By Christopher Graham
Seeding rate can have significant impacts on sorghum yields. Consequently, seeding rates vary dramatically across the sorghum producing regions of the state. To get a better understanding of how seeding rate affects yield, planting rate studies were established at various locations across the central and western portions of South Dakota. In these trials we assessed sorghum yields at planting populations of 30,000, 60,000, 90,000 and 120,000 pure live seeds (PLS)/ac, which corresponds to roughly 2.5, 5, 7.5 and 10 lbs./ac, respectively. All plots received 125 lbs. N and 30 lbs. P at planting and were planted on 30” row spacing.
Results varied dramatically at each site. Pierre (Dakota Lakes Research Farm) had yield increases of nearly 30 bushels/ac by going from the 30,000 to 90,000 PLS/ac. Wall and Kennebec, on the other hand, were far less responsive over that same range. Some of these differences are related to growing conditions and some are related to geographic location. Certainly, longer-term observations would improve seeding rate recommendations.
Grain sorghum yields at increasing seeding rates. Sites represent four site-years in central and western South Dakota: Wall in 2015, Pierre (Dakota Lakes) in 2016 and 2017, and Kennebec in 2016.
Making a few assumptions for the cost of seed and the price received for a bushel of sorghum, it is relatively straight-forward to predict the break-even value where increasing the seeding rate no longer returns any additional profit from these studies. However, there is still a great deal of uncertainty involved in these estimates. For example, it is difficult to account for additional fertility required when plant populations are increased. Row spacing can also have large effects on resource use. Additionally, tillering can have a large effect in some years. Tillering can compensate to some extent for low populations under favorable growing conditions. Cool, moist soils are conducive for increased tillering. However, under drought conditions, the sorghum plant will abort tillers to save the primary shoot. At the same time, higher plant populations require more resources to sustain plant growth and could suffer from a lack of soil moisture under drought conditions. Therefore, the trick is trying to figure out how to balance the two extremes.
At all sites, yields increased with increasing populations. If one were to assume a breakpoint as a return of approximately $1/1000 additional seeds, a planting population of ~80,000 – 90,000 seeds (6.5 to 7.5 lbs/ac) was optimal at responsive sites (ie. Pierre) despite marginal increases in yield even up to 10 lbs/ac. This is assuming $3.30/bu and $130 for a 50# bag of seed. On the whole, these studies do suggest that there is room for profitable increases in planting populations for sorghum where growing conditions are favorable. On-farm experimentation on small strips would be extremely beneficial in corroborating these results. Even at lower yielding sites like Wall, increasing planting populations from 60,000 seeds (~5 lbs) to 80,000 seeds/ac (~6.5 lbs/ac) was still profitable and feasible in most years, particularly under no-till practices. Moving east, however, the probability of receiving a higher yield response to increasing planting populations is much improved.