Drought Effects on Small Grains

By Alyssa Collins

Drought is still the elephant in the room. Early development of corn and soybean has been affected. Meanwhile, small grains have been enduring drought in some way for most of the season, from a mild but prolonged drought in the spring to the most recent severe drought affecting many fields at the flowering and grain filling stages. Here we provide a summary of drought effects we have observed on small grains so far, and expectations for this harvest season in light of the drought.

More unproductive tillers

In PA, for each wheat or barley seed planted, we typically get an average of two to three tillers (including the main shoot) bearing a harvestable head. However, this year’s fields look thinner than usual, with many plants having only one head. Timely planted fields experienced generally good fall conditions and exhibited more than three well developed tillers per plant by early spring. However, the dry weather that followed the end of tillering and continued through most of the stem elongation stages likely triggered competition between tillers and impaired head development in the youngest tillers. Unproductive tillers can be seen in the lower canopy of many fields. Late planted fields may have been additionally limited in overall tiller production.  Regardless of the cause, these thinner stands may result in less uniform head ripening and leave fields more vulnerable to late weed pressure.

More "empty kernels" and slow kernel development

Florets develop into kernels after successful pollination and fertilization. Drought stress at the flowering stage can cause pollen sterility, resulting in unfertilized florets or "empty kernels". The prolonged drought we had in May, when many small grains were flowering, likely increased the percentage of "empty kernels" and will contribute to lower yields.

Note that anther extrusion and pollen shed often still happen even at high levels of pollen sterility, so unfortunately there is no visual cue of this phenomenon. The only way to confirm that a floret was fertilized is to wait for the kernel to start developing. Kernels on the mid-top portion of the head can be checked first as they are the first ones to be pollinated (see this wheat staging guide for pictures of developing kernel). Many have reported that kernel development this year took longer than usual (up to a month between flowering and milking stage) which led to false alarms of extremely high "empty kernel" numbers. In addition to the drought, the cool weather we had in May likely contributed to slow kernel development. In Lancaster, total growing degree days accumulated in May was 25% below the 15-year average. Therefore, for fields that are still coming out of flowering and not showing signs of kernel development yet, it may be worth waiting a bit longer to have a final count on kernels per head. Easier said than done!

Slow grain fill rate and short grain fill duration

When drought occurs at the grain filling stages, the rate of grain fill (grams of dry matter accumulated in the kernel per day) is decreased. This is due to both decreased photosynthesis and decreased remobilization of sugars from other plant parts to the grain. In addition to less dry matter being accumulated in the grain per day, the accumulation period is shorter. This is because drought triggers early senescence. The plant shuts itself down earlier than it would in ideal conditions to ensure completion of its life cycle. This was observed in many wheat fields where all leaves except for the flag leaf are already dead. As a result of the decreased grain fill rate and shorter grain filling period, kernel weight is reduced.

Low disease pressure

Most of our typical diseases are favored by wet weather so it is not surprising that disease severity is very low or even absent in most fields.

Expectations for the 2023 harvest

An early harvest is expected. Many barley fields have reached harvest maturity and many wheat fields show signs of early senescence due to drought. The impact of drought on final yield will depend on the timing and severity of drought. Fields that experienced severe drought during the reproductive and grain filling stages should be the most affected, primarily due to decreased number of kernels per head (high numbers of "empty kernels"). Stay tuned for an upcoming article reporting crop conditions observed during this year's Mid-Atlantic Wheat Tour.