While the potential for yield and profitability is strong, a major limiting factor to adopting reduced tillage in corn production is the concern of less-effective weed control. Because adequate nitrogen availability is essential for corn development, use a legume cover crop that provides both weed control and nitrogen fixation, such as hairy vetch, red clover or medics. Use a burndown herbicide prior to corn planting for early-season weed control when using cover crops. To broaden the number of weed species controlled as well as to extend control into the season, apply a residual herbicide in conjunction with the herbicide used for cover crop termination. A number of pre-emergence herbicides are available that can be applied without incorporation into the soil and that are effective even with plant residue on the soil surface. These herbicides and post-emergence herbicide choices that can be successfully utilized in conservation-tillage corn with cover crops are listed in Table 11.4.
When glyphosate-resistant cotton was made available, reduced tillage became practical since a broad spectrum of weed species could be controlled with a single herbicide. Extensive research on conservation-tillage cotton has demonstrated yield benefits. Moreover, with herbicide-resistant cotton varieties, weed control has been as successful as conventional-tillage cotton. Because of this success, conservation tillage has been widely adopted in the Southeast. This dependence on a single herbicide, however, has led to the appearance of herbicide-resistant weed species that now threaten the feasibility of reduced-tillage cotton production. Currently, research efforts focus on identifying ways to ensure the long-term viability of conservation tillage while controlling established populations of herbicide-resistant weed species and reducing the risk of future development of resistant weeds. Cover crops, along with multiple herbicide modes of action and rotation, have been shown to effectively control weeds in reduced-tillage cotton. Pre-emergent herbicides are especially important in early-season weed control to ensure management of weed species that are difficult to control later in the season. See Table 11.5 for a number of herbicide choices available for use with conservation-tillage cotton.
The vast majority of soybeans in the United States are produced with conservation tillage. This can be attributed to the environmental and economic benefits achieved with reduced-tillage as well as to the commercial availability of herbicide-tolerant soybeans, which have made successful chemical weed control achievable with the use of fewer herbicides. Studies of conservation-tillage soybeans have reported equal or improved yield compared to conventional systems. Studies of soybean systems planted behind wheat or a cover crop such as rye have noted improved weed control compared to fallow and greater yield with a cover crop than with just the previous crop’s stubble. Table 11.6 provides a partial list of herbicides that can be utilized in reduced-tillage soybeans with cover crops.
Concerns over the peanut’s response to reduced tillage, due to its growth habits, have prompted studies to identify successful means of using conservation tillage for peanut production. Inconsistent peanut yield in conservation tillage systems has been reported. Some studies have reported that yields of conservation-tillage peanuts are reduced or equal to conventionally tilled peanuts, while others have reported equal or greater yields in a conservation tillage system. Weed control in peanuts, regardless of the tillage system, can be problematic due to the extended growing season and the crop’s unique growth habits. Generally, peanut production requires an incorporated residual as well as a post-emergent herbicide to provide effective weed control under the slow-closing canopy of peanuts. Moreover, in-season cultivation for weed management cannot be implemented due to the potential to damage developing peanut pods. Studies have shown effective weed control with cover crops in strip-tillage peanut systems that use a dinitroaniline pre-emergent herbicide over cover crop residue. Other effective herbicides used in conservation-tillage peanut systems are listed in Table 11.7.
Much research has been conducted to evaluate wheat productivity in conservation tillage practices. Reports reveal similar or increased grain yield for reduced-tillage compared to conventional tillage systems. With little or no tillage operations, some chemical applications are required in order to achieve successful levels of weed control; however, with herbicide applications, weed species have been effectively controlled below levels that could reduce yield. To offset the herbicide needs in conservation tillage, evaluations of cover crops as ground cover have been conducted. Cover crops such as mustard, peas and lentils have proven to be good choices with little yield differences. Table 11.8 lists many of the herbicide options for use in conservation tillage systems for wheat production.