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Seeking plants with polyculture potential

Choose your species wisely to avoid unintended results

Mike Schellenberg and Alan Iwaasa are evaluating the impact of polycrop mixtures on grazing capacity and soil health in the semi-arid prairie.

When producers discuss the benefits of growing annual polycultures, also known as multi-species mixes or crop cocktails, the talk often revolves around soil-health and environmental improvements, often­­­times taking forage production for granted. Of course, production does occur, but yield and feed quality may be disappointing if the species chosen target soil problems rather than forage and beef production.

“Producers need to be aware of their end goals for the polyculture because production and improving soil are very different objectives,” says Dr. Mike Schellenberg, a range and forage plant ecologist at Agriculture Canada’s Swift Current Research and Development Centre where he and Dr. Jillian Bainard have completed several polyculture trials.

Given that most research on the use of polyculture crops is from humid eastern regions of the U.S. and Canada, they set out to learn whether the benefits seen in those parts actually do occur in the drier Prairie region.

The initial project evaluated the performance of monocultures and polycultures with up to 12 annual species from four functional groups: warm-season grasses (corn, millet, sorghum), cool-season grasses (barley, oat, triticale), legumes (field pea, forage pea, hairy vetch), and brassicas (kale, forage radish, purple-top turnip). Each crop treatment was planted in early June on its same plot three years in a row and meadow brome-alfalfa was grown as the traditional mix for comparison. No fertilizers and pesticides for weed, disease or insect control were used in any year, and approximately 98 per cent of the crop was removed for greenfeed each August.

The change in organic carbon was measured by looking at the percentage of water-stable aggregates (clumps of soil that don’t fall apart in water) in each plot at the start and end of the project. Organic carbon is an important energy source for soil micro-organisms so they can carry out their nutrient cycling role. All of the polyculture plots and the barley monoculture plot gained organic carbon, without rotating crop treatments or leaving much residue behind and under very dry conditions in 2015.

Thus, if you hope to improve the soil, Schellenberg suggests choosing species with the greatest potential to benefit the soil more than the cattle, and leave plenty of plant material behind to feed the soil and protect the surface from erosion.

Alternatively, grazing livestock will leave some nutrients behind in manure and urine and their hooves provide some tillage action to work plant material into the soil surface to speed nutrient recycling.

It’s important to follow up with soil tests to know whether your choice of species is meeting the mark, he adds.

If your goal is production, yield and quality as cattle feed would be priorities. Their work showed that crop biomass was highly correlated with the number of species and functional groups in the mix. Biomass was highest the first year and dropped significantly in each of the next two years, partly due to dry conditions, but mainly because of dwindling soil nitrate levels resulting from the removal of nutrients in the form of plant material each harvest and not replenishing them with annual applications of nitrogen. Soil phosphate, potassium, iron, calcium and copper levels didn’t change much over the three years in any of the plots.

Plots with the least decline in soil nitrates were those sown to mixes that included legumes to fix atmospheric nitrogen and those with brassicas because they accumulate nitrates in plant tissues.

As a followup to evaluate implications for the subsequent annual crop, barley was planted on all of the plots and production was very poor due to the shortage of soil nitrates.

The initial trial showed that forage quality can be maintained or improved by planting mixtures. The combination of acid detergent fibre, neutral detergent fibre, and crude protein were the measures of quality. Overall quality of the mixes was not as high as some of the species on their own, but was higher than many single species offered.

Schellenberg reminds producers to consider perennial forage mixtures if they want a cover crop on the same piece of land for more than a year. Perennials still have a decided advantage over annual mixes because they are already established to take advantage of early-spring moisture and get a jump on weeds.

Equally as important as selecting species for a purpose is choosing varieties adapted to your growing area. They tested most species that producers have been growing in the semi-arid region and found nine provided most of the production. Kale and sorghum didn’t establish and grow well in any of the three years, whereas, hairy vetch performed very well as a high-quality forage while its nitrogen-fixing action benefitted the soil.

Pending funding approval, their next step will be to dig deeper into the proportion of species in mixtures best suited for achieving production and, or soil-building goals in the prairie region. They also see a need for longer-term projects to pull out details and identify cumulative effects in dry soil zones and test mixes and strategies to mitigate plant disease issues.

Diseased plant residues and inoculum left in soils can set the stage for a large-scale outbreak in a susceptible field crop or even another polyculture crop of the same species grown on that field the following year, he explains.

Sclerotinia-infected brassicas in a polyculture, for example, could be a reservoir of infection for disease in a canola crop if environmental conditions are conducive.

Insect pests aren’t commonly a problem in polyculture crops because a good mix of species from the four functional groups makes it difficult for insects to find their host of choice.

Schellenberg says the potential to control weeds is a very promising aspect of polyculture cropping. The more functional groups and more species included in the mix, the better it is for weed control, although the trend was less apparent under dry conditions that limited production and weed growth.

The weed control likely stems from the polyculture species out-competing the weeds for nutrients and space at several levels below and above ground. The shading effect under the canopy that prevents light from reaching the weeds is an important feature, he says.

Additionally, some crops, such as triticale, radish and turnip, and the perennial, western wheatgrass, have an allelopathic effect on other plants. The roots of these plants exude chemicals that harm the growth and development of nearby plants, and it is most often aimed at specific plants. Western wheatgrass, for instance, has an allelopathic effect on dandelion, but not alfalfa and other grasses.

In a followup two-year weed control trial, they found planting single species with differing characteristics in alternating rows didn’t control weeds quite as well as sowing all the species together in rows. Once again, the numbers of functional groups and species were the most important variables.

The bottom line, to date, is that having goals and a strategy for use of polycultures as part of a cropping or grazing system as a whole are very important because what goes into the mix and how the crop is managed can have positive or negative implications for the next crop.

A two-year swath grazing study is measuring the effect of a polycrop mix compared to a conventional annual forage on soil health, beef cattle performance, grazing capacity, and grazing system economics. The conventional forage is Maverick barley and the polycrop is 4010 peas with Union Forage’s Ultimate Blend (hairy vetch, Crusader Italian ryegrass, sorghum, crimson clover, Winfred forage brassica, Hunter forage turnip, Graza forage radish). The sites in the semi-arid brown soil region at Swift Current Research and Development Centre and in the parkland region at the Western Beef Development Centre were planted in June this year for October swath grazing with the schedule to be repeated next year and results expected in late 2018.

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