Soybeans after maize: herbicide residues can stunt growth
A Polish soybean grower says fields for soybeans should be chosen up to two years ahead, because herbicide residues and weed seed banks affect the crop. The Polish Soya Association names several active substances that can harm soybeans.
Tomasz Gdyk, a farmer from Poland's Subcarpathian Voivodeship who has grown soybeans for 10 years, advises choosing a field for soybeans not in spring before sowing, but up to two years in advance. The reason is to avoid herbicide residues (chemicals used to control weeds) in the soil and to account for the weed seed bank in the field. He pays particular attention to fields where maize was previously grown.
According to the farmer, two factors determine soybean success: the purchase price and the yield. Growers have little influence on price, but yield can be affected, including through field selection. Even with this year's favourable prices, he says, it is worth caring for yield.
Herbicide residues in the soil
In maize, Gdyk uses only foliar herbicides, which carry a lower risk of soil residues than substances that act on weeds through their roots. He says residues from other crops often cause hidden phytotoxicity, meaning harmful effects on soybeans: the plants grow more slowly and develop shoots, roots, nodules and pods poorly.
Mariusz Wojtyna, vice-president of the Polish Soya Association (Stowarzyszenie Polska Soja), says most reports of phytotoxic effects concern mesotrione, used in maize before soybeans. Amidopyralid, applied in wheat, can also be dangerous. There are considerably fewer reports about thiencarbazone-methyl in maize and cereals.
Creeping thistle must be tackled before sowing
The second factor is the weed seed bank. A small seed bank and the absence of perennial weeds allow a field to stay clean until harvest, but Gdyk says savings should not be made on weed control. His biggest problem is not goosefoot or nightshade but creeping thistle, which must be dealt with well before sowing, as there are no fully effective methods within the soybean crop itself.
He suggests using clopyralid in cereal stubble against it, while warning of phytotoxicity risk. For him the risk is not high because he ploughs, which moves soil containing residues deeper and reduces contact with soybean seedlings.
The takeaway: a soybean field should be prepared well in advance, and when soybeans move to a new field, it is worth checking what was sown and applied there and what the weed seed bank is.
