Wheat Update: Precipitation, Air Temperatures, Vegetative Conditions As Of March 3

Mar 06, 2017

By Romulo Lollato

Extension Wheat Specialist

Precipitation totals

Total precipitation during the winter wheat growing season from September 1, 2016 until February 28, 2017 ranges from less than one inch in portions of western Kansas to more than 23 inches in south central Kansas. These totals correspond to 28% and 187% of the long-term normal for these regions, respectively (Fig. 1). The majority of the central, eastern, and portions of northwest Kansas have received above-normal precipitation this growing season, while the entire southwest quarter of the state is below normal by as much as 4 inches.

Figure 1. Departure from normal seasonal precipitation from September 1, 2016, to February 28, 2017.

As a consequence of the precipitation shortage last fall in southwest Kansas, many fields in that region have erratic stands, or are just now germinating and, in some cases, emerging. For plants from fields in this situation, such as those shown in Figure 2, producers need to consider several agronomic and economic factors when making the decision to maintain the crop. Some of these include:

Final stand:

Some plants may have initiated the first leaf while still below ground, giving them an accordion-like appearance, as shown in Fig. 2. This probably happened because the sowing depth in this specific field was relatively deep (2 inches). Plants in which the first leaf emerged below ground might never grow through the soil surface and this field might end up with very scattered stand. In cases in which the seed has been dry since last fall and now has moisture, the shoots could very well make it through the surface. Where that occurs, chances are that producers will end up with a decent stand. Sowing depth, seed treatment, and seed vigor will affect the final stand.

Figure 2. Wheat seedlings just now germinating and, in many cases, not being able to emerge as the first true leaf emerged below ground, causing the accordion-like shape above. Photo taken February 27 by Glenn Newdigger and Marin Gleason, K-State Extension agents in Stafford and Edwards counties.

Yield potential:

Based on previous K-State research evaluating spring-planted winter wheat, fields that are just now emerging will probably have about half of the yield potential of a crop that emerged and tillered last fall. Weather conditions will influence this yield potential: If the weather turns cool and moist during the spring, these fields might still yield okay; but if we have a more normal warm and dry year, the yield reduction could be even worse than half.

Weed pressure:

Spring-emerged wheat will more likely experience greater weed pressure due to a thinner crop. In this case, producers need to be aware of herbicide timing and rotation restrictions in case they want to terminate the crop later.

Vernalization:

Fields that are just now emerging might also have problems vernalizing, as some varieties need 6-8 weeks of soil temperatures below 50°F to meet vernalization requirements. In varieties with shorter vernalization requirements, such as Overley, this might not be as big of a problem. It will mostly depend on temperatures from here on and variety-specific vernalization requirements.

The decision of whether to keep the stand

Where the wheat doesn’t emerge until this spring, producers have a tough decision to make. If the resulting stand is decent, the crop will still have a reduced yield potential compared to a fall-emerged crop, as spring tillers are less productive than fall tillers. Producers in this situation should take a stand count to determine final number of plants emerged per row foot. If stand is decent compared to the target stand (Table 1), consider whether having a reduced yield potential is affordable. If the stand is not decent or producers cannot afford having the reduced yield potential, terminating the crop and going into a summer crop would be an option, herbicide restrictions allowing.

Table 1. Target plants per row foot (80% emergence) based on seeding rate, seed size, and row spacing.

Seeding rate	Seed size	Row spacing (inches)
Seeding rate	Seed size	6	7.5	8	10	12
lb/ac	seeds/lb	Target plants per row foot (80% emergence)
45	12,000	5	6	7	8	10
	14,000	6	7	8	10	12
	16,000	7	8	9	11	13
	18,000	7	9	10	12	15
60	12,000	7	8	9	11	13
	14,000	8	10	10	13	15
	16,000	9	11	12	15	18
	18,000	10	12	13	17	20
75	12,000	8	10	11	14	17
	14,000	10	12	13	16	19
	16,000	11	14	15	18	22
	18,000	12	15	17	21	25
90	12,000	10	12	13	17	20
	14,000	12	14	15	19	23
	16,000	13	17	18	22	26
	18,000	15	19	20	25	30
120	12,000	13	17	18	22	26
	14,000	15	19	21	26	31
	16,000	18	22	24	29	35
	18,000	20	25	26	33	40

In order to check how far actual stands are from the target stand, count the number of emerged plants per row foot, and compare that to the values in Table 1. That should give a good estimate. Table 1 shows the number of target plants per row foot depending on seeding rate, seed size, and row spacing, and assuming 80% emergence. If seed size is not known, 14,000 to 16,000 seeds per pound can be used for most wheat varieties in Kansas, except those with rather large or small kernels. For certified seed, considering 12,000 seeds per pound when seed size is unknown is a good estimate. To determine the average number of plants per foot of row, several random plant counts across the field should be taken, given a more or less uniform emergence throughout the field.

Air temperatures

Mean air temperatures during the Jan. 1 – Feb. 28 period ranged from approximately 30°F in northwest Kansas, to close to 43°F in southeast Kansas (Fig. 3). This corresponds to a departure from normal mean temperatures for the same period of less than 3°F in the western tier of counties to as much as 10°F in portions of eastern Kansas.

Figure 3. Mean (upper panel) and departure from normal mean (lower panel) temperatures for the January 1 – February 28, 2017, period for Kansas.

Regardless of the magnitude of the temperature departure, the entire state has experienced above-average temperatures this winter. As a consequence, the crop has had an early spring greenup, with some fields already past jointing in the south central portion of the state. The extent of this early greenup is depicted in Fig. 4, a comparison of vegetative condition index for Kansas between the current growing season and the 28-year average.

Figure 4. Compared to the 28-year average at this time for Kansas, this year’s Vegetation Condition Report for February 21- February 27, 2017 from K-State’s Precision Agriculture Laboratory much of the state has above-normal vegetative activity, especially in the central and south central parts of the state, where precipitation has been more favorable.

The Wheat Production group at Kansas State University is keeping track of the spring development of several varieties. Readers can find the latest measurement of hollow stem elongation in the accompanying article “First hollow stem update” in this issue (March 3, 2017) of the Agronomy eUpdate.

Consequences of warm winter temperatures

The early spring greenup will require producers to make certain management practices, such as nitrogen fertilization, earlier in the spring than normal. Winter wheat turns from vegetative to reproductive development after the vernalization requirements are met early spring; thus, the number of spikelets per head is determined at greenup. Having N in the root zone by then is critical to ensure good head formation and maximize the number of spikelets per head. Another important management practice producers need to be aware of is insect management, as aphids and other pests will be more active under warmer conditions.

Another major consequence of the early start to this growing season is the heightened risk of freeze injury. The average last date for freezing (32°F) temperatures ranges from early April in southeast Kansas to early May in northwest Kansas. Where the crop is developing early, it will be more likely to be in more sensitive phases of crop development if and when a freeze occurs later this winter and spring. Heading and anthesis are not the only growth stages sensitive to freeze injury. Once the hollow stem pushes the growing point above the soil surface, the crop is more exposed to risk of freeze injury. Temperature fluctuations such as the drop observed for most of Kansas during the Feb. 23-25 period (Fig. 5) can also be harmful to the wheat crop if it is already jointed, especially if the air temperatures fall below 20°F and remain there for a few hours. The temperature drop in the Feb. 23-25 period was as much as 70°F in Harper County, falling from approximately 80°F on the afternoon of Feb. 23 to less than 10°F in the morning of Feb. 25.

Despite the increased risk for freeze injury, if the crop escapes freeze injury -- either by a mismatch between freeze occurrence and highly sensitive growth stages or simply due to the absence of a freeze -- an early spring greenup might signify an early grain fill period. During the 2015-16 growing season, the crop was able to avoid three major freeze events due to a mismatch between freeze occurrence and sensitive stages of development, and yields were generally very good since the crop experienced good grain filling conditions. It will all depend on the weather moving forward.