Tillage

Every tillage pass results in soil moisture loss through evaporation. If dry conditions persist at planting in your area, excessive tillage may evaporate precious soil moisture. Keep tillage to a minimum if climatologists forecast dry weather for the rest of the growing season. No-till operators will likely not have a problem finding good soil moisture conditions at normal seeding depths; however, those with some degree of tillage may need to consider deeper seeding depths to reach moisture (see Seeding Depth below) unless their topsoil has been replenished with recent rain and or snow.

Row spacing

Responses to corn row spacing are related to light interception. What I understand about this is that yield is optimized if corn canopies intercept 95% of sunlight at silking. Interestingly, modern Iowa corn hybrids in 30-in. rows at normal plant populations intercept 95% of the light at silking. That explains why we rarely see row spacings narrower than 30 in. improving corn yields over those of 30 in. That means that row spacings of 20 inches, 15 inches, 12 inches and/or twin rows rarely improve Iowa corn yields. On the other hand, yields are rarely reduced with row spacings narrower than 30 in. Remember though, yields are likely reduced in rows wider than 30 in. However, if early-season stress of some kind reduces canopy closure of corn in 30-in. rows, narrow rows may out-yield those of 30-in. rows. Dry conditions during vegetative development could do this.

Seeding rates

Optimum plant populations for normal Iowa conditions range between 34,500 and 37,000 plants per acre. Make sure to add in your normal attrition rate – failure to germinate and/or produce a plant – to compute your seeding rate. Attrition rates in our studies range from 3% to 7%. If your attrition rate is much greater than that, investigate the causes carefully!

One more note on seeding rates: If conditions are dry at planting and you expect them to persist, this is the year to be conservative when adjusting the planter. On your higher-yielding fields, plant on the average or high end of what you’ve done before – or up to the range suggested in the last paragraph. On fields with less yield potential, I’d back off a bit on seeding rates. Remember though, modern hybrids are more able to tolerate stress than older hybrids.

Seeding depth

Standard corn seeding depth recommendations for Iowa are 1.5-2 in. As we mentioned in the early March ICM News, this seeding depth allows for quick emergence and maximum emergence rates as well as proper root system development. As discussed above, soils with moisture levels at field capacity provide the moisture level adequate to germinate seeds.

If soils at 1.5-2 in. are drier than field capacity, you could: plant deeper, adjust planter down pressure differently or plant at 1.5-2-in. depths and hope for rain (see Mark Hanna’s ICM New article for more information). The first two options are most reasonable unless you are certain of timely rain, then the third option may be best.

Corn can emerge from fairly deep planting depths depending on soil texture. For example, if necessary to reach adequate moisture, plant corn as deep as 3-3.5 in. on clay soils, 4-4.5 in. on loam soils and 5-6 in. on sandy soils. Remember though, if soil moisture is adequate at shallower planting depths, there is no reason to plant that deep. An array of problems accompanies unnecessarily deep planting: slower and more variable emergence, increased vulnerability to soil crusting and seedling diseases and insects. All of these reduce yield potential…and the crop is hardly out of the starting blocks.

Allow me to add one more thing on seeding depth. Seeding depth that varies among plants, especially in the same row, reduces yield through uneven emergence and thus uneven competition. This sets up a pecking order that chips away at yields all season long. In my experience, planting too fast is a major contributor to this. Time spent carefully adjusting your planter and speed for conditions specific to each of your fields will pay!

 

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