Greer County Cooperative Extension Service
Plant Populations Very Important - Gary Strickland - Greer/Jackson County Cooperative Extension Ag. Educator/SWREC dryland Cropping Systems Specialist
Grain sorghum, in many cases, becomes a crop to just try or fill in with. But, with proper management this crop can be a good primary or rotation crop in production systems. Some of the primary management considerations for grain sorghum production include variety selection, plant populations, fertility, herbicide choices, and planting date. Let’s look at plant populations for this article.
One of the worst mistakes to make with grain sorghum is over populating the field. This generally happens when planting by “pounds per acre” versus “plants per acre”. Grain sorghum varieties vary in the number of seed per pound anywhere from 12,000 to as much as 18,000 with the average falling somewhere around the 14,000 to 14,500 seed per pound. While today’s varieties don’t vary as much, a 2000 to 3000 seed per pound difference can make quiet a difference in the final field plant population. Limited work done over the past decade has shown that final stands of 2 to 2.5 plants per foot of row, across allrow spacing's (with the exception of narrow row grain drill row spacing), for dryland grain sorghum is adequate in Southwest Oklahoma. So, for 30 inch row spacing (17,424 plantable linear feet of row in an acre) a final stand of ~ 35,000 to 43,000 plants per acre would be sufficient. For a 40 inch row spacing (13,069 plantable linear feet of row in an acre) a population of 26,000 to 32,500 plants per acre would be the final stand.
In general recommendations for Southwest Oklahoma should be somewhere between 32,500 to 46,000 plants per acre, final stand. To achieve this goal use the following equation for calculating number of seed to drop per acre: Desired final stand/(germination%*emergence%); i.e.: 35,000 seed per acre desired stand/(90% (.90) germination % for the seed*95% (.95) emergence %) = 40936 seed to drop at planting. I have used a 95% emergence factor due to today’s max emerge type planters where excellent soil to seed placement is easily achieved. However, if using a grain drill or older row planter then the emergence % percentage may not be as high.
The bottom line, it is better to make an error having a low plant population and incrementally adjusting up in the coming years than having too high of a plant population and having to adjust down. By keeping the plant population lower the grain sorghum plant can compensate by making early tillers with heads the size of the primary sorghum head.
A classic example of this occurred in the 2016 Tipton Grain Sorghum Variety Trail. The target plant populations were not achieved at this location with the resulting final plant stand (~ 17,000 plants per acre on 30 inch row spacing) being less than half of what was desired.
However, 2016 brought an abundance of rainfall (24 inches in season) for the Tipton location and the sorghum plants averaged 4 heads per plant. This is a high tillering amount for the sorghum plant (in my experience the normal is closer to 2 heads per plant) and was indicating that there were enough resources in the field in terms of fertility and moisture for a higher plant population. However, the plant compensated for having these abundant resources in conjunction with a lower plant population and tillered more heavily than normal and produced an average yield of 101 bushels per acre across maturity types. That is 5656 pounds per acre which is an outstanding dryland yield!
Now, this will not happen every year in Southwest Oklahoma where rainfall distribution is typically not this high for the growing season. But you will stand a better chance of making a good yielding crop by not over populating a field and increasing stress on the crop due to limiting the available resources per plant population.
When grain sorghum is over populated in a field it tends to make numerous smaller heads across the production season due to limited resources and stress related to a higher plant population. A lot of time these look like succor heads and tend not to have the same yield potential or quality of crop as a fully developed head.
Certainly other factors come into play in terms of achieving the desired yield goal with a grain sorghum crop (fertility, herbicide choices, variety*maturity selection, planting date) but starting out with the right plant population is one of the best first steps a producer can make.