Seeding by plant population? Don’t get too hung up on it

Crops should be seeded with a target plant population in mind.

These targets can be determined by reviewing research done at various locations.

The target plant populations used in this column are averages from researchers across Western Canada and the northern United States. The experience on your own land under your own conditions and using your own seeding equipment will help you fine tune these rates for what is best on your farm.

Generally speaking, western Canadian farmers have been seeding at rates that are less than optimum.

Increased seeding rates will generally result in several benefits:

  • higher yields
  • better weed competition
  • earlier maturity
  • more even maturity

Much of this is the result of heavier plant stands reducing the number of tillers a plant produces. Most of the yield of any field comes from the main head of the plant. As well, tillers mature later than the main head.

Increased seeding rates will also result in slightly smaller seed size, shorter plants and an increased probability for the crop to lodge.

Three factors must be considered when determining seeding rates:

  • desired plant population
  • seed size
  • expected seed survival

These three factors can help farmers calculate their seeding rate using the following calculation: seeding rate (lb./acre) = (10 X desired plant population (ft2) X 1,000 kernel weight in grams) divided by % survival.

The TKW number is important because seed samples will vary greatly in seed size. This is important with all crops but especially important with those such as peas.

  • 95% germination (75% survival), TKW of 280 grams.
  • Desired plant population: 7 plants/ft2
  • (10 X 7 X 280) divided by 75 = 260 lb./acre
  • 90% germination (70% survival), TKW of 150 grams.
  • Desired plant population: 7 plants/ft2
  • (10 X 7 X 150) divided by 70 = 150 lb./acre.

However, the problem with this calculation is calculating survival rate. This is the largest variable in crops with a fairly tight range of seed size.

How you calculate survivability will significantly affect your seeding rate and may cause surprises when the plant stand is counted. A number of factors can affect survivability, and a number of recommended survival rates can be used.

Some sources suggest that survival will be five to 20 percent less than the germination, and that actual seeding rates can be adjusted higher or lower depending on field conditions, date of seeding, weed pressure, seed placed fertilizer and other pressures that may affect plant stand establishment.

They are correct. These are all factors that affect seedling survival, but there are more.

Good, plump seeds of high quality produce a higher plant survival than smaller, poorer seed. Seeds with a high vigour may require a lower seeding rate than seed with a lower vigour because the survival rate will be higher on the more vigorous seedlings.

The Indian Head Agricultural Research Foundation conducted studies that looked at three seed sizes from a single seed lot.

The researchers found significant differences for small seed sizes as compared to medium and larger seeded plots.

Small seeded wheat crops yielded about three bushels more per acre when a seed treatment was used and, when seeded at a lower rate.

Increasing seeding rates by 25 percent, without a seed treatment, boosted yields by a little more than a bu. per acre.

Adding a seed treatment to the small seed, at the higher rate, increased that yield by less than a bu. per acre over the lighter rate, treated plots.

In summary, treating small seed can cause it to yield as well as large seeded crops.

Seed treatments for large and medium sized seeds made little difference to yield.

Canola research has found that larger seeds increase the emergence and survivability. It shows that a seeding rate of 4.5 to 5.5 lb. per acre improved plant stands, yields and economic returns, independent of seed size.

Seeds in high density seeding situations show higher mortality than when densities are lower.

High seed density occurs when seeding with narrow row spacing and wide shank spacings. For example, in a 1996 paper, Lafond and Derkson showed significant stand reduction as row spacing moved from 10 to 20 centimetres. When crops are put into a stress situation, weaker plants tend to die out. This is also true for later germinating plants.

When density is lower, these plants will survive. This competition may also encourage plants to grow a little quicker and be larger than those without competition.

Heavy textured soil may require an increase because of emergence difficulties. Light textured soils may require a decrease in seeding rate because the soil has less water retention ability and can’t support as many plants.

Higher fertility levels can allow for an increase in seeding rates.

There have been a number of studies that show ammonia based nitrogen (urea) can cause significant reductions in seedling emergence. As well, the salt effect of other fertilizer products can also cause emergence reduction.

The rates at which these occur are quite variable and depend on factors such as the crop grown, seed bed utilization, soil moisture, organic matter content, clay content and pH.

Excessive tillage may cause soil crusting, which will make it difficult for seeds to emerge and thus require an increased seeding rate. On the other hand, studies show an increase or decrease in emergence on no-till versus conventional tillage. Of course, the increase would be in dry conditions.

Seeding equipment has increased considerably over the last 20 years, but we still have a wide variety of seeding tools.

Some growers still seed canola by broadcasting it in front of an air seeder.

This can be effective but may require a higher seeding rate.

On the other hand, modern independent seed row tools in which seed depth is more precisely controlled will likely produce a higher survival rate than systems that do not have an independent depth control.

Packing can improve germination and emergence in most conditions. The difference in packer types shows up when moisture is less than optimum.

Soil texture and conditions and moisture conditions will determine which packer design works best in individual fields each year.

Some openers perform better and thus improve emergence in certain conditions. However, these will vary year to year.

And finally, if you haven’t guessed already, the biggest impact on emergence is the weather that follows planting.

Hot, dry winds or warm temperatures or one cm of rain will dramatically change the emergence of a crop.

As well, there may be other reasons for increasing or decreasing a seeding rate, including:

  • Weed populations: High weed pressures require a higher seeding rate.
  • Available moisture: High moisture levels allow for higher seeding rates.
  • Growth habit: Higher seeding rates can reduce or prevent branching of some plants and reduce yield.
  • Yield compensation ability: Some species such as Argentine canola or mustard will compensate for lower plant stands and allow for a constant seeding rate despite factors that may indicate requirements for higher seeding rates.
  • Yield goals: The higher the yield goal, the higher the seeding rate.

Here are my recommendations:

  • Don’t get hung up on trying to seed the perfect rate. I have never obtained the precise plant stand that I targeted. Within 10 percent is great. A number of factors will affect this from field to field and year to year.
  • Use the best quality seed available.
  • Clean all seed to eliminate all foreign materials and small, shrunken or misshapen kernels.
  • Have germination and vigour tests done on seed that has a germination of less than 85 percent before planting it. A lower germination may not be a big issue, but a vigour test should raises a red flag if there are other issues.
  • Avoid using seed treated with pre-harvest glyph sate unless a seed vigour test has been done.
  • Calibrate your seed drill with each crop.
  • Avoid high seed row applications of urea fertilizers.
  • Adjust your seeding rates on a field and make emergence counts.

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