Section 1 - Stand Establishment, Fertility & Water Use

Base Replanting Decision On Calendar, Stand Some crops such as small grains, canola, sunflower and soybean can compensate for low plant populations that may occur as a result of poor stand establishment. These crops will compensate for stand reduction through tillering, branching or increased head or kernel size. Following are minimum stands of several crops to avoid major yield reductions when making decisions on tearing up the field and replanting.

Around June 10, a corn crop with stands of less than 12,000 plants per acre could be torn up and replanted to a crop like sunflower or soybeans that can be planted at this date and still mature. By June 15, however, the decision may be to keep a stand of 12,000 to 14,000 plants per acre because it would be too late to plant a good alternative.

Uniformity of stand is the key to evaluating a poor stand. Even stands below 12 plants per square foot of barley and oats have yielded near normal because they typically tiller more than spring wheat, which typically tillers more than durum. If there are no large skips in the field, fairly low plant populations of soybeans (75,000/ac), dry beans (50,000/ac) and sunflower (10,000 to 11,000/ac) can still maintain yields. These plants have the ability to branch or flex and fill in space. This is not as true with corn, and weeds also become a bigger problem.

About the only replant choice in mid June is flax, buckwheat, early-short season sunflower or millet. Other options to consider would be to grow a crop for hay, or plant winter wheat in the fall. Remember to take crop insurance into consideration, and herbicide used in the prior planted crop and whether it may create a problem with injury to a crop change.

What about replanting to improve stands? Remember that a replant decision costs dollars and extra time. In many cases the later planted crops will yield less than the early planted and lower stand established crop. The later replant also may put the development of the crop into a more stressful period, such as a high temperature, drought or risks of fall frost. Also, the harvest period would be extended into the fall when weather conditions can be difficult.

Refer to NDSU Extension Circular A-934, “Replanting after Early Season Crop Injury” for further information, available on the web at: www.ext.nodak.edu/extpubs/plantsci/crops/a934w.htm.

Spring Wheat Replanting Guidelines

• If reduced stand is uniform (no big skips or holes) keep stands of 15 plants/sq ft.

• If skips are larger (3 to 6 ft) or holes are 4 to 6 ft in diameter and the stand is 18 plants per sq ft or less, then replant if moisture is adequate.

• After June 1 in ND and northern MN, and May 15 in southern MN, replant with a crop other than wheat or barley since yields are reduced by about 50% when planting after these dates compared with normal planting dates.

The Ideal Spring Wheat Stand

Agronomists generally agree that an optimum spring wheat stand ranges between 28 to 32 plants per square foot (1 acre = 43,560 square feet; thus 28 to 32 plants per sq ft is about 1.2 million to nearly 1.4 million plants per acre. Thirty plants per square foot would be about 1.3 million plants per acre). A stand much below 1.2 million plants/ac may reduce yield potential, while a stand much over 1.4 million plants per acre will increase lodging potential.

Soybean Stand Evaluation

Under normal planting conditions and soil temperatures, soybean seedlings should emerge five to nine days following planting. Early planted soybeans without a fungicide seed treatment are more vulnerable to fungi like Pythium which thrives in cool wet soils. Once soil temperatures warm up, other diseases like Phytophthora root rot and Rhizoctonia can reduce soybean stand. It is critical to evaluate soybean fields after emergence and determine what kind of stand you have. The good news is soybean is a very forgiving crop and has a great capacity to compensate for reductions in stand. University of Minnesota research on stand reduction suggests that substantial reduction in plant stands has little effect on yield (figure 1). It takes a 50% stand reduction to reduce potential yield by 10%. Evaluate stands closely before deciding to replant, also considering the calendar date. Weigh yield loss because of stand reduction against the penalty for delayed planting. U of M research on planting date suggests that substantial yield reduction does not occur until after June 1 (figure 2).

Checking Nodules on Field Peas, Soybeans

Field Peas: Field peas that have been emerged for 2 to 3 weeks should have nodules forming on root hairs of both the primary root and lateral roots. Healthy nodules actively fixing nitrogen for the plant are pink or red inside. White, brown or green nodules indicate that nitrogen-fixation is not occurring. If the pea plants appear yellow and the roots appear not to be nodulated, then it’s suggested to get the field top-dressed with nitrogen. Nitrogen fertilization after planting is not generally recommended, as too high levels of nitrogen fertilizer applied can and will inhibit nodule formation and ability to fix N for the legume plant for later grain development and protein content of the seed. When checking the health of nodules also check root proliferation, and look for any root rot diseases or insect damage. Diseased roots will have low nodule formation.

Soybeans: nodulation can be seen shortly after emergence (VE), but the plant is not actively fixing nitrogen until the V2 to V3 stages. Soybean plants that are 5 to 6 inches tall should have their second unfolded trifoliolate leaflets (V2 stage) totally unrolled. The number and nodules formed on the soybean roots along with the amount of nitrogen fixed increases until the R5.5 stage. Nodules actively fixing nitrogen for the plant are pink or red inside. White, brown or green nodules indicate that nitrogen-fixation is not occurring. Nitrogen fertilization after planting is not recommended as nitrogen fertilizer applied to active nodules will render these nodules inactive or inefficient, depending on the amount of nitrogen applied. Soil nitrogen is utilized over fixed nitrogen, if available in large amounts. Check the health of your soybean nodules and check root proliferation. At V2, soybeans should be rooting down six inches into the soil and by V5 will completely reach between 30-inch rows, making any cultivation at V5 needing to be very shallow.

When checking soybean and pea plants for nodules, don’t pull the roots directly from the soil, since that will slough off nodules and result in an inaccurate count. It’s best to use a small shovel, spade or trowel and dig the soybean roots carefully and shake gently or place in a bucket of water to wash soil off the roots. Nodules then can be counted and examined for viability. It’s suggested to check at a minimum of five sample sites in a field to make a good assessment.

Also, one should go back and check again just before the flowering stage to ensure that the N fertility for the legume will be adequate. Nitrogen fixation ceases with the onset of pod formation in peas and lentils. Good nodulation and N fixation not only benefits the growing crop but also is most beneficial to the crop that will follow in next year’s rotation. These principles and methods of checking for nodules are also suggested for lentils and chickpeas.

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