Alternatives to preemergence herbicides

by Kendra Baumgartner, USDA-ARS, Department of Plant Pathology, Lissa Veilleux, UC SAREP, Department of Plant Pathology, and Janet C. “Jenny” Broome, UC SAREP

In August 2002, USDA ARS plant pathologist Kendra Baumgartner and UC SAREP Associate Director Janet C. “Jenny” Broome signed a two-year cooperative agreement to target funds toward research into the development of sustainable weed management for winegrapes. We are pleased to provide here a summary of some of the results of this cooperation.

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. Several factors, however, including emergence of resistant weed populations, have caused growers and researchers to question this growing dependence on herbicides (Liebman and Gallandt 1997). The California winegrape industry has taken a particular interest in finding alternatives to preemergence herbicides. One reason is the threat 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 the industry is interested in alternative methods is a trend toward more sustainable viticultural practices. In an effort to be more sustainable, some growers reduce their preemergence herbicide. Fear of ineffective weed control, however, is still the largest reason the majority of growers continue to use more conventional methods (Barberi 2002, Bond and Grundy 2001). Research into alternative methods can provide the evidence some growers require before they adopt more sustainable practices.

Warwick (1991) documented herbicide resistance in more than 100 species, highlighting the importance of exploring non-chemical weed control options. Shifts in weed populations, however, are not only associated with herbicides but with cultural practices as well (Liebman and 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 there is reliance 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 and Co., Wittlich, Germany), and MatranTM (EcoSmart Technologies, Inc.) (Baumgartner and 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.
Experimental Treatment	Practice	Date Appliedc	Rate Applied
1. Winter Roundup®/Spring Roundup®	Roundup®a Roundup®	22-02-03 22-05-03	2 qt/acre 4 qt/acre
2. Fall Cultivation/Spring Cultivation	Clemens Clemens	27-11-02 16-05-03	in-row cultivation in-row cultivation
3. Fall Cultivation/Spring MatranTM	Clemens Matran2TM b	27-11-02 22-05-03	in-row cultivation 4 gal/acre
4. Fall Cultivation/Spring Roundup®	Clemens Roundup®	27-11-02 22-05-03	in-row cultivation 4 qt/acre
5. Winter MatranTM/Spring MatranTM	Matran2TM Matran2TM	22-02-03 22-05-03	4 gal/acre 6 gal/acre
6. Cover Crop (Festuca idahoensis)	Clemens and Plant Mow	27-11-03 15-05-03	in-row cultivation, seeding Mow
7. Natural Vegetation	Mow Mow	18-04-03 15-05-03	Mow Mow
8. Spring Cultivation Only	Mow Clemens	15-05-03 16-05-03	Mow in-row cultivation

a Active ingredient: Glyphosate
b Active ingredient: Clove Oil
c Dates 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 MatranTM/Spring MatranTM, Spring cultivation, and the natural vegetation control. Intermediate were the three treatments with a fall cultivation. MatranTM was very effective against broadleaves, but was least effective against grasses. Although MatranTM did not control grasses, the dominant grass weed was the cover crop (Zorro fescue, Vulpia myuros var. hirsuta), a low stature grass. Given that MatranTM controlled the more problematic broadleaves, this treatment was still successful.

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.

Total Aboveground Biomass (g/m2) and SD
Winter Roundup/Spring Roundup
Fall Cultivation/Spring Cultivation
Fall Cultivation/Spring Matran
Fall Cultivation/Spring Roundup
Winter Matran/Spring Matran
Natural Vegetation
Spring Cultivation Only
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.

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 and 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.

For more information contact Kendra Baumgartner, USDA-ARS, Department of Plant Pathology, (530) 754-7461, kbaumgartner@ucdavis.edu.

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.