Do Larger Seeds Mean Higher Grain Yield?
DR. DENNIS B. EGLI
LEXINGTON, KENTUCKY
Will increasing seed size (weight per seed) increase yield? The universal yield equation [yield = (seeds/acre)(weight per seed)] suggests that it will. This equation seems to indicate that the larger the seed, the higher the yield. But it’s not that simple.
To understand seed size – yield relationships we first have to ask - are the differences in seed size under genetic control or are they a result of the environmental?
There are large differences in seed size among species. Canola seeds, for example, are tiny (43,560 seeds per pound) relative to corn seeds (approximately 1,574 seeds per pound).
Genetic differences in seed size within a species are also common. Generally speaking, these genetic differences in size are not related to yield. Wheat, for example, produces very high yields with a relatively small seed (11,340 seeds per pound).
This surprising result occurs because most genetic differences in size are a result of variation in seed-growth rate. Big seeds grow fast, small seeds grow slow. When seeds grow fast, the number of seeds per acre is reduced and there is no effect on yield. Smaller seeds result in more seeds and again the same yield. Selecting a variety that has large seeds will not result in higher yields.
But, of course, there are exceptions to this rule. If seeds are large because they grow for a longer time – have a longer seed-filling period - the large seeds will result in higher yields. Most of the time, large seeds are a result of higher seed-growth rates, but there are exceptions where large seeds are a result of a long seed-filling period. These exceptions will result in higher yields.
Seed size is also affected by environmental conditions during the growth of the crop and these effects usually translate into changes in yield. These effects occur during seed filling, the last stage of the yield production process. The number of seeds the crop produces is determined first, before seed filling so, most of the environmental effects on yield are a result of variation in seeds /acre. Seed size responds to the environment during seed filling, after seeds/acre is fixed.
If environmental conditions improve after seed number is fixed (starts to rain again, for example), seed size and yield will increase. Stress during seed filling will reduce seed size and yield. Unfortunately, as in many situations in life, the downside opportunities are much greater than the upside. It’s more likely that stress during seed filling will decrease seed size than for good conditions to increase seed size.
One reason for the limited upside is that the adjustment of seeds per acre to environmental conditions has already accounted for most of the environmental effects on yield. Seed size can only respond to the changes in the environment after seed per acre is fixed. A constant environment (good or bad) should produce a normal sized seed.
Secondly, there are physical restrictions (set by pod size in soybean or the packing of corn seed on the cob) on how large the seed can be (You can’t fit a golf ball in a soybean pod!) limiting the potential increase in seed size and yield. There are no limits to how small it can be. Many producers, unfortunately, have experienced the year when it quit raining during seed filling and seed size and yield took a big hit.
It doesn’t take a severe drought during seed filling to reduce seed size and yield. We found in greenhouse experiments with soybean that only three days of water stress during seed filling accelerated leaf senescence, shortened seed filling, and reduced seed size (13%) and yield (17%) averaged across two experiments. We may not notice just three days of stress, but it can have an effect on seed size and yield.
The bottom line is that yield is not made until physiological maturity (growth stage R7 in soybean and black layer in corn), so stress during seed filling (water stress, diseases stress, high temperatures) has the potential to shorten the seed filling period, reduce seed size and yield. Improvement in growing conditions during seed filling can increase seed size and yield.
Seed size – yield relationships are complicated in all grain crops. A lot of the obvious differences in size are under genetic control and are not related to yield. Environmental conditions during seed filling can affect seed size and yield. The key question is always - what caused the differences in size – genetics or the environment?
Aways remember – “There are, in fact, two things: science and opinion. The former begats knowledge, the latter ignorance” Hippocrates , Greek physician, and philosopher. (460 – 370 BC). ∆
DR. DENNIS B. EGLI
UNIVERSITY OF KENTUCKY