Rain! Rain! Go away and come back another day (preferably June, July and August)! At this time it would be premature to switch from adapted hybrids to shorter season hybrids. When field conditions delay planting and necessitate switching to less than full-season hybrids, factors to consider in this decision include yield potential of shorter-season hybrids, test weight concerns, drying costs and late-season harvesting capabilities.
Grain corn obtains 90% of its total grain weight by one-half milk line, a maturity stage that even late-planted, full-season hybrids reach in most years. Switching to shorter-season hybrids may be a reason-able alternative from a grain yield perspective if earlier hybrids can produce within 10% of the full-season hybrid’s yield. Generally, this is a more favourable proposition in longer-season areas.
Growing hybrids with a range in maturity provides some buffer against both silking time stresses and end-of-season risks. However, making significant shifts to earlier hybrids should be reserved in the Southwest (>3,200 CHU-M1) until May 30-June 1; in the mid-maturity corn growing areas (2,800-3,200 CHU-M1) until May 20-25 and in the shorter-season areas (<2,800) until May 15-20.
Growing 3,000 CHU-M1 hybrids as the full-season selections allows for switching to hybrids that are 100-150 heat units less without sacrificing excessive yield. If the full-season hybrids are in the 2,800 CHU-M1 range, the odds of dropping to a hybrid 100 heat units less without giving up more than 10% yield are low.
Extensive research across the northern corn belt defines the optimal date when producers should switch away from full-season hybrids. Some of this data is summarized in Table 1- Recommended Dates to Switch From Full-Season Hybrids Across Various Heat Unit Zones. This collection of long-term data took into account yields for hybrids of various maturity ratings as well as deductions for test weight and drying. The switch date indicates the planting date when earlier-maturing hybrids surpass full-season hybrids in terms of net returns (gross returns less drying and test weight losses).
A general rule has been to reduce hybrid maturity by 100 CHUs for every week that planting is delayed beyond the cut-off date for full-season hybrids.
Table 1- Recommended Dates to Switch From Full-Season Hybrids Across Various Heat Unit Zones
| Heat Unit(CHU-M1) | Switch Date |
|---|---|
| >3,200+ | May 30-early June |
| 2800-3,200 | May 20-25 |
| < 2,800 | May 15-20 |
Source: Adapted from R.Iragavarapu. Basing Hybrid Maturity on Long-Term Data. Pioneer Hi-Bred Ltd.
Table 2 shows the effect of planting date and plant population on final grain yield. Yields are based on stands that are normal in terms of uniformity of plant size and distribution. Grain yields for varying dates and populations are expressed as a percentage of the yield obtained at the optimum planting date and population: 64,200-76,200 plants/ha (26,000-30,000 plants/acre). Results will vary depending on location, environmental conditions, hybrid and other factors.
Table 2-Expected Grain Yield Due to Various Planting Dates and Population
| Planting Date |
56,000/ha 22,500/acre |
62,000/ha 25,000/acre |
68,000/ha 27,500/acre |
74,000/ha 30,000/acre |
|---|---|---|---|---|
|
April 20
|
90
|
92
|
94
|
94
|
|
April 25
|
92
|
95
|
97
|
97
|
|
April 30
|
94
|
97
|
98
|
99
|
|
May 4
|
95
|
98
|
99
|
100
|
|
May 9
|
95
|
98
|
99
|
100
|
|
May 14
|
94
|
97
|
99
|
98
|
|
May 19
|
93
|
95
|
97
|
97
|
|
May 24
|
90
|
92
|
94
|
95
|
|
May 29
|
86
|
89
|
90
|
91
|
|
June 3
|
82
|
84
|
86
|
86
|
|
June 8
|
76
|
79
|
80
|
81
|