Lecture 7 Seed Density

Introduction to Sowing Rates for Seed Crops

Understanding the appropriate sowing rate for crops aimed at seed production is crucial for optimizing yield. This involves recognizing the balance between maximizing seed yield per unit area and seed yield per individual plant. Factors influencing sowing rates include crop type, density, and pest pressures, among others.

Yield Response Curves

Density vs. Seed Yield

The classical yield response curves illustrate how relative seed yield varies with density per square meter. Each crop has a peak point of density that produces the highest yield, with lower yields recorded at both higher and lower density levels.

• Increasing Density: As the planting density increases, the yield per individual plant decreases. This highlights the necessity of finding an optimum density that maximizes total yield without compromising individual plant performance.

Varied Crop Needs

For food crops like cereals, the optimum density for seed production is generally equivalent to that used for grain production. However, for crops providing forage or leafy vegetables, the ideal seed density for seed production is significantly lower.

• Example: Rye grass used for pasture may be sown at 15-20 kilograms per hectare, but for seed production, the recommended sowing rate drops to 2-10 kilograms per hectare.

  • Most farmers utilize around 8 kilograms per hectare for diploids.

  • In red clover pastures, a sowing rate of 6 to 8 kilograms per hectare is common, halved for seed production.

Adjusting Sowing Rates

Sowing rates can be fine-tuned not only by adjusting the amount of seed but also by altering row widths. Effective management requires an understanding of pure live seed calculations, which are crucial for determining seeding rates.

Pure Live Seed Calculation

• Definition: Pure live seed (PLS) is calculated by multiplying the seed's germination percentage by its purity percentage and dividing by 100.

• Typical Recommendations: Recommended sowing rates are often based on the assumption of 98% purity and 90% germination. If seeds' germination rates are lower, adjustments to the sowing rate are necessary.

Importance of Seed Form

Additionally, the form of the seed plays a significant role in determining final sowing rates:

• Pelleted Seeds: Seed coatings can vary in weight from minimal (micro-coat) to significant (double weight). This affects sowing rates and plant density.

  • Tetraploid seeds, for instance, weigh more due to an increased chromosome count, necessitating higher sowing rates.

Row Configurations and Plant Density

The distance between rows also impacts plant density and, subsequently, crop yield.

• Example: Sowing at 10 kilograms per hectare in 15-centimeter rows differs significantly from a 30-centimeter row setting regarding plant density.

  • Wider row widths may lower sowing rates but demand careful spacing to maintain adequate density.

  • In white clover, wider rows assist in allowing stolons to propagate and produce flowers, increasing overall yield.

Challenges with Wider Rows

• Wider rows can lead to difficulties in harvesting; crops may lie flat on the ground, leading to soil pickup and contamination with unwanted soil pests, impacting seed quality.

• A specific soil concern mentions ``blind seed disease'' that is facilitated by overly wide rows promoting disease spread before flowering, leading to significant losses as affected seeds appear normal but are devoid of viability.

Research and Trial Designs

Research methodologies such as the Nedler radial design allow for efficient evaluation of sowing density by reducing the area required for density trials, utilizing a radial approach to vary density from the center to the outer parts of a trial plot.

Seed Costs and Availability

The costs associated with high-grade seeds are significantly higher than basic seeds, potentially impacting sowing strategies. Seed companies often prefer cultivators who can efficiently sow at lower rates due to these expenses.

Establishment Effectiveness

Several factors can affect the establishment and effectiveness of seedlings:

1. Seedling Vigor: How robust are the seedlings at germination?

2. Environmental Conditions: Soil moisture, temperature variances, and pest pressures like slugs, along with diseases such as damping-off can jeopardize establishment.

3. Frost Lift Risks: Early planting that encounters frost may lead to 'frost lift,' where seedlings are pushed out of the soil, hindering establishment.

Navigating these variables, particularly with colder temperatures in late planting seasons, encourages some growers to increase sowing rates to compensate for potential losses.

Examples of Sowing Practices

Ryegrass and Carrots

• Ryegrass: Past experiences reveal shifts from 30 kilograms down to 10 kilograms per hectare as growers gain confidence in managing practices effectively.

• Carrots: Growing density varies greatly; seed to seed methods yield far better than traditional transplant systems.

  • When comparing densities, rapidly increasing plant numbers for seed yields can lead to substantial increases in overall yield, illustrated by yield graphs correlating plant density with seed output.

Peas and Beans

In studies, it is noted that while increasing density reduces seed yield per plant, it contributes significantly to overall yield through an increased number of pods. A balance of plant density is essential for pod production, as higher plant numbers often do not equate directly to more seeds per pod.

Considerations for Seed Quality

The maturity uniformity of plants can be adversely impacted by lower planting density which allows secondary tillers to dominate growth dynamics. Effective management of crop density while also maintaining high-quality seed production necessitates careful monitoring of primary flower production over secondary branches.

Roguing

Roguing, or the elimination of off-types in seed crops, is another vital practice, with higher densities complicating identification and removal of undesired plants.

Final Thoughts

The journey toward achieving optimal seed yield is influenced by a constellation of factors—density management, seed quality, environmental conditions, and pest pressures—alongside economic considerations about seed costs and availability. Approaches must be adapted over time based on agronomic success and the confidence of the farmer in their management practices.