By Trevor Philp; Summer Crop Agronomist, Pacific Seeds

Its long been understood that uneven gappy plant stands reduce yield and promote unevenness in corn crops, but it has been assumed that grain sorghum is not affected by this. Grain sorghums ability to tiller and flex its head size has been thought to allow this versatile crop to compensate for uneven stands. A replicated trial conducted at Pacific Seeds research farm at Gatton in Queensland has busted this myth, yield loss of up to $160/ha was recorded for gaps in the stand greater than 45cm even when the neighboring row was perfectly spaced Figure 1. Yield loss recorded in this trial warrants growers spending more time trying to ensure more even spacing.

Figure One. Yield loss caused by variable plant spacing. (Source: Pacific Seeds Research Farm Gatton, Queensland)

Trial Lay out;
The trial was planted on the 15/09/2009, into a full profile; the row spacing was 75 cm and the variety used was MR-Eclipse. Each treatment was hand thinned to the required spacing. Plots then received an early irrigation to ensure good root growth and even tillering. Each plot consisted of 4 rows by 8.5 meters long, 2 out side rows perfectly spaced at 22 cm between plants to aid in compensation, the two inside rows were the treatment rows.

The four treatments were;

  • Untreated, perfect spacing
  • Double, every third plant placed within 5cm of the forth plant
  • 3 in 1 out, miss every third plant
  • 2 in 2 out, 2 plants evenly spaced, 2 missed.

Figure 3. Treatment 3. 3 in 1 out.  (Source: Pacific Seeds Research Farm Gatton, Queensland)

Results and Discussions;
The trial grew very well through the vegetative period, and early tillering was high in all treatments, timely rainfall at booting ensured good grain set even though temperatures were hotter than average. The treatments in this trial had optimum conditions to compensate for the gaps in the stand, while there was some compensation through higher tillering head numbers were still reduced compared to the untreated; Figure 4.
Individual head weight also increased compared to the untreated but not enough to compensate for the reduction in head number. The boarder rows which were perfectly spaced showed no significant improvement, indicating that the inter plant spacing has a higher impact on head and grain number than the row spacing.

Figure 4. % Heads per plot compared to Perfect spacing. (Source: Pacific Seeds Research Farm Gatton, Queensland)

It appears that inter plant spacing is more critical in grain sorghum than once thought, other benefits beside yield improvement exist such as;

  • Better competition against weeds,
  • Improved crop evenness; which will aid in pest and harvest management.

This trial had very good early growth which allowed for maximum compensation, yield loss is expected to be higher when early conditions reduce tillering of grain number is reduced through stress.

It is time well spent insuring planters are set up well for each field and for each seed lot; there is also a good case for investment in precision planting equipment. At $ 160/ha yield improvement it would be easy to justify investing in new equipment, although a planters only as good as the operator.
Key points to ensure even establishment.

  • Machine is well maintained, eg; chains are well oiled, discs and points are not worn
  • Machine is correctly calibrated,
  • Planting speed, slower is better for even depth and seed drop
  • Vacuum planters; appropriate vacuum setting for seed size
  • Good quality seed
  • Treated seed for insect and disease control

Seed depth is correct and press wheel pressure is adjusted to suit the conditions
Soil temperatures are optium for the crop.

Many thanks to Trevor Philp Pacific Seeds summer crop agronamist for providing this trial data.