Control of Common Chickweed in Winter Wheat with Postemergence Herbicides

A total of six experiments were conducted over a two-year period (2018, 2019) at the University of Guelph Ridgetown Campus to assess the efficacy of various herbicides applied postemergence (POST) for the control of common chickweed in winter wheat. Fluroxypyr/bromoxynil/MCPA, pyrasulfotole/bromoxynil, pyrasulfotole/bromoxynil/fluroxypyr, pyrasulfotole/bromoxynil/thiencarbazone, pyrasulfotole/bromoxynil/thiencarbazone + MCPA ester, tolpyralate and flu-roxypyr/halauxifen + MCPA EHE, applied POST, controlled common chickweed only 5% - 42% at 2 weeks after treatment (WAT) and 1% - 23% at 4 WAT and resulted in common chickweed density and biomass that was similar to non-treated weedy control. Fluroxypyr/halauxifen + pyroxsulam + MCPA EHE, applied POST, provided 57% - 82% control of common chickweed and reduced density 93% and biomass 98% compared to the non-treated control. Thifensulfuron/tribenuron, thifensulfuron/tribenuron + MCPA ester, thifensulfuron/tribenuron + fluroxypyr + MCPA ester, tribenuron + thiencarbazone, and tribenuron + thiencarbazone + MCPA ester, applied POST, controlled common chickweed 98% - 100% and reduced common chickweed density 96% - 98% and common chickweed biomass 99%. Based on these results, herbicide treatments which contained tribenuron including thifensulfuron/tribenuron, thifensulfuron/tribenuron + MCPA ester, thifen-sulfuron/tribenuron + fluroxypyr + MCPA ester, tribenuron + thiencarbazone, and tribenuron + thiencarbazone + MCPA ester were the most effica-cious for the control of common chickweed in wheat. In addition, flurox-ypyr/halauxifen + pyroxsulam + MCPA EHE, applied POST, can provide adequate control of common chickweed in winter wheat.


Introduction
Winter wheat (Triticum aestivum L.) is one of the most widely grown cereals in Ontario [1]. In 2019, over 2,000,000 tonnes of winter wheat was produced from 406,000 ha in Ontario with a value of nearly $450,000,000 [1]. Winter wheat production management practices have evolved over the years. In the past, Ontario winter wheat producers used conventional tillage to prepare the seedbed for winter wheat, however, in recent years many growers have changed their cropping system to no/reduced-tillage cropping practices.
No-tillage cropping production system enables producers to directly seed into standing stubble from the previous crop early in the fall, which advances the seeding date resulting in increased plant stand, winter survival and yield, thereby increasing net returns of winter wheat [2] [3] [4]. No-till practices will also improve soil physical structure, soil microbial biodiversity, and water drainage while reducing soil losses due to wind and water erosion [2] [3] [4]. Increased adoption of no-tillage cropping management system by winter wheat producers coupled with reduced usage of soil-applied residual herbicides in soybean has resulted in an increase in winter annual weeds, most importantly common chickweed (Stellaria media [L]. Vill.).
Common chickweed is a creeping annual or winter annual weed from the Caryophyllaceae family that can infest many habitats from lawns to croplands [5] [6]. The common name "common chickweed" originates from its use as bird feed for young chicks [5]. Common chickweed reproduces mainly through seeds or through leafy stems which root at the nodes allows it to spread horizontally [7]. Common chickweed stems can be prostrate, semi-erect or nearly upright, they are much branched and can be up to 50 cm in length. The stem is smooth with swollen nodes except for a line of hair about 1 mm wide which alternates from one side to the other at each node. Common chickweed leaves are opposite, the leaves are stalked near the base and stalkless at the distal ends of the stems, the leaves are small, pale green, oval-shaped with pointed tips. Common chickweed flowers are at the end of the stems and in angles between branches, the flowers have five, small (3 -4 mm), lobed, star-like white petals [8] [9]. Common chickweed generally germinates in the fall or late winter through seeds and flowers in the spring and summer before seed release and dying as the temperature rises [8] [9]. A single common chickweed plant can produce up to 20,000 seeds which can germinate and emerge from depths of up to 2 cm [7].
Common chickweed can grow profusely in poor, compacted and moist soils, and under shady areas below taller plants such as winter wheat [8]. It is a very competitive weed with the winter wheat crop and can cause significant losses in seed quality and yield. Farahbakhsh et al. [10] reported up to 32% yield loss in winter wheat due to common chickweed interference at densities of 20, 40, 80 and 160 plants•m −2 . Vrabel [11] reported a potential net monetary loss of $32.75 to $51.87 ha −1 when common chickweed was not adequately controlled in winter wheat. Common chickweed can also release water-soluble phenolic biochemicals that can cause allelopathic interference to seedling growth of wheat [6]. Common chickweed has also been shown to be a host for non-beneficial insects and virus diseases of crops that can be conveyed through its seed [7] [9] [12]. Therefore, it is critical for winter wheat growers to adequately control common chickweed within their production system to be competitive in the global marketplace. Thifensulfuron/tribenuron, thifensulfuron/tribenuron + MCPA ester, thifensulfuron/tribenuron + fluroxypyr + MCPA ester, fluroxypyr/bromoxynil/MCPA, pyrasulfotole/bromoxynil, pyrasulfotole/bromoxynil/fluroxypyr, pyrasulfotole/bromoxynil/thiencarbazone, pyrasulfotole/bromoxynil/thiencarbazone + MCPA ester, tribenuron + thiencarbazone, tribenuron + thiencarbazone + MCPA ester, tolpyralate, fluroxypyr/halauxifen + MCPA EHE, and fluroxypyr/halauxifen + pyroxsulam + MCPA EHE are postemergence (POST) herbicides that have potential to control common chickweed in winter wheat [13]. There is limited efficacy data on these POST herbicides for the control of common chickweed under Ontario environmental conditions.
Experiments were arranged in a randomized complete block design with three replications. Treatments included a weedy control and thifensulfuron/tribenuron, thifensulfuron/tribenuron + MCPA ester, thifensulfuron/tribenuron + fluroxypyr + MCPA ester, fluroxypyr/bromoxynil/ MCPA, pyrasulfotole/bromoxynil, pyrasulfotole/bromoxynil/fluroxypyr, pyrasulfotole/bromoxynil/thiencarbazone, pyrasulfotole/bromoxynil/thiencarbazone + MCPA ester, tribenuron + thiencarbazone, tribenuron + thiencarbazone + MCPA ester, tolpyralate, fluroxypyr/halauxifen + MCPA EHE, and fluroxypyr/halauxifen + pyroxsulam + MCPA EHE, applied POST, at rates listed in Table 1 Data were analyzed using Proc GLIMMIX in SAS [15]. Herbicide treatment was the fixed effect, and environment (site-year), replicate within the environment and the treatment by environment interaction were random effects. Data from all six environments were pooled for analysis to be able to identify the most effective herbicide treatments for common chickweed control under Ontario environmental conditions. The best distribution was chosen by comparing residual plots, Pearson chi-square/df, and the Shapiro-Wilk statistic from among the appropriate potential distributions for each variable. Visual estimates of common chickweed control were arcsine square-root transformed prior to analysis with the Gaussian distribution. Common chickweed density and dry biomass were analyzed using the lognormal distribution. The non-treated weedy control, assigned a value of zero, was excluded from the analysis but treatment means were still compared to the value zero using the P-value included in the LSMEAN output. Pairwise comparisons were subjected to Tukey's adjustment prior to determining treatment differences at a significance level of 0.05. Treatment means were back-transformed for presentation and a correction for log bias was applied to chickweed density and dry biomass means.

Results and Discussion
At 1, 2 and 4 WAT, there was no visible winter wheat injury from the herbicide treatments evaluated (data not presented).