Linda Benedict, Boquet, Donald J., Blanche, Sterling, Padgett, Guy B., Leonard, Billy R.
Donald J. Boquet, B. Rogers Leonard, Boyd Padgett and Sterling Brooks Blanche
Louisiana farmers often harvest soybeans with a high percentage of damaged seed, resulting in significant marketing discounts. Seed damage is caused by Louisiana’s climatic conditions that encourage high levels of diseases and insect infestations in soybean fields. Also, wet weather and water-saturated soils frequently delay harvest, which contributes to the further deterioration of pods and seed in the field. Methods to minimize damage to seed during the growing season and after seed maturity would be of great value to the Louisiana soybean industry.
Seed quality is the highest as soon as crop development is completed and before any weather-related events cause deterioration. Seed quality is a function of initial quality at maturity, which is the cumulative effect of a variety’s genetics and exposure to season-long growing conditions including diseases and insects. It's also a function of harvest quality, which includes the effects of field weathering after maturity.
Initial differences in quality are derived from differences in maturity date, the producer’s management practices, and the inherent ability of varieties to protect developing seed from diseases such as anthracnose, cercospora/purple seed stain and phomopsis. Resistance to weathering is derived from differences in pod wall structure. Some varieties may have pod walls that are more substantial (physical and chemical attributes) and maintain their integrity longer than other varieties and, therefore, protect seed longer from weathering and, perhaps, from diseases and insect damage. Over time, however, all pods deteriorate and, if wet conditions persist, seed eventually become susceptible to opportunistic fungi and bacteria (Figure 1). A common thought among producers has been that the characteristics of brown pods protect seed better than gray pods, but no research has yet demonstrated this effect.
In 2010, a project partially funded by the Louisiana Soybean and Grain Research and Promotion Board was initiated to identify genetic and environmental factors affecting seed quality and to develop practices to improve seed quality. The objectives are to:
1) Develop a method to evaluate soybean seed quality that relates to state and federal grading standards.
2) Assess differences among varieties in seed quality at maturity and after periods of field weathering.
3) Evaluate the value of fungicides and insecticides at mid and late growth stages for control of seed damaging diseases and insects and improvement of seed quality.
Despite the fact that seed quality is a principal determinant of the value of soybean grain, there is no standard research method for evaluating seed quality that equates with U.S. Department of Agriculture or state grading standards used for marketing grain. Rather, a numerical rating system developed in the 1940s is occasionally used to assess seed quality differences. The system is a subjective visual rating of 1 through 5, where 1 is excellent quality, 2 is good quality, 3 is fair quality, 4 is poor quality and 5 is very poor seed quality (Figure 1). This is a rapid method for quality assessment that accounts for all visible sources of seed damage, but it is unclear how this system correlates to grading requirements used in marketing grain that also assesses internal seed damage. The two systems will be compared to determine if the subjective 1 to 5 rating has value to researchers and producers or if some other system is needed.
In 2010, the initial year of the project, several field experiments were established to address the project objectives. Two experiments were completed to determine the seed quality benefits from applying combinations of fungicides and insecticides at different times during R2 (early flowering) through R7 (physiological maturity) growth stages. A third experiment evaluated the seed quality of 16 varieties expressing independent genetic and phenotypic traits. The three experi-ments were planted under a sprinkler ir-rigation system used to periodically wet the plants to simulate weathering effects on seed after R8 (harvest maturity).
In each experiment, beginning at R8, pod samples were collected at 10-day intervals for 40 days for seed quality evaluation. In addition, the official soybean variety trials at the Macon Ridge Research Sta-tion near Winnsboro and the Dean Lee Research Station near Alexandria were used to study variety effects on seed quality. Pods from selected varieties with variable maturity dates and different phenotypic traits were collected at R8 and at 10-day intervals for 40 days after R8 for seed quality evaluation. A total of 72 varieties were sampled – 10 from Maturity Group (MG) 4.0-4.5, 30 from MG 4.6-4.9 and 32 from MG 5. Maturity Group refers to the length of the growing season with lower number groups maturing earlier than higher numbers. Samples were evaluated subjectively with visual ratings and objectively with official grading criteria.
Conditions that usually cause severe seed quality deterioration in Louisiana did not occur in 2010, and soybean seed quality across Louisiana was the best seen in 20 years. Insect infestations were low throughout the summer, and harvest weather was dry, with no extended rainfall events delaying harvest. Seed that remained in the field for 40 days after maturity retained excellent quality. As a result, the 2010 crop incurred almost no discounts for seed damage, which was highly beneficial for producers. The lack of conditions that typically cause extensive seed damage, however, placed limitations on the seed quality research project.
Disease and Insect Control
In the two studies to evaluate effects of diseases and insects on seed quality, the nontreated controls had excellent quality both at crop maturity and after 40 days of weathering. Application of insecticides and fungicides for pest control had little effect in improving quality. One important finding of the studies, however, was that the application of water with an overhead sprinkler system to simulate rainfall had limited effectiveness in causing seed deterioration under otherwise dry weather conditions.
Pods dried quickly after wetting, not allowing enough seed moisture increase to support growth of pathogens or opportunistic saprophytic organisms. From these results, future studies will require a unique approach to induce seed weathering. Multiple water applications during night time will be needed to create an adverse environment for weathering seed.
Pod color and quality
The color of soybean plants (and pods) results from a combination of the external plant tissue and pubescence colors, both of which can vary from dark brown to gray. Pod and pubescence colors can be shades of brown, tawny, chocolate, tan or gray. The specific combination of tissue and pubescence color imparts the plant/pod color seen at maturity. The color combinations are so numerous a numerical system was developed to describe pod color in which 1 = dark brown, 2 = brown, 3 = light brown, 4 = tan and 5 = gray plants/pods. These numerical designations for pod color allow statistical analyses to determine if there is an effect of pod color on seed quality. These analyses revealed that in 2010, there was no association between pod color and seed quality of varieties. There was, however, a variety effect on quality, with values ranging from an excellent rating of 1.2 to moderately poor rating of 2.9. View Table 1.
This amount of variation in seed quality among varieties, although significant, was less than expected because of growing and harvest conditions. No firm conclusions can be reached about the association between pod color and seed quality until further data is collected under additional plant development and seed weathering conditions.
Varietal resistance to weather damage
One of the project objectives was to determine if there were differences among varieties in tolerance to weather-induced seed damage after crop maturity.View seed quality results of this study in Table 2. There were significant differences in seed quality among varieties at maturity and after 40 days of in-field weathering.
Under the prevailing conditions, the differences were small, ranging from excellent to good quality. There were also significant differences among varieties in their ability to resist seed damage after crop maturity, as measured by changes in seed quality ratings between R8 and 40 days after R8. Weathering damage of 40 days duration ranged from no quality change for a single variety to as much as a one point quality loss for another variety.
These results suggest that measurable seed quality differences among varieties exist, but the overall soybean seed quality in 2010 was not representative of usual conditions in Louisiana. These preliminary results do not, therefore, provide information on varietal performance under weather conditions of a normal year when seed damage potential is much higher.
Growing conditions and excellent harvest weather in 2010 combined to create an almost perfect, but rare, scenario for production of excellent quality soybean seed. Under the 2010 conditions, applications of insecticides and fungicides could not improve upon existing excellent seed quality. Even with the overall excellent seed quality, there were significant differences among varieties in quality at maturity and after weathering. Further research under more typical weather conditions is needed to verify that varietal differences are large enough to warrant consideration in making variety selections and to determine which specific plant traits and management practices affect seed quality.
Donald J. Boquet, Professor of Agronomy and Jack and Henrietta Jones Endowed Professor; B. Rogers Leonard, Professor of Entomology and Jack Hamilton Regents Chair in Cotton Production; Boyd Padgett, Professor, Macon Ridge Research Station, Winnsboro, La.; and Sterling Brooks Blanche, Assistant Professor, Dean Lee Research Station, Alexandria, La.
(This article was published in the spring 2011 issue of Louisiana Agriculture magazine.)