Seedling Vigor: Why Is It Important? (Collins, Edmisten, Stewart, York, & Reisig)

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Seedling vigor (seedling size, health, and growth rate) is the product of several factors related to genetics and environmental influences, and can also be manipulated through management. Naturally, seed size and performance in warm and cool germination tests can be indicators of how vigorous seedlings might be. Soil temperatures at planting and soon thereafter can also greatly influence seedling vigor. Lastly, improvements in vigor can be achieved through adequate protection from early season pests such as thrips and occasionally the use of starter fertilizers (in a 2×2 placement).

As mentioned above, seed size can be one predictor of vigor as larger-seeded varieties tend to be more vigorous, which will be discussed later in this article. It is important to note however, that variety decisions should be based on factors such as lint yield and yield stability, fiber quality, technology packages etc., with seed size considered mostly in regard to how that seed should be managed at planting. As seen from variety trial results, yield potential appears to be independent from seed size, as long as acceptable stands are established….in other words, high yields can be achieved from both large and small-seeded varieties as long as plant stands are adequate or optimal. Smaller-seeded varieties can also be our highest-yielding varieties in some years, however extra care may sometimes be necessary to ensure vigorous seedling growth and acceptable stand establishment, including upwards adjustments in seeding rates, delaying planting until conditions are optimal, use of seed treatments and/or in-furrow insecticides, hill-dropping, etc. Observing cool germ test results is also a good standard practice that is commonly under-utilized. Although only warm germ test results are printed on the seed bag, a grower can easily retrieve cool germ results by recording the lot number and asking the local seed company representative for the cool germ test results for that lot number. As a general practice, it is a wise practice to record the following information for each bag of seed a grower purchases: lot number, cool germ test results, specific seed treatment and treatment location (if treated downstream), planting weather (soil temperatures, air temperatures, and rainfall from a few days prior to planting until full emergence), and save a small sample of seed (1 gallon or so). This information along with a sample of seed can significantly help growers in situations where poor stands are observed.

With regard to cool germination, below is a quote from a publication written by Dr. Sandy Stewart who conducted some research observing the effects and interaction of seed size and cool germination during 2005-2006 in Louisiana:

“The results suggests that the perceived negative effects of a low cool germination value can be overcome by a larger seed size, presumably due to greater stored energy reserves for emergence within the seed. Similarly, results suggest that the perceived negative effects of a small seed could be overcome by a high cool germination value in stand establishment. The lack of interactions among the treatment factors suggests that these findings should be consistent across varieties as well as environmental conditions. Growers do appear to be justified in seeking larger seed lots or higher cool germination lots for planting. However, the results from this study indicate that the combination of a small seed and a low cool germination is more detrimental to stand establishment than either factor alone. Smaller seeded lots should be handled with care, but may tolerate more challenging conditions than previously thought as long as the cool germination is high. Similarly, a low cool germination lot may emerge satisfactorily in challenging conditions as long as it also has a large seed size.”

His findings suggests that cool germ test results may be more important for smaller seed than larger seed, but both large and small seed can achieve acceptable stands if cool germ test results are adequate/optimal, or when planting in favorable environmental conditions or both.

So how do you quantify seed size, and how do you know if it is large or smaller seeded? Seed size is printed on the bag as seeds per lb. The higher the number, the smaller the seed. There is no designation for large versus small seed, therefore seed size is only relative when comparing one variety to another variety or comparing lot numbers. Assuming cool germ test results are similar and favorable for all varieties in a comparison, seed size can be one (of many) predictors of vigor. The graph below illustrates a relatively high correlation between seed size (as printed on the bag) and seedling vigor in five locations of the 2015 On-Farm Cotton Variety Trials. Note: seed quality and planting conditions were very favorable for these trials in 2015.

Correlation between Seed Size and Vigor 2015

So how do certain varieties compare to one another with regard to seedling vigor? The graph below illustrates vigor (fresh weights of 10 seedlings per plot at the 2-3 leaf stage) of all varieties in the same five locations of the 2015 On-Farm Cotton Variety Trials.

Vigor of 2015 On-Farm Varieties

It is important to note that all varieties in these trials resulted in optimal stands and acceptable vigor, and vigor did not appear to translate into higher yields. Planting weather was optimal for these trials in 2015 and severe thrips pressure and herbicide injury was essentially non-existent in field observations. However, if these varieties were to be planted under adverse weather conditions, and encounter severe thrips pressure and herbicide injury, then perhaps some of the smaller seeded or lower vigor varieties would need extra care or attention during planting and soon thereafter to ensure acceptable growth, especially if cool germ test results were less than ideal.

So why is seedling vigor important? Over the years, seedling vigor has always been important in stand establishment, especially in the early part of our planting window when conditions can be relatively cool. However, when planting in excellent conditions, yes, growers may still notice small differences in high and low vigor seedlings, but these differences often disappear later in the season and may not translate into differences in yield. But if the truth be told, this was more common back when we still had aldicarb and glyphosate resistance was not an issue.

In modern times, there are two major factors that can interact to elevate the value of vigorous seedling growth. These factors are the loss of aldicarb, and thus increased challenges in controlling thrips, and the increased reliance on residual herbicides for managing glyphosate-resistant weeds.

Injury from thrips is exacerbated by any factor that slows seedling growth. Reduced growth rates of seedlings allow thrips to feed on developing terminal leaves for a longer period of time and also prolong the time between seedling emergence and the 4-5 leaf stage (point at which seedlings are generally safe from thrips) which intensifies the adverse effects on yield and maturity. With the loss of aldicarb, there is a greater need to promote rapid seedling growth to minimize these losses, through planting high-vigor seed and planting in warm, moist soils when conditions following planting are expected to favor vigorous growth. This is especially important in modern times as many of our thrips control products may expire before seedling reach the thrips “safe stage” if growth is slowed too much. Secondly, as we all know, the success of modern weed control programs relies heavily on effective use of residual herbicides to prevent pigweed emergence. An occasional side effect of such is herbicide injury, which can also slow seedling growth and most commonly occurs when experiencing rainfall at cracking or when seedlings imbibe a high concentration of herbicide as they are trying to emerge. The effects of herbicide injury alone (i.e. in the absence of all other stresses) is generally minimal and seedlings can overcome that injury given a little time. However, the effects of herbicide injury can be exacerbated when seedling growth is slowed due to cool weather or if proactive thrips control measures are not taken.

Let’s be clear on something….the use of residual herbicides is as important now as it ever was. Weed control programs that are too lax (reduced rates or no residual herbicides) will result in poor weed control which will more than negate any improvements in seedling vigor or thrips control. In the current period of low cotton prices, some growers have expressed concern over the cost of some herbicides, especially when they have experienced severe injury in previous years. The effects that herbicides have on seedling vigor and thrips control can be overcome by timely management and are miniscule in comparison to the costs of dealing with weedy fields. Finally, it should be noted that herbicide injury can sometimes look very similar to thrips injury. Be sure you know if thrips are present before you manage what is actually an herbicide problem.

So what can we do?? Given the current challenges we face, the potential impact of planting high vigor seed in conditions favorable for rapid seedling growth is greater now than it has ever been. Therefore, as simple as it may seem, a little extra attention to warm and cool germination tests and avoiding planting during periods of cool, wet conditions can help alleviate many of these issues. The effects of both thrips and herbicide injury should be lessened to some degree when doing so. Secondly, if herbicide injury is observed, research across the belt has shown that a well-timed foliar spray of acephate when the first true leaf barely appears between the cotyledons (15-20 days after planting depending on growth rate) can remove the added stress from thrips and can prevent adverse effects on yield. It is important to note that the timeliness of this foliar spray is extremely important to the success of this approach. It is understandable that growers want to combine sprays when possible to reduce trips across the field. However, if the preemergence herbicide is working, thrips control will often be needed prior to time for the first postemergence herbicide application.

With regard to thrips management practices, many growers have had good success with an insecticidal seed treatment followed by a well-timed foliar spray to 1-leaf cotton. Recent cases of documented resistance to seed treatments across the belt, including North Carolina, have raised concerns of whether or not a seed treatment will suffice in controlling thrips. In 2016, growers should have the option to choose which seed treatment they prefer, whether the seed is treated by the manufacturer or downstream at local distribution outlets. As with any seed treatment, a well-timed foliar spray should be expected in most cases. It is important to note, that a higher rate of acephate or Radiant plus surfactant (Radiant rates of 1.5 oz/A or preferably higher, with better control expected at higher rates) may be necessary in controlling western flower thrips. In many cases, the presence of high numbers of western flower thrips is only realized once the foliar spray has failed and another application is needed.

In recent years, there has been a growing interest in applying liquid Admire Pro (or like product) in-furrow in addition to a seed treatment. There has been a great deal of success with this approach, as data have illustrated prolonged control of thrips on occasion. However, as Dr. Reisig pointed out in the winter cotton meetings, growers may not expect a “one and done” approach to suffice in all cases in 2016. As seen in 2015, there will likely be several scenarios where a seed treatment plus Admire Pro in-furrow will require an additional foliar spray of acephate. Regardless, the success of this program relies heavily on achieving adequate coverage or direct contact of liquid insecticide with the seed as it is planted. There are several methods used to make liquid in-furrow applications of Admire Pro, however research in North Carolina has shown the greatest success with the 9.2 oz/A rate of Admire Pro at application/carrier volumes of 5 to 9 gallons per acre applied through an orifice which provides a direct stream of the insecticide contacting the seed as it is planted. With regard to seed covers, growers should evaluate seed/insecticide contact in their system and make necessary adjustments to ensure direct contact of insecticide to the seed.