Most forage crops are planted in autumn or spring in Alabama, and once a field has been planted, rapid stand development is highly advantageous. A quick, vigorous stand facilitates competition against weeds, expanded leaf area allows capture of more sunlight for food production and a larger root system helps plants obtain water and nutrients more easily. As size increases, plants are less vulnerable to stresses including drought, diseases, insects, and cold or hot weather. Also, the faster seedlings grow, the sooner a field will be ready for grazing or hay production.
Seedling vigor refers to the rate of establishment of plants. An important concept is: small differences in seedling vigor can sometimes make big differences in stand establishment. Seedling vigor can be affected by several different factors.
Genetic Influences - Some crop species have much better seedling vigor than others. In general, annual plants have better seedling vigor than perennial plants, because the survival of an annual depends upon maturity and seed production during the establishment year. Perennials, on the other hand, can survive from year to year due to their ability to perenniate (come back from the roots).
There are sometimes wide differences in seedling vigor among varieties within a given species, as anyone who has conducted variety trials knows. Unfortunately, even though seedling vigor is important, few variety trials rate seedling vigor per se. Due to cost and time limitations, data collection is usually limited to yield and possibly one or two other factors like forage quality or disease resistance.
Seed Source - Some seed is of higher quality than other seed. In some crop species like the small grains, "test weight" (which refers to the pounds of seed per unit of measure) is used to evaluate seed quality. Seed having a high test weight contain more stored carbohydrate and are more likely to produce plants having good seedling vigor. Storing seed for long periods of time (especially if the seed is not stored in a cool, dry place) uses up stored carbohydrates and lowers seedling vigor.
Establishment Influences - Planting techniques can also influence seedling growth. For example, if seed is planted on a date earlier or later than optimum, conditions for growth of the seedlings will also likely be less than optimum. This is one of the reasons why timeliness of planting is so important and is often stressed by agronomists.
In addition, planting depth can, in some cases, have an influence on seedling growth. For example, seed planted too shallowly may not have adequate moisture to grow well during a drought period. Seed planted too deeply may struggle to reach the surface, thus largely depleting food reserves in the seed and delaying seedling development.
Also, in order for young plants to grow well, the fertilizer elements necessary for good growth must be present. Phosphorus is especially important in seedling growth. In addition, soil pH is of critical importance, especially for clovers and other legumes, as a suitable pH level is important for both the legume plants and the nitrogen-fixing Rhizobium bacteria interacting with legumes.
Other - Other factors, some climate-related and some management-related, can also affect seedling growth. Some of these, like the effect a crop may have on the seedling growth of a succeeding crop, are not well understood. Others, like extreme heat or cold, are understood, but there is little we can do about them.
Conclusion - Given the importance of seedling vigor, it pays to do the best job possible when establishing a forage stand. When establishment conditions as well as subsequent growth conditions are good, almost any technique may be successful. However, when stress conditions occur, the stand making the best seedling growth is most likely to survive
Don Ball is an Extension Forage Crop Agronomist with Auburn University.