The Red River Valley is moving into a fourth week of high temperatures and little to no rain. Even a short walk through the field shows the stress cereals are under. Lack of moisture combined with high temperatures is causing wilted leaves, shorter plants and smaller heads, early development and even the shedding of tillers. Unfortunately, this drought hasn’t ended during the optimum timing for yield development. So just how thirsty is your wheat?
Wheat is a cool season crop and yields best when daily maximum temperatures are below 21°C. Cooler temperatures throughout the growing season result in increased tillering and spikelet development and larger, plump kernels with heavier weights. For every 2°C increase in temperatures above 18°C during the 4-5 leaf stage can mean a loss of one spikelet per spike. Plants set their yield early in the growing season, meaning that inopportune heat early in the growing season can impact what goes in the bin. This set yield continues to be impacted by stressors as the plant further develops, flowers and fills the grain.
Last week, many cereals started flowering and are now moving into the grain development phase. These plants are dealing with double stressors during this critical time which will further affect this year’s yields.
High temperatures during flowering can impact the fertilization potential of the spikelets, causing pollination failure or kernel abortion. As plants move into grain fill, high temperatures have plants closing their stomata to preserve moisture. This reduces photosynthesis and the amount of starch sugars moving into the kernel. Temperatures above 32°C will cause smaller, lighter kernels. Fortunately, the protein content of kernels is not affected by heat stress. Heat stress will cause the plant to have a high rate of grain fill but the period will be shortened, resulting in a lower maximum kernel weight.
Ultimately, heat stress means smaller kernels and lower yields but maintains a high protein content. However, most grain harvested this year will likely have high protein levels, meaning no premiums.
Crops need Water to help Beat the Heat!
Plants have a high water requirement during flowering and early grain fill. With high temperatures, even more water is needed for basic plant functions. Lack of water might be seen as bleached spikes or heads and dried, angular leaves. These plants will not have lush, turgid leaves. Cereals might even abort tillers and secondary heads to focus on the health of the main stem to bring it to complete development.
Some parts of the Valley received a bit of rain over the weekend; enough to help the plants for a couple days. A thirsty wheat crop can consume 0.3″ of water a day when temps reach the mid 30’s(C) and only half that amount when temps are in the high teens. An inch of rainfall doesn’t go very far in quenching thirsty wheat. When we talk about available soil moisture, a term often heard is Field Capacity (FC). FC which is the amount of water a soil can hold without losing any to gravity. At FC our clay soils hold roughly 2″ of crop available moisture/foot. Wheat will usually root to somewhere between 3 and 5ft so we can say that at FC we should have approximately 8″ of available water.
We assume that on average we start the season with at least 8″s of soil moisture. Our average growing season rainfall for wheat is approximately 7″ giving us 15″ total available moisture and average yields in the mid 70’s. This year we may have areas that received 5-7 inches of growing season rainfall moisture but we likely started with lower soil levels. Coupled with increased water use due to high temperatures and we see why the crops are struggling.
How do we Determine Available Water?
There are many moisture probes on the market that take the guess work out of determining available water. They typically include a weather station and a soil probe 3′ or more in length that will measure every few cm or inches to determine how much water is in the profile. Coupled with crop stage and weather info, we can estimate rough yield potentials to help with decision making.
Without technology it is still possible to estimate soil available moisture by feel and appearance. Grab a handful of soil and squeeze it to see if it will form a ball or ribbon out between your thumb and forefinger. The tough part is digging down to sample subsoils. Most of us need the exercise anyway?! If your clay soils barely form a ball, there is no water available for plants.
Here is a link to a handy table from the University of Minnesota that you can use to determine how much water you potentially have available :
Thirsty wheat crops this season will continue to struggle with moisture deficits and temperature stressors. All plants are able to adapt to their environmental conditions, within reason. This growing season has provided many hurdles for crops. Many crops got off to an ok start and still look good. Hopefully they can access the moisture needed for proper grain fill. Without additional timely rains this season, we will need to rely on snow trap to help rebuild soil moisture. This means trying to leave taller stubble, reduced tillage and higher levels of residue to cover and protect the soil.