The most critical planting practices affecting sorghum performance are seeding rate, row spacing, planting date, and hybrid maturity. This article on seeding rate and row spacing complements a companion article in this eUpdate issue on planting dates and hybrid maturity.
Sorghum plants can compensate and adjust to diverse environmental conditions by modifying the number of tillers, seeds/head, and seed weight. For sorghum, seeds/head is the yield component that varies the most.
Seeding rates and plant populations
Recommended plant populations range from 23,000 to 90,000 plants per acre, depending on average annual rainfall (Table 1), with recommended populations of up to 110,000 plants under irrigation. Because of sorghum’s ability to respond to environmental conditions, final stands can vary by at least 25 percent from the recommended values without significantly affecting yields.
Table 1. Grain sorghum recommended seeding rate, plant population, and row spacing based on average annual rainfall.
|
Avg. Annual Rainfall (inches) |
Seeding Rate |
Recommended |
Within-row Seed Spacing |
||
|
10-inch rows |
20-inch rows |
30-inch rows |
|||
|
< 20 |
30-35 |
23-27 |
21-18 |
10-9 |
7-6 |
|
20 to 26 |
35-64 |
25-45 |
18-10 |
9-5 |
6-3 |
|
26 to 32 |
50-80 |
35-55 |
13-8 |
6-4 |
4-3 |
|
> 32 |
70-125 |
50-90 |
9-5 |
4-2 |
3-2 |
|
Irrigated |
110-150 |
80-110 |
5-4 |
3-2 |
2-1 |
* Assuming 70% field emergence.
Lower seeding rates can minimize the risk of crop failure in dry environments. Having more than the recommended number of plants per acre results in fewer fertile and productive tillers and thinner stems, reducing yield in drier environments and increasing susceptibility to drought. Sorghum can compensate for good growing conditions by adding tillers or increasing head size. However, insufficient plants/acre will cap yield potential in better environments and a higher final plant population will be needed to maximize yields under high-yielding environments.
Planting date is a consideration when selecting a target seeding rate (the companion eUpdate article “Sorghum planting considerations: Planting date and hybrid maturity”). Seeding rates should be increased when planting late to compensate for reduced tiller production. Increase rates by 15-20 percent if planting later than the optimal window for your location in the state (Figure 1 of the companion article). Late planting generally results in less tillering due to increased in-season plant stress, and tillers that are initiated are less likely to be productive and contribute to final grain yield, thus decreasing the plant’s ability to compensate for inadequate stands.
Row spacing
The other factor that can influence yield is row spacing. Narrow rows increase early-season light interception and water use, provide faster canopy closure, reduce evaporation losses, and improve suppression of late-emerging weeds. Narrow rows can aid in maximizing sorghum yields in high-yield environments. A comparison between wide (30-inch) vs. narrow (15-inch) row spacing showed an overall yield benefit of 4 bushels per acre with narrow rows, with narrow rows out-yielding wide rows in 71 percent of all observations. The most consistent response to narrow rows was seed when yields were above 70 bushels per acre. Research in southwest Kansas showed a variable response to narrow rows over wide rows, with an advantage to narrow rows in two years, no response in one year, and a disadvantage to narrow rows in the other year (Table 2).
Table 2. Grain sorghum yield of four narrow-row (7.5” and 15”) plant populations compared to wide-row (30”) at Garden City and Tribune, KS.
|
Seeding rate |
Garden City |
Tribune |
||
|
(Seeds/acre) |
2016 |
2017 |
2016 |
2017 |
|
20,000 Narrow1 |
87 |
43 |
135 |
81 |
|
40,000 Narrow |
103 |
49 |
136 |
95 |
|
60,000 Narrow |
101 |
61 |
137 |
102 |
|
80,000 Narrow |
97 |
67 |
137 |
101 |
|
Planted2 |
118 |
49 |
130 |
90 |
1Narrow row plots were seeded on 7.5” row spacing in 2016 and 15” row spacing in 2017.
2Planted plots were seeded at 27,000 seeds per acre in 2016 and 20,000 seeds per acre in 2017 at Garden City and at 40,000 seeds per acre in both years at Tribune.
Should populations be adjusted with narrow rows?
Research indicates that different row spacing doesn't necessarily change the plant population required to maximize yield. However, populations on the higher end of the recommended range for a given yield environment would be encouraged with planting on narrow rows. An important consideration is the seeding equipment used to plant narrow-row sorghum. Some equipment designs will struggle to obtain consistency of depth, and seed-to-soil contact is customary with row-crop planters, thus reducing final emergence. It's also important to recognize that the metering systems typically employed for narrow-row seeding generally do not singulate seed, but work on a volumetric basis. With these types of equipment, it is important to follow manufacturers' recommendations for metering modifications for low rates of small seed. Also, producers should consider differences in seeds/lb for the sorghum hybrids that they plant and perform calibrations when changing seed sizes.
Should row spacing be adjusted for planting dates?
Research in Kansas has shown that there was essentially no difference in yield between 15- and 30-inch rows for late-May plantings. However, there was a 10-bushel yield advantage for 15-inch rows for late-June plantings.
Planting depth
Seed placement is also a critical factor when planting sorghum. The optimum seed placement for sorghum is between 1-2 inches deep. Shallower or deeper planting depths can affect the time between planting and emergence, affecting early-season plant uniformity. Seed should be placed deep enough to ensure placement into uniform soil moisture and temperature conditions while not exceeding the capability of seedling emergence.
Summary
Suggested resources for grain sorghum from K-State Research and Extension
“Narrow-row Grain Sorghum Production in Kansas” MF2388 https://bookstore.ksre.ksu.edu/pubs/MF2388.pdf
“Sorghum Growth and Development” poster (updated in 2023)
https://bookstore.ksre.ksu.edu/pubs/MF3234.pdf
Lucas Haag, Northwest Area agronomist
lhaag@ksu.edu
Logan Simon, Southwest Area Agronomist
lsimon@ksu.edu
Tina Sullivan, Northeast Area Agronomist
tsullivan@ksu.edu
Tags: planting sorghum grain sorghum seeding rate row spacing