Determining the minimum biologically effective dose of herbicides for weed control in corn-soybean rotations

Project Code: PRR06-380

Project Lead

Gilles Leroux - Laval University

Objective

To determine the weed management effects of rotating herbicide tolerantcorn and soybean cultivars and minimum biologically effective doses of glyphosate and glufosinate herbicides applied in rotation

Summary of Results

Background

With the advent of transgenic, herbicide-tolerant crops, glyphosate and glufosinate are increasingly being used in corn and soybean crops. Due to their effectiveness and broad-spectrum action, the use of these herbicides is a powerful weed management tool. When rotated, these herbicides which have different modes of action could prevent the development of resistance to glyphosate in weeds. Moreover, they have relatively low toxicity compared with herbicides commonly used in agricultural production, and the use of reduced doses may offer considerable advantages.

This project aimed at assessing the impact of reduced doses of herbicides applied in rotation at various crop alternations of two transgenic, herbicide-tolerant corn and soybean cultivars on weed control and yield. The goal was to establish an optimal weed control strategy with significant agronomic, environmental and economic benefits as a result of reduced herbicide use.

Methods

The trials were carried out at the experimental research farm of Laval University in Quebec over three years, in 2006, 2007 and 2008. The efficacy of two herbicides (glyphosate and glufosinate), each applied at four rates (0X, ½ X, ¾ X and 1X) relative to respective label rates (1X), was tested in two crops of corn and soybean cultivars resistant to glyphosate (Roundup Ready® [RR]) and glufosinate (Liberty Link® [LL]). A total of 32 treatments, including combinations of eight crop alternations of two cultivars, two herbicides and four application rates were established on the same plots over the three years of the study. The crop alternations included: 1) Corn RR- Corn RR- Corn RR; 2) Corn RR- Corn LL- Corn LL; 3) Corn RR- Soybean RR- Corn RR; 4) Corn LL - Corn LL - Corn LL; 5) Corn LL - Corn RR - Corn LL; 6) Corn LL - Soybean RR - Corn LL; 7) Soybean RR - Corn RR - Corn RR; 8) Soybean RR - Corn LL - Corn LL.

All herbicides were applied at post-emergence using a precision sprayer. Label rates of reference were 0.50 and 0.90 kilograms/hectare for glufosinate and glyphosate, respectively. Weed suppression was assessed by measuring the dry weight of aerial biomass of weeds collected from sample sections of 50x50 cm in each treated plot. Soil cores were collected and analyzed in each plot both in spring 2006 and after harvest in 2007 to determine the initial composition and changes occurring to the weed seed bank following the treatments. Seed yield was measured each year for both corn and soybean at maturity.

Results

The results show that glufosinate is less effective than glyphosate in controlling weeds. All crop alternations using glufosinate show about 30% lower levels of annual grass control than the control crop alternations. The same is true for lamb’s quarters and common ragweed, except with the Corn LL-Corn RR-Corn LL and Corn LL-Soybean RR-Corn LL.

Overall, the use of glufosinate leads to yield reductions as compared to glyphosate. With respect to rates used, crop alternations using glufosinate show much greater yield reduction at low rates. For crop alternations using glyphosate alone, the half- rate causes significant yield losses compared to the full rate, but the three-quarter rate does not impact yield.

From an agronomic perspective, the Corn LL- Corn RR- Corn LL and Corn LL-Soybean RR- Corn LL alternations provide the most viable solution for replacing the systems using glyphosate alone, unless a serious annual grass problem exists. Reducing the glyphosate rate to less than three-quarters is not feasible due to the adverse effects on weed control, crop yield and weed seed bank. Reducing the rate of glufosinate is not recommended.

The economic analysis confirms the benefit of the Corn LL- Corn RR- Corn LL alternation, even at a reduced glyphosate dose, but the Corn RR- Corn RR- Corn RR alternation at full rate remains the economic baseline. With respect to the environmental impact quotient, crop alternations using glufosinate alone are less advantageous in reducing pesticide risk than those using glyphosate alone.

In conclusion, alternating between glyphosate and glufosinate every other year appears to be the best compromise observed in this study. Reducing the rate appears possible in years when using glyphosate, but not when using glufosinate. However, it is important to always closely monitor the fields to observe any changes in weed population and seed bank dynamics.

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