field observations, these new treatments were assumed to provide better weed and cover crop management.
Planting and harvest times are provided in the ‘Summary’ section of this report. Herbicide applications were conducted during approximately the first and last weeks of July in the inter-seeded tomato trial and during the second and last weeks of August in the sole cover crop trial. This year concludes one season of data collection. One more year of trials is required to draw conclusions based on treatments results. Lack of emergence in sesbania, in both trials, was a setback. This was likely due to heavy rain following planting, which was in turn succeeded by a dry week, resulting in soil crusting that could have hampered emergence of the small seeded sesbania. Sesbania will however, be tested again next year before it is considered unsuitable for the Northeast.
It was observed this year that the variation in herbicide injury (on cover crops) among the different treatments did not manifest correspondingly in cover crop biomass accumulation or tomato yield. The possibilities from this have yet to be interpreted completely, but, a compensatory mechanism might exist in the cover crop that will give us flexibility with herbicide rates, which could vary with type and magnitude of weed presence. An interesting result (also mentioned in the ‘Objectives/Performance targets’ section of this report) was a significant increase in tomato yield with increase in cover crop biomass. This was slightly unexpected, but very encouraging; especially because of such short-term benefit.
Impacts and Contributions/Outcomes
Excellent biomass production and ground cover, flexibility with herbicides and application rates and positive influences on vegetable yield were some promising outcomes. These factors, together, improve/ maintain agro-ecosystem sustainability and commercial yields. Inter-row cultivations were not carried out in the tomato trial, but absence of yield losses in tomato suggests that weed suppression was adequate. Further, without compromising cash crop yield. Considerable amounts of cover crop biomass was accumulated and returned to the soil. It is also likely that, during the heavy thunderstorms that occurred during this season, the cover crop cover reduced soil erosion. The field trials could not be demonstrated to extension agents, growers and others because the Field Day at the research farm was cancelled due to heavy rains. The results from this year’s experiments, including the information in this report, will be presented at the Northeastern Plant, Pest and Soils Conference in January 2016 at Philadelphia.
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