Similar to the no or little effect of cover crop treatments on insect pest population densities, my research findings cannot confirm that off-season cover cropping reduces crop damage in the subsequent vegetable crop. In contrast, many researchers observed that crop damages can be minimized in a mixed cover and main crop interplanting. These researchers argue that cover crops in a mixed stand interfere with host locating capability and oviposition of insects, masking the main host crop or obstructing the odor profiles , hence reducing pest pressure and damage on the main crop. However, Finch and Collier ; Finch et al. argue that there is little support for general masking theory, except when visual cues are restricted .Cover crops in my trial were used as off-season cropping rotation consequently a major masking effect would not be expected. The most positive effect of the off-season cover crops was enhancement of parasitoids and insect pest parasitization levels, although Furlong et al. state that the real role of cover crops in manipulating population dynamics of insects and natural enemies still remains unclear. There were greater pest parasitization levels on the cover crop treatments for all years compared to the fallow system. The most likely reason for greater parasitization on cover crop treatments was greater pest population densities on these treatments and that the parasitoids were responding strongly to host population density. It is also possible that natural enemies could be more abundant in diverse vegetation systems because of the continuous variety of microhabitats or food resources . A long season cropping period may allow naturally occurring biological control agents to sustain higher population levels on alternate hosts or prey and to persist in agricultural environment throughout the year .
Within the individual insect pests, vertical grow system parasitization was significant for cabbage loopers and diamondback moths, but not the cabbage worms. However, since insect population density and broccoli leaf damage occurred regardless of the patterns of pest parasitization, such natural pest-parasite relations did not off-set and stabilize broccoli insect pests or its leaf damages. Never the less, tachnid flies, chalcid wasps and braconid wasps are some of the major parasitoids against broccoli insect pests.Regardless of the occasional greater pest population densities and higher vegetable crop damages on the cover crop treatments, higher broccoli marketable yield was obtained from plots that received summer cover crop treatments than crops from the summer fallow plots . Therefore, growers should consider the holistic contributions from cover crops than their effects on insect pest population density. This study determined that off-season cover crops suppress weeds , enhanced beneficial saprophytic nematode populations , and enhanced the soil environment . Cover crops may provide many benefits; however, they are not do-it-all wonder crops . Growers need to make proper selections of cover crops considering many factors that may include benefits to ecosystem biodiversity, contribution to the productivity of agricultural systems and compatibility with the main vegetable crop.Modern agricultural systems involving monocropping have become productive, but only because of their high dependence on external chemical inputs . Questions are being raised about the growing dependence of modern farming on chemicals and other non-renewable resources .
There is also an increasing consumer demand for safe agricultural products and hence, a need for services that may help producers, processors, and distributors adapt to changing consumer preference . Such practices require adoption of alternative management practices or enhance functional biodiversity and sustainable production . The use of cover crops is a step towards a sound practice that may accommodate the changing needs of consumers and increase confidence in the quality of agricultural produce. Accordingly, there has been a growing interest in using short-season annual legumes and others as cover crops in vegetable production systems . Cover crops in farming systems improve soil health, reduce environmental pollution, and improve crop yields Walp have been identified as the best candidate for summer cover-crop rotation with winter vegetable crops and improve soil fertility and crop yield . Although several studies have been conducted on cover cropping systems, their use in vegetable crops have rarely been studied or the research has been mainly on winter annual cover crops with very little research on summer cover crops. However, summer cover crops can produce biomass, contribute nitrogen to cropping systems, increase soil organic matter, and suppress weeds , and they are compatible with both organic and conventional farming practices whether incorporated or used as surface mulches . Although cover crops are important components of a sustainable crop production system, their beneficial effects depend on the selection of appropriate cover crops and their management . This research is aimed at evaluating the effect of summer cover cropping on the subsequent vegetable crop. It is hypothesized that incorporation of cover crop plant material provides a valuable source of N and enhances crop growth and yield. Summer cover crops are used between spring and fall vegetable crops . I hypothesized that cover crops would increase soil nutrition, with subsequent improvement of broccoli yield.
Two types of summer cover crops, a legume and a nonlegume, were compared with the standard practice of summer fallow in a Mediterranean type climate.A three-year field study was conducted from 2007-2009 at the University of California South Coast Research and Extension Center in Irvine, CA on a loamy-sandy soil. The field site was loamy sand with a history of root-knot nematode infestation. Three summer cropping treatments were employed: 1) French marigold , 2) cowpea , seeded at 56 kg/ha, and 3) a summer dry fallow as the untreated control. Cowpea was chosen because it is a drought hardy legume, resistant to weeds and enhances some beneficial organisms . Marigold was chosen because it is known to control nematodes . Each treatment plot was 12 m long x 10.7 m wide and laid out into 14 planting rows. The cover crops were direct-seeded in the last week of June in the center of the planting rows of each plot, watered through drip-tubing and grown for three months. The fallow control plots did not receive water during the summer. Each cover crop treatment plot was planted with the same cover crop in each of the three years of study. Plots were separated from each other with a 3 m wide buffer bare ground. The three treatments were replicated four times in a completely randomized design. At the end of the summer cropping period , the cover crops were mowed at the soil line, chopped, and the residues left on the ground. Concurrently, industrial grow alternate rows of each of the cover crop treatments were incorporated into the soil at about 0.4 m intervals using a hand-pushed rotary tiller.The fallow plots were not tilled. Plots for cover crop and broccoli planting are shown in Figure 1a. At the beginning of the subsequent cropping season , broccoli seedlings were transplanted in double rows into the tilled strips of the summer cover crop and fallow plots at an inter and intra-row spacing of 13 and 35 cm, respectively . Broccoli transplants were drip irrigated and fertilized with emulsified fish meal at 5 gallons/acre rate. Broccoli was chosen because it is a high-value vegetable crop that is sensitive to weeds, insect pests, nematodes , and requires high soil nutrients . All plot treatments were maintained in the same location for all three years of study in order to assess a cumulative effect of cover crops over time.Soil nutrient concentrations oscillated between sampling periods and years. During the second year , only soil Ca and Na concentrations and cation exchange capacities were slightly higher for the cover crop treatments at ACCP sampling. However, none of the soil nutrition was different between the cropping treatments at ACCI sampling of the same year . Soil potassium , Na, and CEC was higher for cover crop treatments at ABH sampling for 2008, relative to the fallow treatment . In 2009, the only time soil nutrient contents were visibly different for the cropping treatments was at the ACCI sampling and all soil nutrients were at the same level for all cropping treatments at other sampling periods of 2009. The pH of the soil for both study years ranged from 7.9 to 8.2 and was not different among the cropping treatments. In addition to the above nutrient types, soil NO3 showed unique responses based on cropping treatments . Soil NO3 was consistently higher for the cover crop treatments relative to the summer fallow, but not until after cover crop incorporation of2008. Soil NO3 level declined and was not different among the cropping treatments at ABH sampling of 2009 . In relative comparisons soil NO3 levels were higher in 2009 than in 2008. Soil SO4, and percent cation saturations were higher for the cover crop treatments, compared to the fallow, but not until 2009. Mn and B were higher in the fallow than in the cover cropped plots at ACCI.
Soil nutrient concentrations, particularly NO3 and SO4 were generally greater when the cover crop was a cowpea than marigold . Some, but not all of the soil nutrient enrichment from cover cropping is reflected in the nutrient uptake of the vegetable crop . As for the plant nutrients, higher N, S and K were detected in the shoots of broccoli grown on the summer cowpea plots compared to the fallow treatments. However, these nutrient increases were only in the 2009 crops, but not in 2008 . Other nutrients such as Mg, Ca, and Na showed higher levels in broccoli shoots that were grown on the summer cowpea plots as early as 2008 and also in the shoots of the 2009 crops. Al and B were also higher in broccoli shoots from the summer cowpea treatments, compared to the marigold and fallow treatments . In general broccoli benefitted from higher nutrient uptakes from treatments that had a summer cowpea than marigold and least when broccoli was grown on summer fallow plots. As with the soil and vegetable crop nutrient conditions, broccoli growth differed depending on the summer cropping systems. Broccoli grown on the summer cover crop plots were taller and had more vigorous growth than those on the summer fallow plots. During almost all sampling weeks, broccolis crops were consistently taller for the two cover crops than the fallow treatments. Consistent with nutrient status, crop height growth was highest for those from cowpea, followed by marigold and least for crops grown on the summer fallow . The increase in height of broccoli grown on the cover crops is more prominent after the third week of sampling for all study years and more pronounced for the 2008 crops. Broccoli canopy spread was similar to the crop‘s height responses in that broccoli on the summer cover crop treatments for all years had relatively broader canopy, but were most significant for the 2008 cropping year . The mean number of leaves per individual plant was also variable. Once again these crop growth parameters differed based on sampling years and cropping treatments . During all trial years, broccoli grown on the summer cowpea fields had relatively higher mean leaf numbers per plant than any other cropping treatments, particularly at about 8 weeks after broccoli transplant. Mean leaf number production and variation between cropping treatments were clearly visible for the 2008 and 2009 cropping seasons than for the 2007 broccoli . Broccoli shoot biomass determination from destructive crop sampling at harvest time showed that there was no significant broccoli shoot biomass gain from cover cropping for the first year rotation, although broccoli grown on the summer cowpea were relatively heavier than the other two cropping treatments . Vegetable crops of heavier shoot biomass from cover cropping were observed for 2008 and 2009. During those latter two years, broccoli shoot biomass was heavier for those from cowpea followed by marigold and least for crops grown on the summer fallow . The 2008 broccoli shoot biomass for crops from summer cowpea and marigold were about 43% and 23% higher, respectively than these grown following a summer fallow treatment. Although broccoli shoot biomass for 2009 was generally lighter than the 2008 crops, similar trends as for the 2008 crops was observed for treatment effects. Accordingly, broccoli grown on cowpea, followed by those on marigold had heavier biomass than crops grown on the summer fallow field.Finally, all effects of cropping treatments is expected to show the benefits of cover cropping with the evaluation of the marketable yields of the vegetable crop.