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soybean field
KEEP IT OR REPLANT? Soybean stands can be deceiving to the eye, especially in narrow rows where stands can be greatly underestimated.

Take a good look at bean fields

Soybean Source: The first step in making an informed replant decision is determining the initial plant stand.

Every spring as soybeans start to emerge, we talk about the importance of taking time for early-season scouting. In a year like 2019, with many of us dealing with marginal soil conditions and less-than-ideal temperatures, checking our fields will be critical. A recent Iowa State University ICM blog post from Alison Robertson, Daren Mueller and Ethan Stoetzer cut to the chase quickly: “There's a high probability some portions of the state will see reduced stand counts.”

While soybeans are fairly forgiving if a stand isn’t perfect and are incredibly resilient when hit by rough weather or pests, getting them off to a strong start is still key to maximizing yields. Soil crusting, seedling diseases, frost, hail, herbicide injury and other unfortunate events can impact early-season stands and leave us wondering if a stand is “good enough.”

Where do we start?

A lot of factors go into deciding if a stand is “good enough,” but it starts with getting accurate stand counts. Of the various methods, the most common is probably counting the number of bean plants per foot of row and comparing that to a chart listing populations based on average plants per foot of row.

Another option that many find easier in narrow rows and especially drilled beans is the hula hoop method. Using a factor from the chart below, multiply the number of plants found within a hoop by the corresponding factor to get your stand numbers.

Multiplication factors for hula hoop method

For the tech savvy, smartphones open the door to plenty of crop scouting apps that will walk you through the process and do the math for about any row spacing or hoop diameter.

No matter what method you use, it’s most important to get a good look at the entire field and take enough stand counts to get a reliable number. In most fields, 10 to 15 counts is solid. If things look good, I may take fewer. If we have thin stands and are trying to make tough decisions, more counts may help.

Look at big picture

Take a “big picture” view of your stand after emergence (or following frost, hail, herbicide drift or something else that impacts your stand). If plants didn’t emerge, or emerged and were subsequently killed by some event, often the “leave it or replant” decisions can be made quickly.

With damaged plants — from storms or herbicide drift, for example — decisions aren’t easy. But often waiting a few days before making any replant decisions is wise. Sometimes we know right away, and sometimes it may be a week or so before we can really tell how well a stand might recover.

Flagging a few representative areas so you can reference their progress day-to-day helps you gauge if plants are recovering adequately or not. Fields typically either look noticeably better or worse after a few days to a week, giving you more insight when making tough replant decisions.

While doing stand counts, look at how uniform your plants are. We don’t need the picket fence beauty of a corn stand since soybean branching can adequately compensate for up to approximately 1-foot gaps resulting from early stand loss or vegetative injury. The table below does a good job of illustrating how the size of plant gaps, and their timing, can impact yield.

How gaps in plant stands/thin stands affect soybean yield potential

With good stand counts in hand, you can estimate the yield potential based on population and planting date. If the stand loss is fairly uniform and remaining plants are healthy, it generally takes a population of less than 75,000 plants per acre to pay to replant in mid- to late May, and less than 50,000 to 60,000 in mid- to late June. If you are below that range and considering replanting, charts like the one below are available to assess the yield potential of a later-planted soybean stand.

Approximate percent of yield at various planting dates

Replant factors 

If you have to replant and are much past your area’s optimum soybean planting window, a few things to consider are:

Canopy. Look for varieties that provide a bushier type canopy; many seed companies have ratings to help guide you.

Maturity. You can probably stick with your original soybean maturities if you plant or replant by around June 1 in northern Iowa, and gradually a little later into June in southern Iowa, where we often don’t think about changing maturities until mid-June. A recent article, Late Soybean Planting Options, from ISU’s Mark Licht and Sotirios Archontoulis goes into more detail on the topic.

Seeding rate. If you are replanting after early June, consider bumping your seeding rate up 10% over what you normally plant and if possible replant in narrow rows (15 inches or less). A higher seeding rate combined with narrower row spacing can help speed plant growth and potential canopy closure, and increase yield potential in late-planted beans.

Weeds. Watch thin stands or later planted beans for weed control issues. You may need to shift postemergence herbicide plans or make another trip to keep fields clean if they don’t canopy well. Also, check your herbicide labels to determine if there are any possible replant issues from products you applied earlier.

Most of the charts and data we use look at soybean stands and replanting from a yield perspective; growers and insurance agents remind me there are some other economic factors to account for. Seed, fuel, labor and machinery expenses have to factor into the decision, and insurance may trump those. Alejandro Plastina, our ISU Extension economist, has some great information on his website about multiple-peril crop insurance.

Hopefully, the weather will improve as we wrap up bean planting, and our scouting trips will find stands that are “good enough.”

McGrath is the research and Extension coordinator for the Iowa Soybean Research Center at Iowa State University. Email him at cmcgrath@iastate.edu.

 

 

 

TAGS: Planting
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