Grain Filling – The Final Stage in the Production of Yield

Dr. D.B. EGLI 

LEXINGTON, KENTUCKY

Grain-filling is the final event in the production of yield by a grain crop. The vegetative plant has stopped growing; the production of the leaves that power plant growth and the roots that nourish them is finished. The number of grains the plant will produce is fixed and the tiny grains are ready to start growing. Grain filling is the main event, all of the preliminary events are over. The preliminary events are essential, but the heavy lifting for yield occurs during grain filling.

‘Deciding’ how many grains the crop will produce is usually considered a critical event that, in large part, determines yield, but I think we make a mistake if we discount the importance of grain filling. At the beginning of the grain-filling period yield is essentially zero, no yield has been accumulated – all of the yield is produced during grain filling. 

It is difficult to exactly define the start of grain filling (when the grains start their rapid accumulation of dry weight) , but growth stage R5 is a good approximation for soybean, while growth stage R2 approximates the beginning for corn. Grain filling ends at physiological maturity (maximum grain dry weight) which occurs at approximately growth stage R7 in soybean and R6 in corn. 

Yield is a function of the total rate of grain growth (pounds per acre per day) and the duration of grain growth (days). The rate is determined by the capacity of the plant, via photosynthesis and remobilization of stored nutrients, to supply the raw materials for grain growth. Duration is defined by how long the grains keep growing. It is not surprising that both rate and duration are usually directly associated with yield. The faster and the longer the grains grow, the greater the yield. The length of the grain-filling period is under genetic control. In fact, there are many historical examples where plant breeders selecting for yield, inadvertently selected for a longer grain-filling period.   

The grain-filling period has several interesting characteristics. First, it is relatively short – only 30 to 40 days in most crops. This amounts to 25 to 33% of the total growth cycle (planting to  maturity) for a crop that takes 120 days to mature. Way over half of the total growth cycle is taken up by preliminary activities. These activities are necessary, but all of yield is produced in the last 30 to 40 days. Producing high yields in such a short time puts a lot of stress on the plant to supply the necessary carbohydrates and other nutrients. Any disruption of plant growth during this period can reduce yield.

The second interesting characteristic involves the plant’s initiation of leaf senescence shortly after the beginning of grain filling. During senescence, the plant breaks down its photosynthetic machinery in the leaf and translocates the nitrogen and other nutrients to the developing grain. Destroying its photosynthetic capacity just when the crop finally starts to produce yield doesn’t seem to be the best strategy to make high yields. Senescence, however, is not all bad; remobilizing the breakdown products to the grain leaves less nitrogen and other nutrients in the stover, resulting in a more efficient use of nutrients.

The length of the grain-filling period is affected by temperature – as temperature decreases, the length of the grain-filling period increases. This relationship may explain why remarkably high yields often occur in relatively cool climates (e.g., high wheat yields in Europe). A famous crop physiologist at the University of Kentucky, the late W.G. Duncan, once theorized that the ideal environment for high yield was a high elevation dry climate with irrigation (water not limiting). The high dry climate provided high levels of solar radiation (fewer clouds) and high temperatures during the day to maximize photosynthesis, but lower temperatures at night  resulted in a longer grain-filling period. High photosynthesis and a long grain-filling period equals high yield. 

Water stress during grain filling will accelerate leaf senescence, shorten grain filling, and reduce yield. In this case, water stress acts as a ‘hidden’ stress because the visual aspects of senescence (leaf yellowing and abscission) proceed normally,  just faster, so, it’s not noticeable (unless well-watered plants are available for comparison) until harvest when the smaller grains and lower yields are obvious. Interestingly, once water stress accelerated leaf senescence in our experiments, watering the soybean plants to relieve the stress did not stop the accelerated senescence. Apparently, only a few days of stress (3 days in our experiments) are needed to accelerate senescence, shorten grain filling, and reduce yield. Water stress during seed filling may be a more important yield-limiting factor than commonly thought.

All yield is produced during grain filling. The old adage that yield is ‘made’ early in seed filling is not necessarily true – the potential is there, but stress can easily reduce it. Yield is not ‘made’ until the seeds reach physiological maturity.

Adapted from Egli, D.B. 2021. Applied Crop Physiology: Understanding the Fundamentals of Grain Crop Management. CABI. 178 pp. ∆

Dr. D.B.EGLI: Emeritus Professor, University of Kentucky