Ultra narrow row (UNR) cotton has row spacings down to 20 cm and plant populations are usually above 200,000 per hectare, while conventional cotton is sown in rows one metre apart and has a plant population of about 100,000 per hectare. UNR cotton has advantages in cooler regions where the growing season may be too short for conventional cotton to fully mature, causing a reduction in lint quality.

With its high plant population, UNR cotton needs less bolls per plant to achieve the same yield as conventional cotton. Because UNR cotton does not have to wait for late formed bolls to mature, crop growth can terminate earlier which is an advantage in cool regions.

For most plant species, the optimum spacing is where the distance between rows equal to the distance between plants in each row, which allows maximum access to sunlight, water and nutrients. With 250,000 plants per hectare, the optimum spacing is rows 20 cm apart, and plants 20 cm apart in each row.

For many growers the row spacing for UNR cotton is set by the type of seeding machinery available. Row spacing on Monosem seeders, which are popular, can be changed but the narrower the row spacing, the more seeder units required, and the more expensive the machine.

Apart from a reduction in machinery costs, there are other potential benefits from row spacings greater than 20 cm. With 40 or 50 cm row spacings there is potential to use shielded sprayers and less expensive chemicals. Also, it may be possible to use different harvesting techniques.

Currently harvesting is a problem with UNR cotton. Because of the narrow row spacing, UNR cotton is harvested with a stripper machine which puts more trash and bark in the lint than spindle pickers. Therefore the lint quality of UNR cotton is often reduced.

From the plant’s point of view, narrow rows are best, but wider row spacings have advantages for crop management. So the question is: would using wider row spacings cause a significant reduction in yield?

To quantify the effect of row spacing on UNR cotton yields, trials were conducted in 2000–01 and 2001–02 at Trangie Agricultural Research Centre in the Macquarie Valley. A seeder built as part of a Cotton Research and Development Corporation project on UNR cotton allowed row spacings of 20, 30, 40, and 50 cm.

Because plot width varied with the row spacing, the whole trial was sown on the flat in a border check flood irrigation bay. There were 2.5 metre spaces between every second plot so a ground spray rig could apply insecticides, herbicides and growth regulants.

The seeding rates with the different row spacings were adjusted so all treatments produced the same number of plants per hectare. The number of plants established was 226,000 per hectare in 2000–01 and 216,000 per hectare in 2001–02. Because all plots were in the one irrigation bay, they all received the same number of irrigations, the same insect sprays, and the same growth regulant (Pix) sprays, while defoliants were applied at the same time.

Seed cotton yield was measured by hand picks from each plot, with the number of bolls counted at the same time. In 2000–01, when the outside rows were included in the pick, the 20 cm row spacing yielded more than 30, 40 or 50 cm spacings (Figure 1). The outside rows, with less competition, had larger plants than the rows in the centre of the plot, especially with the narrower row spacings.

Therefore the higher yield of the 20 cm row spacing was probably an error caused by the gaps between plots in the experimental situation. In the following season (2001–02), when the outside rows of each plot were excluded, there were no significant differences between row spacings in seed cotton yield.

Plant height and nodes per plant showed no difference between the four row spacings in either season (Table 1). Total dry matter in 2000–01 was greatest for the 20 cm row spacing. In 2001–02 the advantage of the 20 cm spacing only became apparent late in the crop’s development.

There was no significant difference between 30, 40, and 50 cm spacings in either season. The number of bolls per square metre followed the same pattern as seed cotton yield. In 2000–01 the 20 cm spacing had more bolls per square metre, but in 2001–02, when edge rows were excluded, row spacing had no significant effect.

Conclusions

Row spacings of 20 cm in UNR cotton do produce plants with more dry matter per square metre than wider row spacings, but advantages in seed cotton yield are small. It is likely that benefits from using wider row spacings, such as different harvesting techniques, more weed control options, or reduced cost of seeding equipment, would outweigh any yield penalty.