Soybean School: Does row orientation impact weed pressure and yield?

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By François Tardif

Many master gardeners would tell you that to maximize your vegetable yields, rows need to be oriented in a north-south manner. This is supposed to maximize light interception by the crops, which in turn maximizes photosynthesis.

In contrast, east-west rows have more shading and this negatively impacts yields. Is the same true for field crops? Could this approach be used to improve weed management in a crop like soybeans?

In the future, crop row orientation may be used as part of an integrated system to help combat weeds in field crops like soybeans, says Francois Tardif.

The effect of row orientation is complex and is influenced by row spacing, crop height, latitude, and growing season.

Both row spacing and crop height will affect how much shading one row will shade and reduce light availability for a neighbouring row. Latitude’s influence is in the angle of the sun. For example, in the summer afternoon, sun angle will be 70 degrees at Elora, Ontario, while it would be 80 degrees in South Carolina. For each degree of latitude you move north, the sun angle declines by one degree. The steeper the angle, the less shading you have.

For field crops. most studies have shown that under temperate climates there is a yield advantage to have north-south rows. This is especially true for wide rows, as wider spacing means more crowded plants on the row and more shading.

The effect of row orientation to reduce weed impact has been examined in Australia. Research done in that country from 2002 to 2005 showed that row orientation had an impact on weeds, especially with wheat and barley. The effect was negligible with other crops such as canola and peas. In the absence of weed control, orienting wheat east-west reduced the biomass of annual ryegrass (the main grass weed in that country) by 51% compared to north-south rows. A similar effect of 37% was seen in barley.

Would the same effect occur under our Ontario climate? In this episode of RealAgriculture Soybean School, OMAFRA weed specialist Mike Cowbrough takes us on a tour of a 2017 Elora Research Station trial designed to help answer that question. In the first year of the trial, soybeans were planted in 15-inch rows. Plots were north-south or east-west and had either complete weed control with herbicides or were left to fend themselves against the weeds.

In the case of the weed-free plots we saw no difference in yields. With the weedy plots, yields were reduced by about 50% which is to be expected. While there was no statistically significant difference due to row orientation on yield, we did observe a difference in weed species composition.

East-west rows had significantly more wild mustard that north-south rows. This was unexpected. It is possible that the increased shading in the east-west plots has kept the inter-row spaces cooler which would favour a cold-adapted species such as wild mustard. Unfortunately, we did not take any detailed measurements on weed species composition. The trial is being repeated in 2018 to see if the effect holds true and to establish any impact on specific weed species. Whether other crops would show the same effect still remains to be seen.

In the future, if field size and layout permit, it is possible that crop row orientation might be used as part of an integrated system to help combat weeds.

Professor François Tardif is a weed researcher at the University of Guelph’s Department of Plant Agriculture.

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