The Math Behind Fall Seeding Rates
Every fall there is talk of increasing or decreasing seeding rates, but why? It is all about the SWHEAT spot. The SWHEAT spot refers to the number of heads measured per square yard. In an ideal stand, we want to see 450-600 good heads per square yard says Phil Needham. This provides optimal yield potential with minimal lodging risk.
Hitting the SWHEAT spot all starts in the fall! Using a proper seed source with a consistent and known seed size with a guaranteed germination is imperative. Make sure equipment is properly calibrated to put out the desired rate. Having residue chopped and spread evenly from the combine will prevent hair-pinning which will greatly reduce uneven emergence. Lastly, a starter fertilizer source containing phosphorus to ensure the crop has enough nutrition to get up and off to a good start is key. Establishing a strong root system in the fall is crucial for nutrient uptake and winter survivability.
Determining the right seeding rate depends on a few different factors such as planting date, seed size, previous crop, field conditions and variety. Although more is perceived to be better, more seed can actually negatively impact yield in certain situations. As an example, planting early September with a high seeding rate increases the risk of lodging as the plants have more time to tiller, making the canopy very dense and crowded. This could put the head count in the bottom right hand corner (1,000+ heads/square yard), which we do not want! Whereas, if we are planting late October – into November we need to make sure seeding rates remain on the high end to compensate for lack of tillering due to a reduced growing season and possible plant loss through the winter. In this scenario, if we end up with 20 viable plants per foot of row with at least one tiller, we will be in the SWHEAT spot.
The rule of thumb is to target 1.6 million seeds/ac at the optimum date for the region you are in. If we are planting before the optimum date, reduce rates by 100,000 seeds/ac/week. If planting past the optimum planting date, increase by 100,000 seeds/ac/week to a maximum of 2.2 million seeds/ac. See map below to determine the optimum planting date for your region. The main reason for increasing or decreasing rates comes down to stand and tiller management to maintain 450-600 good heads/square yard.
- Planting Early -> Less seeds/ft of row -> Longer Growing Season -> More Tillers = 450-600
- Planting Late -> More seeds/ft of row-> Less Growing Season -> Less tillers = 450-600
With new varieties on the marketplace, some offer small seed size which allows for significant seed savings because we only need 120lbs/ac or less to get the desired final stand. For example, if we are planting 150 lbs/ac because that is what the drill has always been set for, using a variety that is 14,000 seeds per pound late September we would be putting down 2,100,000 seeds per acre. The lodging risk would be through the roof and the risk of snow mould increases as well as unnecessary seed costs.
Now that we have determined our seeding rate, we need to make sure the rate we have the drill set for is what’s going in the ground. If the drill is not calibrated properly, the drill could easily be off 10-20% from the chart setting. The best method is by using a scale, whether it is a digital scale on the drill or a scale on the tailgate of the truck, just make sure any scale you use is calibrated. The second method is by counting the seeds per foot of row. Either tie up a closing wheel or use a heavy rubber mat similar to one that Phil Needham used at our drill clinic last summer in the photo below. Count the number of seeds per foot of and make the necessary adjustments to get the proper seeding rate. Note; if using a mat, seed firmers will scatter the seeds making them very difficult to count.
Maximizing the potential of every plant is key for success. Keeping stands in the SWHEAT spot is a balancing act between being too thick or too thin. Serious wheat growers strive to hit the SWHEAT spot year after year to capture the most yield and ROI. Are you hitting the SWHEAT spot?