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Replant decision deserves more than a quick glance at the field

Agriculture.com Staff 07/06/2010 @ 5:21pm

A quick glance at your field probably won't give you the information you need to decide whether to replant soybeans this year because you're likely to underestimate your existing plant population. How do you know when it's time to replant? Several university experts offer advice including an interactive tool that could help you decide.

Before you act, you should think about any factors that caused stand reduction, the percentage of population lost, and the costs of replanting.

Dave Nicolai, University of Minnesota Extension crops educator, offered the following advice in a recent Extension release:

Accurately estimating soybean plant population after wet, cold field conditions is important before making replant decisions.

Plant population should be based on an accurate stand count, along with factors such as yield potential of the existing stand, date of replanting and the real cost of replanting. The existing stand will be determined by evaluating uniformity of stand and overall health of plants.

Only some areas of the field may require replanting if the majority of the field seems to have enough viable plants remaining. It is important to wait three to five days after a crop has been damaged (or has emerged) before replanting. Injury can look very serious the day after the event, but recovery may be possible.

Previous University of Minnesota studies have shown that a final stand as low as 78,000 plants per acre has consistently yielded more than 90% of the optimum plant population (optimum population is listed at 157,000 plants per acre). The reason is that soybean plants can compensate for missing plants and reduced stands by branching out to make up for a thin stand.

But keep in mind that the lower the stand count, the more that weeds will become a problem due to less shading, especially later in the growing season. If a reduced stand is saved, weed control must be a priority. A final low plant population of 39,000 by itself will result in 75 percent of optimum yield.

The first step to determine the population of live plants remaining in your field is to count the actual number of living plants per foot of row in several areas. Multiply your average number of plants per foot of row by 17,424 for 30-inch row widths or 23,764 for 22-inch row widths to obtain total plant population. Convert this number to a percentage by dividing it by 157,000.

Incidence of soil-borne fungal pathogens like Pythium spp. and Phytophthora, known to cause important losses in germination and seedling stand, may increase under water logging. Another seedling pathogen - Rhizoctonia - is not favored by oxygen-depleted environments such as flooded soils.

Flooding and pathogens will have a greater impact when poor-quality seed is used than when the seed is not mechanically damaged and is free of seed-borne pathogens. For more information on seed and seedling pathogens, see "Fungal Seedling Diseases in Soybeans" from the University of Minnesota:

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