The growth benefits in height, canopy formation and leaf numbers per plant is a reflection of the nutrient use efficiency from the cover cropping treatments. In contrast, some previous researchers have observed higher mean number of leaves and heavier stem dry weight from a bare soil than when a rye cover crop used suggesting that rye cover cropping treatments resulted in broccoli marketable yield losses. The negative consequences from cover cropping may have been from cover crop vegetable intercropping, hence live competition for available resources. In this experiment I did not observe any negative consequences of cover cropping on any of the three year growth or yield components of the subsequent vegetable crop. 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. The observation once again suggests that a single year cover cropping rotation is not sufficient enough to benefit dry mass accumulation by a subsequent vegetable crop. Shoot biomass gain from cover cropping of the latter years was consistent with the observation of increased soil and crop nutrition. The increase in crop biomass with increasing years of cover cropping reveals that repeated cover cropping results in the buildup of cover crop effects. As for crop shoot dry biomass, vertical cannabis broccoli marketable yields were not significantly different between the cover cropping and fallow treatments for the first year cropping. Such responses were seen in almost all broccoli growth parameters and suggest that a one-year cover cropping rotation is of no net and ultimate benefit to broccoli. Increase in marketable yield from cover cropping was significant in the subsequent study years.
Similar to the higher marketable yield observed from cover crop residue supplemented tomatoes , I observed vigorous growth, higher shoot biomass accumulation and higher marketable yield of cover crop residue supplemented broccoli.In contrast, Hoyt observed a reduction in yield of broccoli planted into desiccated barley cover crop and attributed it to lower soil temperatures in the cover crop treatment. The reduction of soil temperature with the use of cover crop mulches and residues has been discussed as a possible limitation delaying crop harvest for several days . However, I observed that broccoli crops grown following summer cover cropping were heavier and had vigorous crop appearances compared these on a fallow field. These benefits however were more eminent after the second year of cover cropping rotations, indicating a buildup effect of the cover crops. Broccoli seems to have benefited from previous summer cover cropping during its second and third year trials. Higher fresh broccoli marketable heads were obtained during the second year from both harvest times and the total marketable heads. An increase in marketable yields starting the second year indicates the necessity of repeated cover cropping rotations to be beneficial. The increase in both marketable head numbers and fresh weights of the marketable heads occurred during the third year, further revealing the importance of longer and repetitive cover cropping rotation. The generally lower yield for the 2009 trial, relative the previous year, however was due to crop damage by other herbivorous pests and unexpected flowering, reducing number and fresh weights of marketable broccoli heads. Yet relative yield comparisons were valid.The more inclusion of the cover crops in the cropping rotations, the higher was the crop yield benefit both in number and weights of marketable broccoli yield. These findings confirm the recommendations that vegetable farmers can grow cover crops during the off-season and benefit from the harvest of the subsequent crop.
Ngouajio et al. suggests that cover crops can be used in diverse cropping conditions as they are compatible with both organic and conventional farming practices by either incorporating or using them as surface mulches. Improvements in soil physical, chemical, and biological environment from the use of cover crops are the reasons for the improved yields of subsequent crops, although crop yields may vary from crop to crop and agroecological regions. The positive response of the subsequently grown crop is also attributed to the transfer of nutrients from cover cropping and less immobilization nutrients . Similar to our findings, Hively and Cox ; Fageria et al. 2005 observed a higher corn yield following white clover and red clover cover crops. Marketable yield of sweet corn was approximately doubled by hairy vetch in 2 of 3 years compared to an unfertilized, no cover crop control . Burket et al. observed a 58% higher average broccoli yield when grown with no fertilizer N, but following a legume cover crop. In general, the response of broccoli as a vegetable crop to cover cropping rotations was positive associated with nutrient, growth and yield output of the crop. If properly managed, then it is most likely that the cover cropping system can sponsor its own soil fertility, crop protection and productivity. Such low input farming systems with improved crop productivity and profitability can be easily adopted by farmers and becomes very useful in organic farming systems where the use of synthetic fertilizers in not acceptable. Cover crops in farming systems improve soil health, reduce environmental pollution, and improve crop yields and maintain sustainability of crop production . Such sustainable production of agricultural products achievable through cover cropping must be based on holistic agricultural management that encourages interdependent and diverse properties. For higher cover crop use efficiency farmers should also deal with selection of appropriate cover crop species with desirable socioeconomic considerations and ultimate vegetable crop yield improvement. It must also involve lower production costs with no adverse effect on crop health and the environment.Management of weeds is a challenge in crop production.
Weeds can interfere with the cultivated crop by competing for light, water and nutrients, which can lead to reduced yields and reduced economic return on investment to the grower . The approach for integrated management of weeds consists of combined inputs from cultural, mechanical, biological and chemical control methods. Cultural practices like clean seed, clean equipment, and proper field preparation are commonly integrated with mechanical practices like tillage, mowing or cultivation for control of weeds. Biological practices are less common in weed management. Therefore, chemical control is the following option to integrate for weed management . Chemical practices are the use of herbicides to prevent weed emergence or to cease growth of weeds until plant death, in most cases. Herbicides continue to be important tools to integrate in weed management programs because of their cost effectiveness, rapid action and flexibility with management, when used appropriately, which have allowed for increased crop yields to be achieved . A successful weed management program can be accomplished when cultural, mechanical and chemical management are integrated. In the California rice production system, herbicide resistance has been a continuing challenge due to continuous rice cultivation year after year, a historically limited number of herbicides available and the overuse of the available herbicides for weed control . From 2015 to 2021, there were 661 suspected herbicide-resistant weed reports and nearly 53% of watergrass populations recorded multiple-resistance to up to four modes of action . The presence of herbicide-resistant weeds leads to a reduction in weed control with the available herbicides and reduced yield. The most recent herbicides registered in California rice include pyraclonil in 2024, florpyrauxifen-benzyl in 2023, benzobicylcon in 2017, carfentrazone in 2006 and clomazone in 2004 . However, grow racks these herbicides have varying degrees of control over different weed species and producers are limited in control options . There is a need for new herbicide tools to maintain the viability of the current herbicides for future years by practicing herbicide rotations and mixtures . However, the registration of new modes of action in a crop or region is influenced by many factors like the crop injury potential, weed control efficacy, environmental concerns or lack of economic incentive by the manufacturing companies . Because not many new herbicide modes of action have been developed recently and herbicide resistance is increasing, new potential rice herbicides can be evaluated from other cropping systems or by revaluating or reformulating older chemistries. There has been success in introducing herbicides from larger agronomic crops to high value specialty crops through the Interregional Project Number 4, a US federal program . Similarly, evaluating older chemistries for new crops can be successful; however, the environmental effects are of greater concern because old chemistries tend to be less environmentally safe . Various characteristics are important to consider when evaluating a potential herbicide for a new crop like crop safety, weed control spectrum and persistence in the environment.
To ensure a greater potential for success when evaluating new herbicides, a hypothesis-driven research approach should be taken. Pendimethalin [N–2,6-dinitro-3,4-xylidine] is a mitotic inhibiting herbicide from the dinitroaniline chemistry that inhibits seedling growth shortly after germination . Pendimethalin controlled herbicide-resistant grass populations in the greenhouse and has relatively few reports of resistant weed populations . Preliminary greenhouse work indicated pendimethalin was effective in controlling several recently collected herbicide-resistant grasses from California rice fields . Therefore, pendimethalin could be a valuable addition for management of herbicide-resistant weedy grasses. Pendimethalin is registered for use in dryseeded rice and commonly applied to the soil surface after drill-seeding rice relatively deep in the soil . In dry-seeded systems; however, rice injury from pendimethalin is influenced by soil moisture, where higher soil moisture leads to greater injury levels . Characterization of pendimethalin in water-seeded rice, where moisture is always present, has not been evaluated because of the perceived risk of rice injury . There is no previous research that has evaluated pendimethalin formulations at different rates and timings in water-seeded rice. Therefore, the objective of these studies was to evaluate and characterize pendimethalin in water-seeded rice. The objective of this chapter is to review the literature on pendimethalin, pendimethalin use in rice production systems and background related to characterizing pendimethalin for water seeded rice. This review will provide greater background to the research studies outlined in the following dissertation chapters. The review will begin with a history and background of the dinitroaniline chemical family, pendimethalin use in rice, environmental fate of pendimethalin in rice production systems, and future directions for characterization in water-seeded rice.The dinitroaniline chemistries have been available herbicides since the 1960’s. The first dinitroanilines were synthesized by the Eli Lilly Research Laboratories and included trifluralin, benefin, nitralin, isopropalin, oryzalin, profluralin, butralin, ethalfluralin, fluchloralin andprosulfalin . Trifluralin was the first commercialized compound in the US and was used in soybean and cotton as a pre-plant incorporation for grass weed control at rates of 1,000 to 2,000 g ai ha-1 . Currently, trifluralin and pendimethalin are the most commonly used dinitroanilines in the US and worldwide for weed control in cereals, cotton, soybeans, vegetables, ornamentals and fruit and nut trees . Pendimethalin was developed by American Cyanamid in the 1970’s, previously named penoxalin . BASF would later purchase the American Cyanamid’s agrichemical business and take possession of pendimethalin in the 2000’s. Pendimethalin was moderately less volatile than trifluralin, which lead to relatively greater soil persistence and longer weed control activity .Characteristics of Dinitroanilines. The physical, chemical and biological properties of herbicides can help broadly predict their behavior in the environment, weed control efficacy and handler safety. The 2,6-dinitroaniline chemical structure is the base structure that defines the compoundsin the dinitroaniline chemical group . The additional chemical structures on the base structures will affect the specific characteristics of each compound. The common characteristics in all dinitroaniline compounds is a low water solubility, typically <1 parts per million , and most are soluble in organic solvents. The nitro groups decrease the water solubility by creating hydrogen bonds with alkyl groups of other compounds like soil or organic sediments, which creates lipophilic aggregates . The lipophilic nature appears to make the compounds susceptible to bioaccumulation in the environment; however, they have a high affinity for organic sediments or organic matter in the soil . Most compounds are non-ionizable, except for oryzalin . These characteristics tend to lead to the classification of dinitroanilines including pendimethalin as low risk for contaminating surface or ground water and low risk to environment contamination and human health .