Options for fall weed control

Details

Published on Thursday, October 16, 2014

By Elmer Kaskiw

Manitoba Agriculture, Food and Rural Development

There has been some good harvest progress over this past week, although much of what has been harvested has been taken off at damp and tough moisture levels.

The overall cereal harvested is now estimated at 90 per cent complete although there are some pockets which still have substantial acres remaining to be harvested. Much of the remaining cereal harvest includes late seeded oats and wheat. Quality loss is now being noted in most spring wheat samples. Oat yields have been generally coming in at above long term average yields with average bushel weights. 

The canola harvest is now also estimated at 90 per cent complete. Canola for the most part has cured well after the recent rains with no significant issues with green seed and generally above long term average yields being reported. The flax and soybean harvest are just beginning with early yield reports of 25 to 35 bushels in flax and 30 to 40 bushel range in soybeans with generally good quality and minimal green seed issues in soybeans. 

Fall weed control

There have been a number of calls this past week regarding fall weed control and whether control measures were still advisable, given the recent back to back nights of frost having reached lows of -5 to -7 Celsius. Field observations indicate that the previous below normal single digit cool to cold temperatures and lows near the freezing mark did an excellent job of hardening these plants off. Most grasses and many of the broadleaves including Canada thistle are only showing minor injury as a result of these recent frosts. The cool temperatures prior to these frosts resulted in a hardening process which increases their tolerance to severe frost. 

The hardening off of plants to frost generally occurs in two phases. The first phase of hardening takes place at temperatures around the freezing mark when plants accumulate carbohydrates or plant sugars because of a decrease in the rate of respiration. The second phase takes place at mild to moderate frost and is accompanied by the loss of water in plant cells and a thickening of cell walls where the cell contents become insensitive to more severe frosts and the pressure of ice formation. As a result, there is still plenty of healthy plant tissue available to facilitate herbicide uptake especially after the return to warmer temperatures. 

Flax harvest

The hardening off of plants this fall to fall frost has also created issues with green flax straw that wasn’t desiccated prior to the significantly cooler temperatures. These plants, especially in the lower canopy, are still quite green, making it difficult to cut, especially when they begin to take up moisture when temperatures cool. This leaves only the short afternoon hours where harvest can occur and the plant has dried down to a point where it can be cut and combined. Unfortunately little can be done at this point to enhance dry down since desiccation will no longer be effective, leaving only further, more severe frosts as an option to dry down the plant further. 

Elemental sulphur 

application

Growers who have been growing canola on an every second year basis have been drawing down sulphur levels which may be leaving soils in a sulphur deficit. Sulphur levels can be difficult to soil test for, since sulphur is quite mobile in the soil which can result in some extreme variability in soil test results. Since sulphur applications at canola seeding are also generally seed placed with phosphate requirements higher sulphur rate can lead to seed toxicity under adverse growing conditions. One option to help alleviate sulphur shortages is to use elemental sulphur. Elemental sulphur is generally applied in the fall since it has more time to oxidize and will be more readily available when the crop needs it, although full conversion and availability to next year’s spring crop is not guaranteed. Some spring applied sulphate fertilizer will still be required. Growers who apply elemental sulphur in fall are encouraged to continue this practice on a regular basis as part of their sulphur fertilizer program. A history of elemental sulphur use builds up the oxidizing bacterial population, which will speed up the conversion to plant-usable sulphate.