Alternatives to Preemergent Herbicides

Dr. Kendra Baumgartner, USDA-ARS, Department of Plant Pathology, University of California, One Shields Avenue, Davis, CA 95616, (530) 754-7461, kbaumgartner@ucdavis.edu; Lissa Veilleux, UC-SAREP, Department of Plant Pathology, University of California, One Shields Avenue, Davis, CA 95616, (530) 752-6745, lmveilleux@ucdavis.edu

For the past several decades, advances in weed management were largely due to advances in herbicide technology, which has undoubtedly led to improvements in crop productivity and farm labor efficiency. However, several factors, such as emergence of resistant weed populations, have growers and researchers questioning this growing dependence on herbicides (Leibman and Gallandt, 1997). The California winegrape industry has taken a particular interest in finding alternatives to preemergence herbicides. One reason is the threat that preemergence herbicides pose to water quality. Preemergence herbicides have been found to contaminate both surface and ground waters due to their persistence in the soil (Pimentel et al., 1992). Growing concerns over water quality and impending legislation may make some preemergence herbicides unavailable. Alternatives may soon be a necessity. Another reason that the industry is interested in alternative methods is a current trend toward more sustainable viticultural practices. Some growers reduce their preemergence herbicide use in the name of sustainability. However, fear of ineffective weed control is still the largest reason the majority of growers cling to more conventional methods (Barberi, 2002; Bond & Grundy, 2001). Research into alternative methods can provide the evidence that some growers require before adopting more sustainable practices.

Warwick (1991) documented herbicide resistance in more than 100 species, highlighting the importance of exploring non-chemical weed control options. However, shifts in weed populations are not only associated with herbicides, but with cultural practices as well (Leibman & Gallandt, 1997). For this reason, it is important to combine weed management strategies in order to avoid shifts toward weed species that become difficult to manage when relying on a single weed control practice.

Our research examines several alternative weed control practices used alone and in combination. We address two very different, yet equally important, aspects of using these practices. First, we address their effects on weed establishment. Second, we address their effects on vine yield, growth, and nutrition. An understanding of both aspects is important to growers who desire effective weed control practices that do not adversely affect yields.

In 2002 and 2003, we examined three weed control practices: glyphosate (Roundup^®, Monsanto), in-row soil cultivation with a Radius Weeder^® (Clemens & Co., Wittlich, Germany), and Matran^TM (EcoSmart Technologies, Inc.) (Baumgartner & Veilleux, 2004). All three were tested alone and in combination to give a total of seven experimental treatments, including a non-treated control ("natural vegetation") (Table 1). An eighth treatment, an in-row cover crop, was established, but data was not included in the analysis due to poor cover crop establishment.

Table 1: Weed control practices associated with experimental weed control treatments.

^aActive ingredient: Glyphosate
^bActive ingredient: Clove Oil
^cDates presented are from applications made in the first year of the study, Fall 2002 to Summer 2003.

Preliminary results from this research are encouraging. We detected significant differences among treatments after only one year of applications (Figure 1). The most effective weed control treatment, based on total weed biomass, was Winter Roundup^®/Spring Roundup^®. Least effective were Winter Matran^TM/Spring Matran^TM, Spring cultivation, and the natural vegetation control. Intermediate were the three treatments with a Fall cultivation. Matran^TM was very effective against broadleaves, but was least effective against grasses. Although Matran^TM did not control grasses, the dominant grass weed was the cover crop (Zorro fescue, Vulpia myuros var. hirsuta), a low stature grass. Given that Matran^TM controlled the more problematic broadleaves, this treatment was still successful.

Figure 1. Effect of weed management on total aboveground biomass of weeds. Columns with the same letter are not significantly different at P ≤ 0.05, Tukey’s test.

Although there were differences in weed control efficacy among treatments, there were no effects on grapevine yield or growth in the first year of this study. In analyzing grapevine nutrition, we found significantly lower potassium levels, although not low enough to be considered deficient, in the three treatments with a Fall cultivation.

We will continue this research in the 2004/2005 growing season. Weed studies are best conducted over several years in order to truly understand how weed populations are adapting to specific practices. Some practices that seem effective in the short-term may lead to shifts in weed populations that are difficult to manage in the long-term. For example, in the first year of our study, we found that the Winter Roundup^®/Spring Roundup^® treatment had the lowest weed biomass, but it also had a significantly lower weed diversity index, as did the other treatment with Roundup^® (P = 0.001; data not shown). In fact, there were only two weeds present in these treatments, Zorro fescue (Vulpia myuros var. hirsuta) and panicle willowweed (Epilobium brachycarpum). Continued monitoring is important in order to determine if these two weeds become increasingly difficult to control with Roundup^® at this site.

More than one year of research is also necessary to assess the full impact of the practices on vine yield, growth, and nutrition. As previously mentioned, there were significant effects of weed management on petiole potassium concentrations; all treatments with a Fall cultivation showed lower levels of petiole potassium. We know that grapevine fine roots are more abundant closest to the vine trunk (Cheng & Baumgartner, unpublished data). It is likely that in-row cultivation severs these fine roots, cutting off an important source of nutrient uptake. Continual monitoring is important in order to determine if other nutrients will eventually be affected by in-row cultivation.

Literature Cited

Barberi, P. 2002. Weed management in organic agriculture: are we addressing the right issues? Weed Research 42: 177-193.

Baumgartner, K; Veilleux, L. Alternatives to pre-emergence herbicides in North Coast Vineyards. Proceedings of the 56th Annual Conference of the California Weed Science Society. Sacramento, CA. January 13, 2004. (accepted for publication, 1/20/04).

Bond, W; Grundy, AC. 2001. Non-chemical weed management in organic farming systems. Weed Research 41: 383-45.

Cheng, X; Baumgartner, K. 2003. Root system distribution and arbuscular mycorrhizal colonization of grapevines in a California vineyard. 9th International Soil Science Society Conference Abstracts: 30.

Gross, KL. 1990. A comparison of methods for estimating seed numbers in the soil. Journal of Ecology 78: 1079-93.

Liebman, M; Gallandt, ER. 1997. Ecological management of crop-weed interactions. In Ecology in Agriculture. LE Jackson, ed. San Diego, CA, USA. p 291-343.

Pimentel, D; Aacquay, H; Biltonen, M; Rice, P; Silva, M; Nelson, J; Lipner, V; Giordano, S; Horowitz, A; D’Amore, M. 1992. Environmental and economic costs of pesticide use. Bioscience 42: 750-761.

Warwick, SI. 1991. Herbicide resistance in weedy plants: Physiology and population biology. Annual Review of Ecological Systems 22:95-114.