Roundup Best Practice: Seed Germination

Effect of glyphosate on seed germination

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.

Caution: Roundup should not be used on seed crops of any type. For crops grown on contract, particularly malting barley, and anything for human consumption check any end user restrictions on use of glyphosate pre-harvest.

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 we do not recommend direct drilling into 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.