Dry heads on green stems in wheat: A source-sink issue
Some producers have reported that the grain is dry in their wheat crop, but green stems are keeping them from being able to harvest smoothly (Fig. 1). A few different factors may be playing a role in maintaining green stems on a dry wheat head, and they relate back to the plant’s source-sink relationship. The source (or machinery) is the sugar factory (leaves and stems), while the sink is the developing grain responsible for pulling those recently produced sugars from the leaves and stems.
Figure 1. Green stems on mature wheat heads. Photo taken June 23, 2017 by Melanie Schlatter, Lebanon, Kansas.
- Good management practices: greater “source”
When crop management strives to provide good grain filling conditions, the crop remains green for longer periods of time as compared to a crop that is not well managed. Examples include greater nitrogen rates and foliar fungicides, which will often delay plant maturity (Fig. 2).
Figure 2. Maintenance of green leaf area due to an additional 40 lbs N/acre and two foliar fungicides (upper panel) versus no extra fungicide (lower panel). Photos by Romulo Lollato, K-State Research and Extension.
- Late-moisture, cool conditions
Grain filling conditions for most of April, May, and early June were very favorable with above-average precipitation and below-average temperatures. These conditions will favor the maintenance of the photosynthetic machinery of the plant for a longer period of time as compared to hot, dry conditions, which hasten crop development.
- Frozen or hailed-out crop: lower “sink”
If the number of kernels is decreased by environmental factors such as a spring freeze that damaged floret fertility and development, or by hail damage that physically removed spikelets from the spike, the sink strength of the crop is decreased proportionately to the number of grains decreased. With less strength to pull the sugars from leaves and stems, there is a chance that stems might remain green while grains are ripe and ready for harvest.
Another reality in many fields this year is late-developing tillers that also have green heads at this time. Areas of Kansas that received significant rainfall in late May might show growth of late-developing tillers. Where this has occurred, there is essentially a second canopy of green heads usually slightly below the main canopy of ripe heads (Figure 3).
Typically, heads that form this late in the season add very little to the overall yield of a field. If these late, green heads are not close to being ready to harvest when the majority of the crop has dried down, then it’s best to start harvesting the field anyway. Waiting for the green heads to mature would risk grain losses due to shattering or hail damage.
Most of the immature grain and green plant parts will go out the back of the combine when the crop is harvested, but enough green material may go into the bin to increase the dockage and overall moisture level of the load of wheat to some extent. Combine settings can help minimize problem, but not eliminate it.
Producers who are harvesting wheat with some green heads present should take special care to measure the moisture content of the grain if they plan to store it on farm, and dry the grain aggressively if the moisture content is high.
Figure 3. Green heads from late-developing tillers interspersed with mature heads nearing harvest in Ellsworth County. Photo by Brent Goss, former agent, K-State Research and Extension.
Romulo Lollato, Wheat and Forages Specialist