Ensuring that the wheat crop is sown at the optimum date and seeding rate are two steps needed to ensure that the maximum yield potential can be attained for a given growing season (Figure 1). Sowing date affects yield potential due to stand establishment, soil and air temperatures to which the crop is exposed, tiller formation, disease pressure, etc. Optimum seeding rate depends on sowing date and its adjustment is crucial to ensure the crop will maximize its yield potential.
Figure 1. Best management practices to be adopted before and after planting to ensure maximum yield potential can be attained in a given growing season. Graphic by Romulo Lollato.
A) K-State recommendations
Optimum sowing date for winter wheat is quickly approaching for a large portion of Kansas (Figure 2). Depending on geographical location, optimum sowing window can start as early as September 10th and last until the end of September (northwest Kansas), or it can start as late as October 5th and last until October 20th (southeast Kansas). This gradient in sowing dates, with earlier dates in the northwest, is a function of temperature. Northern regions will have cooler air and soil temperatures earlier in the year as compared to southern regions.
Figure 2. Optimum planting dates for winter wheat according to geographical location within Kansas. Figure adapted from KSRE publication L-818, Kansas Crop Planting Guide.
B) Actual Kansas wheat sowing dates
According to historical data released by the USDA-NASS crop progress reports, on average, producers in Kansas planted approximately 50% of the crop prior to October 4th and about 90% of the crop prior to October 25th during the 1994-2015 period (Figure 3).
Figure 3. Average percent wheat area planted in Kansas after September 1st. Data represents average and standard deviation for percent planted area during the 1994 – 2015 period as reported by the USDA-NASS Crop Progress Reports (https://www.nass.usda.gov/Publications/National_Crop_Progress/).
Although 50% of the fields are, on average, planted by October 4th, there is large year-to-year variability in percent planted area within the aforementioned date range (see error bars on Figure 3). This year-to-year variability is led by sowing conditions as extremely moist or dry soils may keep producers from sowing at the optimum planting date.
The largest variability of area planted in Kansas in the period 1994-2015 occurred between September 20th and October 15th. During this period the difference in area planted between the earliest and the latest years on record was above 40% (Figure 4). In other words, while 50% of the wheat area was sown by September 21st in the earliest year on record, only 7% of the area was sown by the same date for the latest year on record. In the latest year, 50% wheat area sown was only achieved October 11th. The variability in planted area was lower at earlier planting dates (before September 20th), probably because most producers tend to wait until the optimum planting window with a smaller acreage planted early. Year-to-year variability in planted area also decreased towards the late planting window (after October 15th), as most of the acreage had been planted by that time in most years.
Figure 4. Percent wheat area planted in Kansas after September 1st for the earliest and latest years on record between 1994 and 2015 as reported by the USDA-NASS Crop Progress Reports (https://www.nass.usda.gov/Publications/National_Crop_Progress/). Range in area sown is shown as light purple area in the main graph. Inset shows the difference in percent area planted between the earliest and the latest sowing years on record.
C) Considerations of wheat growth affected by sowing date
Figure 5. Difference in fall growth as a consequence of planting date following summer crops. Photos were taken December 8, 2016 from neighboring fields which were sown in early October in no-till following corn (left photo) or late October no-till following soybeans (right photo). Photos by Romulo Lollato.
Optimum seeding rate varies with geographical location in Kansas, following the existing east-to-west precipitation gradient. If sown at the optimal date, optimum seeding rate should be about 1,125,000 – 1,350,000 seeds per acre in the eastern portion of the state, where annual precipitation is above 30 inches, or under irrigated conditions (Figure 6). Seeding rate should be decreased to 900,000 – 1,250,000 seeds/acre in the central region, where annual precipitation ranges between 20 and 30 inches; and a further decrease in seeding rate should occur in the western third of the state where annual precipitation is less than 20 inches, for a final seeding rate between 750,000 and 900,000 seeds per acre in that region (Figure 6).
Seeding rate should always be discussed along with planting date, and in many times with soil fertility status as well. As mentioned above, later planting dates will decrease the potential number of fall tillers formed and grain yield will be more dependent on the main stem and maybe one or two tillers formed during the fall. Thus, seeding rate should be increased as planting date is delayed (for more information see “Management adjustments when planting wheat late” in eUpdate Issue 536). On the other hand, producers with a history of manure application and very high soil phosphorus and organic matter levels have been observing a yield increase from reduced plant populations. The reason behind this response is that high phosphorus levels and increased overall fertility resulting from long-term application of manure can increases the wheat tillering potential, decreasing the need for high plant populations.
Figure 6. Optimum planting rates for winter wheat according to geographical location within Kansas. Figure adapted from KSRE publication L-818, Kansas Crop Planting Guide.
Romulo Lollato, Extension wheat and forage specialist