A comparison of surrogate and real weeds: Do surrogate weeds accurately reflect real weeds across several flex-tine harrows?

                    Johnny Sanchez and Eric Gallandt

Graduate Research Assistant in Ecology and Environmental Sciences, University of Maine, Orono, ME, USA; Professor of Weed Ecology, University of Maine, Orono, ME, USA

Interim report: February 26th, 2021

Take-home points:

  1. There was no difference between surrogates and real weeds in field cultivation susceptibility in 2019, while in 2020 real weeds were significantly less susceptible to cultivation than surrogate weeds
  2. Differences in efficacy results for each species were true for all tools
  3. Rates of efficacy across tools for artificial weeds did not reflect those of wild radish, but were similar to surrogate weeds
  4. Tiny Treffler out-performed all other included hand- and tractor-drawn tine harrows in both years

Tools for physical weed control are highly heterogenous in design, adjustment, and effectiveness (Gallandt et al. 2018). Moreover, the effectiveness of mechanical weeding implements can be considerably affected by variations in soil conditions, weed growth stage, weed density, and weed population (Duerinckx et al. 2005; Mohler 2001; Rasmussen et al. 2008). A better understanding of how specific design characteristics interact in real-world contexts, is important for the development of improved cultivation tools (Kurstjens and Perdok 2000).

Surrogate weeds are often a key component of studies of physical weed control (Gallandt 2010; Kolb and Gallandt 2012; Melander and McCollough 2020). Surrogate weeds are crop species, closely related to common agricultural weeds, that can respond similarly to weed control tactics (McCollough et al. 2020). Bred as crops, surrogate weeds lack complications such as complex patterns of dormancy (Malik et al. 2010), variable stands (Myers et al. 2005), and intricate emergence patterns (Egley and Williams 1991) often found among real weeds. The objective of this study was to evaluate the ability of broad leaf, cruciferous surrogate weeds to accurately reflect the cultivation susceptibility of related real weeds across six different flex-tine harrows with varying designs and degrees of cultivation aggressiveness.

Four species of commonly used surrogate weeds, including condiment mustards (Guillenia flavescens L.), (Brassica juncea L.), and (Sinapis alba L.) and canola (Brassica napus L.), were broadcast at a rate of 1,500 seeds m-2 and raked into the soil to simulate a stand of broadleaf weeds (Brown and Gallandt 2018; McCollough et al. 2020; Olsen et al. 2005). To ensure a sufficient stand of weeds, wild radish (Raphanus raphanistrum L.) was also sown in each plot at a target density of at least 50 plants per m^-2 (Gerhards et al 2020; Vanhala 2004). In 2020, 35 mm long wooden golf tees (Golf Tees Etc.) were also included (Kshetri et al. 2019).

To include a wide breadth of designs, six flex-tine harrows were used in this study: Lely tines on a Williams tool bar (Lely, Pella, IA), Tiny Treffler (Man@Machine, Netherlands), Johnny’s Selected Seeds Tine Weeding Rake (Johnny’s Selected Seeds, Fairfield, ME), Two Bad Cats Tine Weeder (Two Bad Cats LLC., North Clarendon, VT), Terrateck Light Tines (Terrateck, France), Terrateck Double Wheelhoe with tines (Terrateck, France). Weed control efficacy and crop mortality were determined by the percentage of plants killed in the permanent subplots (Evans et al. 2012; Kolb et al. 2010).

There was no difference between surrogates and real weeds in field cultivation susceptibility in 2019, while in 2020 real weeds were significantly less susceptible to cultivation than surrogate weeds (Figure 1). Differences in efficacy results for each species were true for all tools. Rates of efficacy across tools for golf tees did not reflect those of wild radish but were similar to surrogate weeds. The Tiny Treffler out-performed all hand- and tractor-drawn tine harrows in both years (Figure 2).

As the two years of this study had contrary outcomes, concerning the accuracy of the reflection of wild radish by cruciferous surrogate weeds, the results of this study are largely inconclusive. While adverse weather conditions and different soil compositions may have led to the overall lower rates of weed control in 2020, the flex-tine harrows performed in nearly the same rank orders, suggesting that these factors did not affect the mean efficacies of each species relative to each other. Despite the extensive use of surrogate weeds in physical weed control research, as McCollough et al. (2020) note, there is simply a dearth of research comparing the competitive ability and response to weed control in the literature. Furthermore, inconclusive results suggest a need for increased thoughtfulness when selecting surrogate weed species and may advocate for similar studies to be conducted using other commonly used surrogate weed species.

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Figure 2.