GRAIN GROWING CLASSROOM

A Closer Look at Wheat Dry-down
(Or Lack Thereof)

By Jochum Wiersma
U of M Small Grains Specialist
wiers002@umn.edu

Last summer’s cool weather, including a particularly chilly August, resulted in a wheat crop that for many matured and ripened very slowly, ultimately delaying harvest by weeks.  Some growers have commented that their grain was ready, but that green straw prevented them from harvesting the ripe grain.

Let’s look a bit closer at the issue of grain dry-down, and what we might learn from 2004 and some of the more recent literature.

Before I continue to talk about HRSW, I first would like to draw some parallels to corn. Maturity of corn is expressed as the relative maturity and is closely correlated to growing degree days.  The maturity is measured from emergence through physiological maturity. Physiological maturity is defined at the growth stage at which the maximum amount of dry-matter has accumulated in the developing kernels. In corn, the physiological maturity is marked by the formation of the ‘black layer’ under the outer layer of the kernel tip.  Kernel moisture averages 34% at physiological maturity, but hybrids can differ as much as 10% in kernel moisture content.

The dry-down rate of corn is defined as the rate hybrids will lose kernel moisture after reaching physiological maturity in the field. Hybrids differ in their dry-down rate. Hybrid corn companies will actually advertise that their hybrids have superior dry-down rates when compared to hybrids with the same relative maturity rating. Dry-down is important in corn for a very obvious reason; the imminent arrival of adverse weather as winter approaches.

In hard red spring wheat, breeders and agronomists do not report maturity in the same way it is reported in corn.  Rather, a variety’s maturity is rated according to when the variety reaches anthesis (the beginning of flowering). Thus, in hard red spring wheat, we do not consider the length of grain fill when reporting (relative) differences in maturity between varieties. Likewise, the differences (if any) in dry-down are never recorded.

Why the difference in reporting maturity between corn and wheat? I can only speculate but there are some possible explanations. First, spring wheat is a cool annual season. This has a couple of consequences for the semi-arid production in the northern plains. It is important that the wheat crop has flowered and completed most, if not all, of the grain fill before summer temperatures reach above the upper limits of the photosynthetic apparatus (plant growth) of wheat at 92⁰F. Thus, heading date is an important characteristic to measure. A second, and much more mundane, reason is the relative ease to determine the heading date compared to either physiological maturity or harvest ripeness.

There is enough anecdotal evidence to suggest that HRSW varieties differ genetically for the length of grain fill. For example, the variety 2375 was always known as a wheat variety that finished relatively fast compared to other spring wheat varieties. In addition, the dry-down rates of wheat varieties differ too. 

Mart Pool and Fred Patterson at Purdue University reported already in 1958 that varieties of soft red winter wheat differed significantly in the rate of drying to 14% moisture in the field after rains or dews.  They also reported that awned varieties increased both the moisture losses and gains of ripe standing wheat. 

Furthermore, research at the University of Saskatchewan showed that HRSW reached 17% grain moisture 6 to 14 days after reaching physiological maturity.  The rate of dry-down was a function of the environmental conditions.  High soil moisture, low temperatures, high relative humidities, and/or precipitation slowed down the dry-down of the grain.

Most years these differences among varieties are irrelevant; the average temperatures in August force the crop to reach physiological maturity and harvest ripeness rapidly.  The extremely cool August in 2004 simply did not have the heat units to ripen the spring wheat crop.

In addition to the very cool August, in combination with differences in length of the grainfill and dry-down of varieties, another factor may have played a role in the delay of harvest ripeness. Some recent research in Germany and the UK is shedding new light of the effects of two fungicides commonly used in the EU on wheat.

The research from Germany concluded that both azoxystrobin (Quadris) and epoxiconazole (a triazole class fungicide not labeled in the U.S.) delayed senescence of wheat plants in a greenhouse study.  The researchers suggest that this delay in senescence is due to an enhanced anti-oxidative potential protecting the plants. Both compounds were equally effective in delaying the senescence of the plant when applied at flag leaf or heading but azoxystrobin was more efficient when applied at the 4 to 5 leaf stage.

In field trials with the same two compounds, the UK group found no clear indications that the yield increases resulted from the reported physiological effects on the crop. The authors concluded that the value of any physiological effects to growers under normal conditions of moderate to high disease risk would be small in relation to the large fungicidal effects that occur.

The above mentioned results do, however, beg the question whether the physiological effects of fungicides reported in the greenhouse study played a role in the delay of maturity experienced in this region this past year. First it should be noted that epoxiconazole is not labeled for use in the U.S. Azoxystrobin is labeled for use in wheat as a stand-alone (Quadris) and in a prepackaged mix (Quilt). Without any additional research and testing of the other labeled strobilurin or triazole fungicides, it would be premature to extrapolate the conclusions to other labeled active ingredients.

Glyphosate is used by a number of growers as a harvest aid in spring wheat and feed barley. In the next issue, we’ll look at observations on pre-harvest glyphosate use, including some research data from last year.

U of M small grains pathologist Char Hollingsworth contributed information to this article. Research and outreach efforts of Wiersma, Hollingsworth, and others are supported in part through the Minnesota wheat checkoff administered by the Minnesota Wheat Research and Promotion Council.