Lower population densities of common purslane and all weeds combined were observed at harvest time for cover crop treatments in 2009 compared to summer fallow . Among the cover crops, vegetable crops that had cowpea as a summer cover crop had fewer population of weeds than the marigold . Compared to the previous years, the lowest weed population densities were observed at any sampling date in 2009. There were no significant differences in the population densities of common purslane at the harvest time sampling of 2009 . Cowpea as a cover crop showed stronger weed suppression capabilities than marigold.Biomass accumulation of individual weeds was related to the specific weed population densities. During all years, common purslane attained the highest dry biomass accumulation depending on the cropping treatments. At early sampling of 2007, common purslane attained 100 and 36 times higher dry mass in the fallow , relative to the marigold or cowpea cover crops, respectively . Dry mass accumulation from all weeds combined for the early sampling were also reduced by 37 times when the summer cropping was either a marigold or a cowpea compared to the fallow treatment. Similar to the weed population densities, reduction in weed biomass was stronger for the broadleaves than on grass weeds . Weed biomass accumulation during the mid and harvest time samplings did not vary among the cropping treatments, drying rack for weed attributing the cover crops and initial hand weeding that might have already depleted weed seed banks and new weed germination.
Cover crop suppression of purslane biomass accumulation and all weed biomass accumulation was also observed for 2008. Cowpea and marigold as cover crops reduced purslane biomass by 6 and 20 times, respectively at the mid sampling compared to the same time sampling on a fallow treatment . Similarly, dry mass of all weeds at the mid and harvest time sampling were significantly lower for the cover crop compared to the Fallow treatment . Stronger suppression of on biomass accumulation of broad leaves than grasses is consistent for this year as well. The only exception from the first year trial was the suppression of the dry biomass of a grass weed, Eragrostis barrelieri, under the cover crop treatments at the second year harvest time sampling . Weed biomass was generally lower for 2009 than either 2007 or 2008, suggesting that repeated years of cover cropping rotations and hand weeding may provide increased weed suppression in the subsequent vegetable crop. Greater biomass accumulation in all weeds combined was observed in the fallow plots at early , mid and harvest time samplings compared to plots that had summer cover crops with stronger suppression on broad leaves than grasses weeds.The greenhouse weed seed germination tests showed poor responses for all three years. Even the most dominant weed, Portulaca oleracea germinated poorly or failed to germinate at all. Among these germinated, none or only very few weeds showed variation among cropping treatments for both 2007 and 2008 . Therefore, this portion of the research finding is inconclusive and has been omitted.The time required for the initial weeding in 2007 was not different among the cropping treatments , probably due that the cover crops did not provide efficient weed suppression at this initial stage of crop rotation.
Differences among cropping treatments on supplemental hand weeding duration appeared at the second hand weeding in 2007 and all other weeding periods of the subsequent years. Longer weeding hours were required for the initial weeding on the fallow plots for 2008 and 2009 compared to the cover cropped plots. The combined initial and second round time spent on hand weeding was higher in the fallow plots of 2007 , 2008 , and 2009 . At all hand weeding periods of all years, longer time was spent weeding in the marigold than in the cowpea treatment, showing stronger weed suppression of cowpea as cover crop than marigold. The relative total weeding time in the fallow plot to the time required in a cowpea plot was about twice in 2007, 2.5 times in 2008, and 2.8 times for 2009, indicating stronger reduction in labor with increasing years of cover cropping rotations.The three consecutive experimentation years revealed that common purslane was the most prevalent weed both in population density and biomass accumulation. Adler and Chase states that common purslane is a prolific seed producer that can rapidly colonize warm, moist sites. Either cowpea or marigold used as summer cover crop suppressed common purslane. While individual weeds of the other species were not responsive to the cover cropping treatments, the combined population density and biomass of all weeds was significanly reduced under the summer cover cropping treatments compared to the summer fallow. Hutchinson and McGiffen also observed sufficient levels of weed suppression with cover crop mulches in desert pepper production.Weed suppression with off-season cover cropping treatment was more robust when coupled with supplemental hand weeding, because post-weeding population and biomass accumulation of weeds were lower under the cover cropping treatments than the fallow.
Therefore summer cover cropping can provide long-term weed suppression, even after the establishment of the vegetable crop. The lower weed population and weed biomass during the subsequent sampling periods of all years relative to the early sampling stages reveal that supplemetal hand weeding is critical and reduces early stage weed pressure on vegetable crops. It also shows the importance of integrating cover cropping rotations with supplemental hand weeding for more efficient weed management. Weed suppression during the early growth of a vegetable crop is desirable as most crops suffer serious weed competition during their early growth stages. Eliminating or minimizing early stage crop-weed competition may help a crop to make vigorous growth, develop dense canopy faster and suppress emergence and growth of weeds in the subsequent crop growth season. A long-term weed supression and nitrogen contribution from foxtail millet [Setaria italica Beauv.] and cowpea [Vigna unguiculata ] cover cropping has been observed to produce greater total marketable yield of bulb onion . Consistent and prominent cover crop weed suppresion was observed against broadleaf than grass weeds, suggesting that cover cropping is more beneficial in agricultural fields dominated by broadleaf weeds. In a similar experiment, Wang et al. observed a higher suppression of broadleaf weeds than grasses using sun hemp mulches. While cover cropping consistently suppressed weeds during all years, pipp mobile storage systems the more cover cropping rotation , the stronger was the weed suppression. These findings suggest that cover cropping has incremental effects with increasing years of cover crop rotations. Among the cover crops used, cowpea, if used as a summer cover crop, could provide more weed suppression than if the cover crop was marigold. Many researchers showed that cowpea provided excellent suppression of weeds when used as an intercrop or organic mulch. The stronger weed suppression and hence less supplemental hand weeding time in cowpea cover crop is probably attributed to the nitrogen fixing ability and more nutrient supply potential, and enhancement of the subsequent vegetable crop growth and facilitating the vegetable crop to suppress weeds on its own at its subsequent growth stages. Enhanced soil quality is one of the reasons contributing to the suppression of weeds within the subsequent vegetable crops . The off-season cover cropping that leaves crop residues on soil surfaces increase soil N level and supress weeds in vegetable crops . Weed seed greenhouse germination was generally very poor and showed mixed responses among the different weed species. Some weed species germinated more in soil samples collected from the cover crop plots while others were relatively higher in the fallow treatment. Therefore, the effects of cover cropping for the portion of soil weed seed population densities are inconclusive and the ―potentially higher‖ weed seed bank hypothesis within a fallow summer cannot be confirmed.
The amount of soil used for germination tests may have not been sufficient enough or our greenhouse conditions might have not provided optimum condition for weed seed dormancy breaking. A further study is recommended to verify the potentials of cover cropping in reducing soil weed seed banks. The study on effects of cover cropping on supplemental hand weeding duration revealed that cover cropping reduce the time it may take for supplemental hand weeding. The reduction of supplemental hand weeding time within the cover crop treatments compared to the fallow treatment was consistent for all three years. The total amount of time needed during the whole crop growing season was almost double in the summer fallow plots compared to the summer cover cropped fields. Reducing supplemental weeding needs decreases production costs and provides higher economic return from vegetable crop production. Although Kumar et al. suggest that there is no clear mechanism by which cover crop residues may suppress weeds, there are many possible mechanisms listed for weed suppression using off-season by summer cover crops and their residues. Adler and Chase suggest that cover crop residues suppress weeds through its modification of soilmicroclimate and physical impedance of weed seed germination or serving as a physical barrier and inhibiting light penetration . Others suggest that the exclusion of light is an indirect mechanism by which cover crop residues may suppress weeds and that the actual mechanism is through the reduction of soil temperature fluctuations useful in breaking weed seed dormancy and germination. For example, in the absence of fluctuating soil temperature, Portulaca oleracea , Chenopodium album and Amaranthus retroflexus failed to germinate. Cover crop residues may also suppress weeds through stimulating or suppressing of soil microbial populations which deplete soil weed seed banks . The off-season cover crop stand may utilize water and nutrients that would otherwise be used by weed species and hence provides a mechanism by which off season cover crops could suppress weeds in the subsequent crops. Some cover crops inhibit weeds through allelopathy . If there were allelopathic effects from the cover crops used in this trial, the allelopathic compounds must have been more specific to the broadleaved weeds, because the cover crops used were more suppressants to broadleaves. Selective phytotoxicity of allelopathic compounds to broadleaf weeds has been discussed by Ercoli et al. 2007; Jelonkiewicz and Borowy 2005; Hill et al. 2007; Adler and Chase 2007. Benzoxazinoid compounds specific to rye cover crop mulches suppressed different sets of weed species . A proper and detailed understanding of allelopathic cover crops may aid scientist to develop effective biological weed control and reduce future reliance on synthetic herbicides . Although both marigold and cowpea significantly suppressed weed pressure when used as cover crops in a subsequent vegetable crop relative to the fallow summer, none of the cover crops provided a complete control of weeds without supplemental weed management options. Therefore cover crops may not be considered as the sole control of weeds, but as an integrated component and holistic approach of weed management options. The greater effectiveness of cover cropping as component of integrated weed management strategies was emphasized by Dıaz-Pe´rez et al. . Within the cover crops, the species that possesses rapid growth and large biomass production characteristics may provide more weed suppression. Finally, weed management using cover crops is ecologically friendly and if coupled with some traditional weed control methods could eliminate or reduce reliance on chemical weed control. In this respect, cover crops may be particularly appealing and useful for organic crop production systems where chemical weed management is not an option. Since the off-season cover crops are not grown simultaneously with the major crop, there would not be a resource competition threat as when the cover crops are inter-cropped with the main crop. To make efficient use of cover crops, growers must also identify the adaptability of cover crops to their local farm condition, the weed species and the economic considerations of agricultural systems. One must also confirm that the specific cover crops and residues could suppress diverse weed species with no or little interfere with the major crop. The USDA–AMS emphasizes that the screening of cover crops along various crop productions may fulfill the National Organic Regulations and Guidelines that require preventive measures, safe crop production practices and use of competitive crops as a first line of defense against weeds.Plant-parasitic nematodes cause severe damage to vegetable crops and cause an estimated annual yield loss of $77 billion to $125 billion worldwide .