Optimum cereal seeding rates are key to achieving top end yields for any operation. The reality is that the practice to identify and hit the optimum plant stand often gets thrown by the way side, or isn’t fully understood. There are a number of things to be taken into account, and every operation is going to need to factor in different components to hit these plant stand targets. But why is plant stand important? Weed levels, herbicide efficacy, wireworm/cutworm impact, maturity, fusarium head blight, ergot and more can all be much more significant issues on your farm if you aren’t seeding at the right rate! Here’s how being on top of seeding rates can change your operation for the better.
Target plant stand
The first thing to understand is that a seeding rate is the amount of seed needed to be put down to achieve a target plant stand. Seems basic, but a seeding rate is not your target plant stand. A seeding rate, when done correctly, is taking into account mortality, which can come from drill issues, moisture issues, insects, disease, etc. and adjusts the rates needed to target plants per square foot. Gone are the days where we simply say “I seeded a bushel and a half” or “I seed 90lbs/ac”. The seeding rate really should be broken down into seeds per square foot. For example, you can express your seeding rate as “I seed at 120lbs with my 42gram thousand kernel weight to achieve a plant stand of 30 plants per square foot.”
Thousand Kernel Weight
That leads us into the thousand kernel weight formula discussion. There are 3 key factors used to calculate a seeding rate: thousand kernel weight, germination percentage (or vigor) and mortality. All of these can be done by sending in a sample to your seed testing lab of choice.
Thousand kernel weight (TKW) is a weight in grams of 1,000 seeds from your seed lot to help understand the plumpness of kernels. Seeding a seed lot of wheat that weighs 36g (TKW) at 120lbs/ac is going to give you many more seeds than a seed lot that weighs 47g seeded at 120lbs/ac.
The second factor is germination percentage is the percentage of seeds that are capable of germinating and producing a viable first leaf. Also, extremely important in seeding rate. A 98% germination seed lot seeded at 120lbs is going to have more seeds come up than a 78% seed lot (all other things being equal). Vigor is an often noted, and is the percentage of seeds that germinate in cooler temperatures (5-7 degrees C), whereas germination is the amount that germinate at closer to ideal conditions (18 degrees C). Some prefer to use vigor when seeding earlier in the season.
Mortality is the third factor in the thousand kernel weight formula. The general mortality number used in the formula is 10% for cereals, but you can better understand the factors hindering plant stand on your farm. If you determine that on average you lose 4 plants per square foot than what you calculated out in your formula, then you have the ability to fine tune seeding rates over a given period of time.
The Thousand Kernel Weight Formula
So what exactly is the formula? It looks like this:
Lbs/ac seeding rate = target plant stand in plants per square foot x thousand kernel weight (tkw) /germination (in decimal form) / mortality (in decimal form) / 10.4
Here is an example:
If you have wheat with a TKW of 40g, a germination of 95%, an estimated mortality of 8% and a target plant stand of 30 plants per square foot the formula looks like this:
30 x 40 / 0.95 / 0.92 / 10.4 = 132lbs/ac
Variable Rate Seeding
Using variable rate seeding prescriptions removes the guesswork for determining optimal seeding rates throughout the field; allowing farms to achieve higher yields, better grade, and also enjoy easier combining.
When using the conventional constant rate for seed applications, you’re using average rates for the whole field even though there are varying growing capabilities within their fields. By switching to Decisive Farming’s patented Optimize RX-S variable rate seeding, a prescription is created that varies seeding to optimize performance.