ISSUE 7 June 12, 2003
SMALL GRAINS IN THE REPRODUCTIVE PHASE OF GROWTH
It has generally been a cool and wet spring. Small grains that were planted in April should now be nearing the seventh leaf stage. There is, of course, a substantial area of the state planted much later, even a few fields just now starting to emerge! In the past week I have received several calls from growers wanting to know about top-dressing N to wheat. Some were interested because they felt they had the beginnings of an exceptional crop and they wanted to make sure that it would finish well, others because of generalized yellowing and poor early growth. In much of the state persistent rainfall has delayed many field operations. Those who want to topdress N now wonder if it is too late?
Previous research has shown that the best the best time to topdress N is during the 3-5 leaf stage. For more details see Frazen and Goos, 1997:
This means that for late plantings now is the time to topdress N if its needed. For early plantings, however, the question is, now that the crop is in the 7th leaf stage (or later), will applying N result in a yield increase? Most data I have reviewed for spring and durum wheat suggests the best response to top-dressed N occurs when it is applied early (3-5 leaf stage). When applications are delayed until after anthesis there is very little increase in yield and applications between these two stages give an intermediate response. Grain protein, on the other hand, usually increases with late applications of N. Therefore, late application (from the boot stage through early grain-filling), will do little to increase yield, but may improve grain protein by 0.5 to 1.5% (depending on the level of N deficiency you have at the time of application). Now the intent of this article is not to discuss top-dressing of N to small grains per se, as there are other good references for that topic. The intent is to provide information on the growth and development of small grain plants so that you can better understand why good management early in the season is so important.
As the small grain plant reaches about the 4th leaf stage (4 leaves with visible collars on the main stem), it switches from developing vegetative structures (leaves and tillers) to producing reproductive structures. Of the three major phases of plant development (vegetative, reproductive and grain filling), the reproductive phase is the single most important phase in determining the final yield of the crop. It is during the reproductive phase that the number of kernels per given area are fixed; kernels per given area are determined by spike numbers, spikelets per spike and grains per spikelet. Figure 1 demonstrates the effect of stress (in this case shading) during the growth cycle of spring wheat on kernels per unit area. You can note that the largest reduction in grain numbers occurred when stress was applied during the period 30 to anthesis through to anthesis. Nitrogen stress has the same dramatic effect on yield potential during the reproductive phase. Since there is always a lag between the time that N fertilizer is applied and when it actually is taken up and available in the plant tissue where growth is occurring, topdressing before the reproductive stage begins will help ensure timely availability of N to the plant when it needs it. Temperature during reproductive development also significantly influences yield potential. The warmer the temperature the faster the rate of development meaning the rate of progression from one growth stage to the next is increased. With relatively warm temperatures, the rate of development exceeds the rate of growth, so that fewer reproductive structures are developed (i.e. fewer kernels per given area developed).
Stress during vegetative development has a limited impact on yield if this stress is not too severe. Furthermore, during most growing seasons, filling grains after anthesis is less problematic for the plant than is maintaining kernel numbers per given area prior to anthesis. So what are the take home message from this unsolicited lesson in plant physiology. First, the need to make sure that N is available to the plant at the beginning of reproductive development so that your yield potential can be optimized. Given the difficultly most growers have experience in topdressing N due to persistent rainfall this year, applying most of your N prior to planting is a good strategy; it will give you more flexibility in meeting the needs of your crop if you should decide to apply more later. The other take home message is to plant early. We canít control the weather, but by planting early we can improve the chances that the developing crop will have more favorable temperature during its reproductive phase of development.
MONITORING CORN DEVELOPMENT
Corn growth has been slow this spring because of the cool weather. Furthermore, much of the corn looks yellow or in some cases purple. Be patient, once we receive warmer weather the corn plants will green-up quickly. If you are interested in learning more about early growth and development of corn, I would suggest that you read an article recently posted by Dr. Nielsen of Purdue University
NDSU Extension Agronomist - Cereal Crops
CHECK SOYBEANS FOR NODULES
Soybean plants that are 4 inches tall should have their first unfolded leaflets (V1 stage). Nodulation, the symbiotic relationship of bacteria on the soybean roots, can be seen shortly after emergence, but the plant is not actively fixing nitrogen until the V1 to V2 stages. The number and nodules formed on the soybean roots along with the amount of nitrogen fixed increases until the R5.5 stage which is the seed forming stage in the upper pods of the soybean plant. 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. Nitrogen fertilization after planting (other than pop-up or early, limited fertilization) is not recommended as nitrogen fertilizer applied to active nodules will inhibit nodule formation and its ability to fix N for the legume plant for later grain development and protein content of the seed.
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. Use a small spade or gardening tool to dig up plants to inspect nodule formation and donít pull the plants as the root hairs will slough off along with the nodules you are checking. Its suggested to check at least 5 different sites of the field for nodule development. Also when checking for nodules and examining roots look for root rots or any other signs of damage by insects or for soybean cyst nematodes. We donít have any yet in North Dakota but one can never say never as perhaps they just havenít been identified. They are in Minnesota and South Dakota fields close to our southeastern and southern borders.
CROPS AND SATURATED SOILS
Recent heavy rains in Grand Forks, Walsh and Pembina counties and other areas have caused flooding, ponding, and plant damage across low field areas. Prolonged soil saturation affects crop growth and yield. Corn is very sensitive to flooding in the early vegetative stages (especially prior to the 4th or 5th leaf stage). In early growth stages, corn or soybeans can survive for only two to four days under water in anaerobic conditions. Moderate water movement can reduce flood damage by allowing some oxygen to get to the plants, keeping them respiring and alive. Drainage within one to two days increases the chance of survival. The most susceptible crops to flooding are dry beans and potatoes. Only a day or two of flooding can cause plant death or damage as so that full recovery is never attainable.
The injury extent to seedlings is determined by the plant stage of development at ponding, duration of flooding and the air/soil temperatures as well as if auxiliary buds are present on damaged plants. If temperatures are warm during flooding (greater than 77 F), plants may not survive 24 hours. Cool temperatures may prolong survival. However, cold, wet weather favors disease development. Seed treatments are effective, but limited in protection. Seedling development slowed or delayed two to three weeks allow soil-borne pathogens a greater opportunity to cause damage. Seed rots, seedling blight, corn smut and crazy top affect corn plant development later even though ponding occurred earlier. Delayed soybean growth allows diseases such as Fusarium root rot, Phytophthora rot and Pythium rot to establish and weaken or destroy seedlings. Limited hybrid and variety resistance to these diseases and difficulty in predicting damage makes evaluation difficult. Carefully assess damage before deciding to replant or before tearing up the existing stand.
On surviving stands, remember that favorable weather for plants after ponding is important. Cultivation, once soils are dry enough, will open and aerate surface soil and promote root growth. Be careful working the soil. Working wet soil causes compaction that retards root and crop growth.
An additional nitrogen application in corn may be necessary in fields that show signs of yellowing or uneven growth. A test for nitrate when corn plants are still six to twelve inches tall can determine if more nitrogen is needed and side dressing may be a good option.
Scout more intensively in previously ponded areas . Maintain a good weed control program so that crop plants are not robbed of nutrients and moisture later in the season.
CHECKING SOYBEAN, SUNFLOWER AND CORN FIELDS
Crop scouting in corn, sunflower and soybeans requires knowledge of pest and crop biology, pest identification, correct sampling methods and a decision on economic thresholds. The goal is to give an overview of crop condition with as complete, accurate and unbiased an assessment of the pest problems in the fields as possible.
Frequency of scouting depends on the crop and pests present or expected. Corn often should be monitored at least weekly until pollination is completed, at which time frequency can be relaxed to once every ten days. Soybeans should be checked at least once a week during the initial vegetative stages can be relaxed after pods are well formed. Start monitoring for soybean aphids at the V3 to V4 stage and watch very closely at the start of flowering.
For sunflower, fields should be walked weekly from emergence until stage R6. Early scouting should be for any signs of cutworm damage, wireworms or reduced stands. Next one needs to start watching for sunflower beetle feeding, stem weevils laying eggs, and thistle caterpillar foliar feeding. At the R (reproductive) stages of sunflower one would look for midge, bud moth larva, seed weevil( R5.1) and sunflower moth plus banded moth.
In order to clearly evaluate a field, use scouting patterns across each field for efficiency. An "S-shaped" walking pattern in square or rectangular fields works best and in irregularly shaped fields figure a pattern that covers a representative area of the field. Don't expect the edge of a field to have the same pest populations as other areas. Don't sample field edges unless it is also specifically needed such as with stalk borer or weed scouting. For large fields over 100 acres, consider splitting the field into two or more separate scouted areas based on geography, previous cropping or soil type. Enter the fields at different sites during the weekly visits.
Know as much about the field as possible before scouting. Besides field location, have cropping history, crop yields, pesticide use, fertilizer applications, soil type, soil test records, major pest problems in the past and anything else that can make your current job easier.
Devise a scouting form to record your scouting finds. Even if pest damage is not found, record the general crop health and growth stage. This will help you monitor changes in the field throughout the season.
When scouting crops keep on hand the following equipment: scout forms, clipboard, pencils, pocket knife (for splitting stalks and stems, looking at roots and cutworm scouting), magnifying glass or hand lens for accurate pest identification, bags (as well as plastic vials and labels) for collecting pest specimens for help in identification or later clarification, mechanical hand counter, measuring tape, small spade or hand trowel, cooler with ice in your vehicle if you wish to keep samples for others to look at and identify. A good digital camera also will be useful for documentation of problems or crop injury.
Stand counts in fields should be made the second week after crop emergence. Take counts for the same distance in several locations within the field and determine by use of conversion factors the stand on a per acre basis. If losses are seen later in the season, another stand count may be required.
Occasionally, unknown field problems are seen when scouting . In these cases, it is important to diagram where the problem occurred in the field and to collect plant samples so proper identification can be made. Collect a variety of plant samples including roots and also collect some healthy plants for comparison.
Happy field scouting!!
Duane R. Berglund
NDSU Extension Agronomist