The Effects of Pre-harvest Roundup on Seed Crops
If a crop is sprayed with glyphosate when it is still immature there may be translocation into the seed and subsequent germination will be affected.
In a cereal crop where ripening is relatively even the application should be made when all the seed is mature. This is because the application must be done at less than 30% moisture in the grain, (see recommendations on timing). Spraying earlier will stop further grain fill, leading to shrivelled grain and increasing the risk of residues of glyphosate in the grain.
Malting barley grains germinate as part of the malting process and it is particularly important to be sure the moisture content is below 30% to preserve the viability of the seed.
Crops which have indeterminate flowering have non-uniform maturation of the seed. Timing for pre-harvest treatment of peas, beans, linseed, oilseed rape etc. is always a compromise and to avoid the more mature seed being over-ripe application usually takes place when a proportion of the seed is still immature i.e. average seed moisture content is 30%. Hence germination of any seed retained will be affected. Seed which has taken up glyphosate may produce a shoot, (from seed reserves) and pass a germination test but can still be affected once it reaches cotyledon stage and starts to photosynthesise. So, even after adjusting seed rates, crop stunting and failure is possible.
The Effect of Debris on Germinating Seeds
When large amounts of debris break down in the soil, especially in wet conditions or where land is poorly drained, organic acids and phenols can be released as the environment becomes anaerobic, (no oxygen). In the same way large amounts of decaying foliage in aquatic situations can lead to de-oxygenation of the water and fish death. This phenomenon is often seen where large amounts of straw or chaff are left behind the combine and can result in poor germination and emergence of the following crop, when seeds germinate from the anaerobic layer. This can be minimised by thorough cultivation to disperse or bury the debris before drilling.
Where large amounts of weed material are killed by glyphosate and the target weed has a mass of roots, stolons or underground rhizomes, (like Couch), this can lead to the same effects described above on germinating seeds.
There is no direct effect from the glyphosate, but because it is translocated to all parts of the plant, the resulting death of all the plant material at the same time, can cause anaerobic conditions leading to a slight crop check. Cultivation will minimise such effects, but direct drilling and minimal cultivations into large amounts of trash are at the most risk.
It is for this reason that Monsanto do not recommend direct drilling in to long leys in the autumn where there is a surface mat.
Advice for use of Roundup
After spraying Roundup on high density populations of perennial weeds and establishing with direct drilling or minimal cultivation, there is risk of poor crop establishment unless thorough cultivation is carried out to disperse or bury the decaying organic matter.
Consolidation of loose soil, adequate fertiliser to aid breakdown and appropriate measures against insect and fungal damage are also advised to achieve the optimum results.
Autocast Oilseed Rape Establishment
Autocast establishment of oilseed rape has been around for some years, though is less used now than 5 years ago. With an even spread of straw and plenty of moisture shortly after harvest it can work well. However, there is the potential for unreliable and uneven establishment where straw and chaff spread is uneven. There is a high degree of inherent variability with the autocast system, and poor germination can occur irrespective of a pre-harvest glyphosate treatment, due to high straw levels, compaction, slugs, and weather conditions.
Straw breakdown is slower over Autocast crops, particularly if anaerobic conditions are present, under thick mats of straw on the surface. The straw is not in intimate contact with the soil so breakdown of glyphosate residues left on the outside of the stems will not be rapid, due to a lack of both microbes and moisture. This means a theoretical danger of subsequent rain washing residues from outside of the straw onto the emerging oilseed rape seedlings – if this occurs the effect would be the same as spraying and could cause damage. The highest risk is where no rain falls between spraying and harvesting/autocasting. Heavy rain between the two operations will reduce the risk of subsequent damage.
Conventional drilling, direct drilling and many other reduced cultivation techniques, can all be used successfully after pre-harvest application of Roundup, but Monsanto does not recommend Roundup with the original Autocast mounted on the combine.
- Straw from a crop receiving a pre-harvest treatment is not recommended for use as a mulch. This is effectively what is being achieved with the original autocast system, where seed is broadcast behind the cutter bar on the combine and then covered with a layer of straw and chaff.
- When establishing subsequent crops straw should be at least partially mixed with soil, but does not need to be completely buried. Any system which involves even a light cultivation or soil disturbance e.g. the Opico Variocast or Terracast seeders mounted onto various cultivation equipment ensures that any glyphosate residue remaining on chopped straw is broken down rapidy by microbial activity
Note: Under all other cultivation systems straw and any glyphosate residue will be broken down rapidly by microbial activity even when only partially mixed in the soil by cultivation.