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Underground Herbicides

This article written by Dr. Reynold Bergen, BCRC Science Director, originally appeared in the June 2021 issue of Canadian Cattlemen magazine and is reprinted on the BCRC Blog with permission of the publisher.

When I was a kid, my dad found Russian knapweed in a pasture along an irrigation canal. He explained that it was important to catch this weed quickly, because it can spread very aggressively. Russian knapweed reproduces using seeds as well as by buds growing from its roots (somewhat similar to the sod-forming grasses in last month’s column). But Russian knapweed roots also release a chemical that weakens other plants, like an underground herbicide. This superpower is called “allelopathy”, and helps knapweed establish itself and spread. His explanation was much more interesting than pulling out all the plants later that day.

Western wheatgrass

Weeds aren’t the only plants that can do this. While collecting field data for a project aimed at developing new tame and native varieties and mixtures, researchers at Agriculture and Agri-Food Canada’s Swift Current Research Station and the University of Saskatchewan noticed that there seemed to be fewer weeds in plots that contained western wheatgrass (on its own or in mixtures). This led them to conduct a greenhouse study to learn whether forage plants can also make their own herbicides. These results were published in 2017 (The potential of seven native North American forage species to suppress weeds through allelopathy; Canadian Journal of Plant Science

What They Did:

They seeded five native grasses (western wheatgrass, bluebunch wheatgrass, nodding brome, little bluestem and sideoats grama) and two native legumes (purple and white prairie clover) in individual pots. Each pot contained only one forage species. The pots were shaped like ice cream cones with a small hole in the bottom so that excess water could be collected. That water was used to irrigate different pots containing dandelions, foxtail barley or scentless chamomile every day for a month. The same forage pots were used for three experiments. The first experiment used water collected from three-month-old forage plants that had never been clipped. The second experiment was done two months later after the plants had been cut to an inch in height, just before they flowered. In these two experiments, each weed pot received water from a single kind of forage. In the third experiment, the forage plants were allowed to regrow, then cut to an inch in height again, just before flowering. This time, each type of weed received water from a single kind of forage, as well as each possible multi-forage combination. Some weed pots also received tap water in each experiment. Weed shoots and roots were dried and weighed and compared among the different forage, forage mixture or control treatments.

What They Learned:

These native forages did fight back against weeds. In the first experiment, water collected from each of the young, unclipped forage plants was able to significantly reduce the shoot weights of weeds compared to tap water (dandelions were set back by 33-53%, foxtail barley 14 to 46% and scentless chamomile 38 to 55%). Each forage also reduced weed root weight. Some forages were able to set back dandelion and foxtail barley root weights to a greater degree than weed shoot weight. Scentless chamomile root weights were set back the same as shoot weights (or sometimes less), regardless of the native forage.

Water collected from older forages that had been clipped once also significantly set back the shoot weights of dandelions (33 to 67%), foxtail barley (18 to 64%) and scentless chamomile (34 to 67%) compared to tap water alone. Most forages set back dandelion roots even more than the shoots but had less impact on foxtail barley or scentless chamomile roots. For these weeds, the roots were either set back the same or somewhat less than the shoots.

Combinations of older forages that had been clipped twice were all able to significantly set back shoot weights and most were able to set back root growth of dandelions and scentless chamomile to a similar degree.

So What Does This Mean… to Me?

Chemical weed control is difficult in mixed pastures or rangelands because it’s hard to find herbicide(s) that can control different weeds without harming one or more of the desired forage grasses or legumes. This research suggests that healthy forages with healthy roots can battle weeds (especially dandelions) on their own. The key here is healthy roots. Plants are like anything else; when they are run down and stressed they will be less able to compete and fend off competitors, predators, and disease. Avoiding overgrazing – by providing adequate recovery time after grazing – is key to maintaining a healthy forage stand, especially when growing conditions are stressful (e.g. drought). Overgrazing (under-resting) doesn’t just reduce above-ground growth. It also reduces root growth. When a plant’s stems, leaves and roots have been “pruned” like this they are less able to compete with weeds, and may produce and release less ‘natural herbicide’. And if the weeds and other invasive plants aren’t grazed, they’ll continue to get larger and stronger, gaining the upper hand and spreading much faster in an overgrazed than in a well-managed pasture. The spread of weeds, like dandelions, may be an early warning sign of overgrazing or under-resting.

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