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Targeting plant stands for optimum yields

Knowing plant stand targets can determine seeding rates, and help make decisions about reseeding if things go south

A good plant stand is essential to boost yield, so it’s important to use seeding rates that will provide the optimal number of plants per acre or square foot.

Optimal plant density depends on crop, intended end use (i.e. grain, silage or forage), growing region and growing conditions (i.e. average rainfall).

High plant densities have advantages including increased competitiveness with weeds, stand uniformity, faster and more even maturity, and increased yield potential. In cereals, heavier plant stands result in fewer tillers, which may increase uniformity in flowering and maturity — and hopefully increase yield and quality.

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This is particularly important for wheat because when crops flower and head out uniformly, growers can better manage fungicide and insecticide applications to combat wheat midge and fusarium head blight.

However, very high plant densities, especially in cereals, but also in canola, soybeans and others, can increase disease risk, especially in high-yielding environments like the Red River Valley.

When targeting higher plant densities in any crop, agronomists advise growers to, where possible, select semi-dwarf and shorter-straw varieties with stronger stems to resist lodging.

They should also ensure growing conditions and field selection are conducive to higher plant populations — i.e. choose fertile, well-drained fields, and evaluate available soil moisture, temperature and risk of disease.

Fertility, weed and pest control measures must be managed for higher plant populations throughout the season. Growers also need to calculate the economic return of increasing seeding rates to target higher plant populations.

Calculating target rates

Once growers know the range of target plant population for the crop, (which can be found at the websites of Manitoba Agriculture, commodity groups, local agronomists or seed retailers), they need to choose their target. The higher end of the range can give more protection against adverse stresses early in the season such as hail, disease or pests.

The best way to calculate a seeding rate to achieve the desired plant stand is to use 1,000 kernel weight (TKW), which is the weight in grams of 1,000 seeds. Seed size and weight vary with different crops and varieties and even within the same variety. When producers use TKW they can account for any variations to calculate seeding rates, calibrate seeding equipment and estimate shatter or combine losses.

It’s also critical to know the germination rate of the seed lot to determine the amount of dormant or non-viable seed. This can change from harvest to seeding time. After poor conditions across much of Western Canada last fall, seed testing for germination is especially important. If growers find their seed has poor germination, they may need to consider a higher seeding rate to compensate.

They also need to determine likely seedling mortality. It can be caused by dry seed (especially in the case of pulses or soybean), disease, insects, fertilizer burn, incorrect seeding depth, frost damage or extremely dry soil conditions.

Seedling mortality varies. In cereals it’s typically between five and 20 per cent, while in canola it averages 30 to 60 per cent. The best way to estimate mortality rates is to keep detailed records of every year’s emergence rate.

Next, growers must determine the TKW of the seed lot and estimate the expected seedling survival rate — the germination percentage for that seed, minus a percentage to allow for seedling mortality, which also varies from crop to crop. A common range for estimated cereal seedling survival is 80 to 90 per cent, calculated by subtracting five to 10 per cent mortality from 90 to 95 per cent germination.

Alberta Agriculture sets the equation for calculating seeding rate as follows:

Seeding rate (lb./ac.) = desired plant population/ft² x 1,000 K wt. (g) ÷ seedling survival rate (in decimal form such as 0.90) ÷ 10

Example:

Wheat seeding rate (low end target) = 16 plants/ft² x 35 g ÷ 0.90 ÷ 10 = 60 lb./acre (high end target) = 30 x 35 ÷ 0.90 ÷ 10 = 112 lb./acre

Metric Equation 2 seeding rate (kg/ha) = desired plant population/m² x 1,000 K wt. (g) ÷ seedling survival rate (in decimal form) ÷ 100

Soybeans and pulses

Manitoba provincial pulse specialist Dennis Lange said when targeting plant stands for pulses and soybeans, it’s important to address two critical factors: germination and dry seed.

“Typically, if growers are buying certified seed, they know that it’s going to get at least 85 per cent or higher germination,” he said. “If the seed moisture on peas, for example, is 11 to 12 per cent and growers see a few splits, they need to maybe factor that in.”

In that case, Lange suggests they do a soak test. Take 100 seeds, let them soak for five minutes in room temperature water and see if there’s any seed coat separation. This is where the seed coat peels off or has nicks in it. Then count seeds with coat separation.

“If under 10 per cent of the seeds are affected, they should be fine,” Lange said. “If they are upwards of 15 to 20 per cent, I would recommend increasing the seeding rate to compensate.”

Lange said that even if germination is good, wear and tear on the seed during transport and seeding will increase damage to the seeds. This also applies to soybeans, dry beans and peas.

The Hula Hoop test

It’s hard to assess plant stands and calculate mortality by simply looking.

“Growers might look at a stand and say it’s a little thinner, but it looks fine, meanwhile they seeded 210,000 seeds and only have 100,000 come up,” Lange said. “That begs a question why.”

There could be several reasons. Germination may have been poor, or the seed could’ve been dry. It could also come down to equipment. Although a 15 to 20 per cent mortality rate on pulses and soybean is a rule of thumb, mortality can vary depending on the seeding equipment.

“If a grower is using an air seeder, mortality could be 20 to 25 per cent, whereas if he or she is using a planter, it might be 15 to 18 per cent,” Lange said. “It’s something that growers need to check and it’s very simple to do.”

To assess the mortality of a pulse or soybean crop, Lange suggests using a simple, 28-1/4-inch Hula Hoop to do an in-field plant stand assessment after emergence. “Drop it every so often and do a plant stand count.”

With a 28-1/4-inch hoop, he said, the multiplication factor is 10,000. So, if there are 14 plants in the hoop, there are 140,000 plants per acre.

“It’s quick and easy math; you know that if you’re starting at 200,000 seeds and you have upwards of 140,000 seeds, you can figure out what your mortality is.”

Then it’s a question of what to do if the plant stand is less than optimal.

“Typically, if a grower’s down to 80,000 plants per acre, it’s still not a writeoff in soybeans if they’re growing a herbicide-tolerant bean because they can keep it clean through the season and still (achieve a decent yield),” Lange said.

If they’re growing peas and aiming for seven to nine plants per square foot, but find they’re down to three or four plants, things could get challenging, he said.

“They don’t have the same weed control options, and with a thinner stand, weed competition and those sort of factors will play in.”

Lange said he likes to recommend pulse and soybean growers use seeding rates that will build in a bit of insurance.

“For a seeding rate on peas, for example, I would typically shoot for nine to 10 seeds per square foot, because that way it gives (some leeway) to end up with the seven to nine plants per square foot.”

Targets for canola

Weight of canola seed can vary widely, so using TKW is the preferred method for calculating a seeding rate to achieve target plant stands. Current varieties of herbicide-tolerant canola can vary from 3.5 grams to more than 6.5 grams per 1,000 seeds.

The general target plant stand for canola is five to eight plants per square foot. Research suggests that canola growers need to calculate a seeding rate to achieve approximately 10 seeds per square foot, given that average seed survival rates are between 50 to 60 per cent. This should produce the target plant stand and allow for some plant mortality during the growing season.

In pounds per acre, a grower targeting six plants per square foot with an estimated 40 per cent mortality rate would need to seed at 5.8 pounds (2.6 kilograms) per acre if the TKW of the seed is six grams. If the seed weight is four grams per TKW with the same mortality rate, the seeding rate would be 3.8 pounds (1.7 kilograms) per acre to achieve the same plant density.

Other research has shown the yield potential of canola drops significantly when plant densities fall below three to four plants per square foot.

The canola council suggests considering seed size, seedling survival rates and target plant populations when setting seeding rates can minimize risk and save money.

Reseeding decisions

Recommended plant stand targets are different for every crop and there is usually a range that’s useful if the emerging crop hits early difficulties and requires a decision about reseeding. A crop at the lower end of the range might still yield more than reseeding later in the season.

MASC’s website has a chart showing the relationship between seeding date and yield response based on yield data (see resources sidebar).

“Between the third and fourth week of May, for example, is where canola falls below the 100 per cent yield potential line,” said Rejean Picard, farm production extension specialist with the province. “Even though the farmer may have early-season damage, as we get further into the season, the decision is, ‘Is it worth reseeding that crop to potentially achieve less yield rather than keeping the stand?’”

Manitoba Agriculture has a downloadable, Excel-based Crop Reseeding Decision Tool on its website to help estimate the return per acre of reseeding different crops based on their inputs and field conditions (see resources sidebar).

There are other factors to consider besides economics, Picard said.

“They have to look at rotation; what was the crop before and are they going to reseed the same crop or potentially go with a different crop,” he said. “They have to look at the timing, and the fertility… for example, if they seeded soybeans, how much fertilizer will they need to use if they are looking to reseed canola?”

Herbicide residue is another factor.

“If they applied something like a Group 3, they won’t be able to go with a cereal, so will have to stick with a broadleaf, so chances are they will have to reseed canola,” Picard said.

“Also, the field conditions, type of soil and expected moisture are factors,” he added.

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