Replanting soybeans: Variety selection, seed rate

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Where soybean stands have been severely damaged or lost, producers may be thinking about replanting. The later it gets, the more difficult that decision becomes. With later-than-optimal planting dates, should producers make any changes in management strategies? Should a shorter-season variety be substituted as we move into late-June planting dates?

As planting is delayed, the situation begins to resemble doublecrop soybean production. The soybean crop following wheat is usually planted 2 to 6 weeks later than the optimum date for maximizing yields. Since planting is delayed, often until the end of June or early July, one is tempted to switch to a shorter-season variety to ensure the crop will mature before frost.

Although planting a late-maturing variety may increase the likelihood of frost damage, switching to a substantially earlier-maturing variety should be resisted. This is for two reasons:

1) Early-maturing varieties planted late in the season will usually have limited vegetative development (flowering earlier), short stature, and low yield potential.

2) Any variety will have fewer days to flowering, pod development, and maturity when planted late compared to earlier planting dates.

As planting dates get later into June, the day length has begun or will soon begin to shorten and nights will start getting longer. This change in photoperiod causes plant development to speed up. As a general rule, for every three days delay in planting, maturity is delayed by one day.

Since soybean development is hastened in later plantings, the highest yields in a late-planted or doublecropped system are often achieved by using the same variety, or one only slightly shorter in maturity, as what is used in full-season production. Sometimes a slighter later maturing variety is preferred for late or doublecrop plantings, just to encourage more canopy development before flowering.

Other management practices can be affected by late planting, however. Because late planting shortens the period for vegetative growth and reduces canopy development, increasing the seeding rate alone or in combination with narrow row spacing can help the crop compensate by providing the opportunity to produce more pods in the canopy.

Seeding rates can be increased by 30% to 50% in high-yielding environments if planting is delayed until late June or July. Although past research has demonstrated no consistent benefit for narrow row spacing (less than 30 inches) in Kansas, narrow rows may have an advantage in late plantings in the eastern half of the state.

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