There were a record number of unseeded acres last year. This means not all soybean seed intended for the 2019 growing season was used. Can carry-over seed (grown in 2018) be used successfully in 2020? Before answering that question, it must be stated that soybeans are one of the least storable seeds when compared to many other crops. This is largely because of their high oil content. The length of time a seed lot remains viable is influenced by the initial seed quality, seed moisture, relative humidity, and temperature. Studies have shown that higher temperature and relative humidity during storage will increase the rate of deterioration. Maximum seed quality is reached at physiological maturity and begins to decline after that point. Seed deterioration is impossible to stop and cannot be reversed. Only the rate of deterioration can be slowed by controlling the storage environment. One study showed that seed viability remained high (>92%) over a 20-month period in cold storage (10˚ C) but declined in warm storage (25˚ C) to 78%. The seed stored in a warehouse with fluctuations in temperature and relative humidity dropped to almost 0% over the same 20-month period (Gladys Mbofung et al. 2013 “Effects of Storage Temperature and Relative Humidity on Viability and Vigor of Treated Soybean Seeds”, Crop Science, Vol. 53, May-June.). Therefore, it’s not considered good practice to keep soybean seed from one year to the next, unless it has been stored in a controlled environment and tested.
If carry-over seed is to be used this spring a germination (warm test) and vigour (cold) test should be conducted to determine the viability of the seed. Results from 67 seed lots tested in Michigan this spring showed a significant decline in the viability of the carry-over seed compared to new seed: see Table #1.
Table 1. Average germination test scores for carry-over soybean seed and new soybean seed grown in 2019.
|Sample source||Average warm germination (%)||Average cold germination (%)|
|Carry-over seed from 2019 (44 lots)||71.8||40.0|
|New seed grown in 2019 (23 lots)||96.9||91.3|
Source: Michigan Crop Improvement Association
At first glance an average germination score (warm test) of 71.8% for carry-over seed seems reasonable but the cold test results show just how poor the seed quality can be under stressful conditions (40%). So, can this seed be used? The answer depends on the level of risk a grower is willing to accept. Ideally, this seed should be used for a cover crop and new seed should be used this spring. If old seed must be used, adjust the seeding rate up to compensate for the seeds’ poor quality. There is no exact rule on using either the warm or cold test values to calculate an adjustment factor. It’s worth mentioning that only the warm test is officially recognized in Ontario. Test results often vary widely from sample to sample anyway so the results should only be used as a general guideline. In the above example seeding rates should likely be increased by at least 30-40 percent.
When using carry-over seed, do not handle the seed more than necessary as the seed quality will decline further. Do not re-treat the seed. This will do more harm than good. Since blending carry-over seed with new seed will cause more damage through the mixing process, this is not considered a good practice. Field conditions should also be favourable for fast growth
On a positive note, soybeans can adapt to a wide range of plant stands. If a reasonable stand (>100 000 plants/acre) can be achieved high yields are still attainable even when using poor quality seed, assuming growing conditions are favourable.