Utilizing Cool Germ With Prevailing Conditions, Planting Depth (Collins & Edmisten)
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We’ve been getting a few questions about how to best utilize cool germ for making planting decisions. As mentioned in several previous articles, growers should utilize the NCDA&CS Cotton Seed Quality Testing Program to document their warm and cool germ for every lot of seed purchased. Additional, growers should utilize the Cotton Planting Conditions Calculator to see the current planting conditions ratings for the 5-Day forecast.
Results from the cool germ test can be more variable than the warm germ test, across seed samples and over time. The cool germ test includes a qualifier (radicle length of 4 cm) in order for germinated seed to be counted, whereas the warm test counts seed as germinated or not. There are other reasons for the higher variability of the cool test that we will not discuss here, but it is important to understand that the cool test is not as consistent as the warm test.
So when does cool germ matter, and how should we utilize it? If the planting conditions calculator AND the long-term forecast suggest that conditions are good or excellent for planting cotton, warm germ should be considered, but cool germ can generally be ignored. If planting conditions are rated as poor, cotton shouldn’t be planted at all, regardless of warm or cool germ. When conditions are rated as marginal or adequate, that’s when cool germ matters most. Under such conditions, the best thing to do is wait on better conditions. Keep in mind that rapid emergence and stand establishment is the ultimate goal. Vigorous, rapid-emerging (5 to 7 days after planting) seedlings not only establish optimal stands but are more resilient to thrips and/or injury from residual herbicides. Slower emerging seedings are often more likely to encounter greater thrips injury and or stunting from herbicides, seedling disease, etc. Waiting for better conditions to plant can and should be done right now, but once May 10th or 15th arrives, it’s go-time for most growers, especially larger acreage growers. If marginal or adequate conditions prevail during mid-May and a grower is forced to plant in order to meet crop insurance deadlines, only the highest cool germ seed and largest seed should be planted, with potential adjustments made. Seed size is determined by the number of seed per lb, and is printed on the bag or can be extrapolated from information on the bag. Seed sizes greater than 5000 sd/lb are considered smaller seed, whereas seed sizes in the low 4000’s sd/lb are considered larger. If the highest cool germ seed on-hand is 80% cool germ, plant it shallow (1/2″ at most, assuming good soil coverage is achievable) and at optimal seeding rates (42,000-44,000 sd/A), ideally with larger seed. If the highest cool germ seed on-hand is 65% cool germ, adjust seeding rates 10-15% higher, plant shallow, and hill-drop seed if crusting is expected or historically a problem, also ideally with larger seed. Avoid planting seed with less than 50% cool germ or smaller sized seed during these periods.
Occasionally, a particular lot# of seed may have a high or acceptable warm germ percentage, with a low cool germ. Such seed is still perfectly suitable for planting, but probably not during periods of cool temperatures or when conditions (short and long-term) are less than ideal. In other words, wait on better conditions to plant that seed. Our general advice to growers is to document both warm and cool germ for every lot they purchase, and when forced to plant in less-than-ideal conditions, plant the highest cool germ seed on-hand, preferably larger seed as well.
Why does planting depth matter? Some folks learned a difficult lesson with regard to planting depth last year. Most commonly, deep planting is done to “chase” moisture. Although we understand the logic behind it, deep planting is often disastrous in terms of stand establishment and early season vigor. Deep planting only prolongs the time between planting and emergence, adding stress to developing seedlings and very often resulting in poor stands. When chasing moisture, moisture is already limited, and what moisture is available at deeper planting depths will not likely last very long. There may be enough moisture at those depths to sprout the seed, but that same moisture usually expires before seedlings can emerge, and emerging seedlings can, and often do, die when this occurs. The only ways that deep planting can establish acceptable stands is when planting conditions are good or excellent, when crusting is not a problem (very rare in NC soils), and when subsequent rains occur. However, in all of these scenarios, planting shallower results in either the same, or more commonly, better stands than deep planting. The point here is that growers should plant shallow in nearly all cases. Even when soil moisture is limited, better stands can still be expected from shallow planting into completely dry soil while waiting on a rain.
So how do we define deep vs shallow planting? Cotton should be planted at 1/2 inches deep, assuming good soil coverage can be achieved. Occasionally we see situations where stiffer soils in no-till systems encounter problems achieving good soil coverage and furrow closure, therefore cotton can be planted 0.7 inches deep in these situations. Anything deeper than 0.7 inches deep is considered deep planting and stand establishment problems are expected or likely at these depths.
The illustrations below are a scenario from this year, from some seed quality work that we are doing. Achieving at least 10 DD-60s per day for the 5 days following planting are considered to be ideal planting conditions. As seen in Figure 1, it is important to note that this particular trial was planted under ideal temperatures and moisture for the first 3-4 days following planting, then it encountered a cool spell for 4-5 days, similar to what we are experiencing right now, and then warmed up again.
The left side of each photo shows cotton planted deep (1 inch deep) and the right side of each photo shows cotton planted shallow (1/2 inch deep). Figure 2. illustrates emergence at 7 and 14 days after planting for some of the best-quality seed we evaluated in this trial (larger seed, high oil content, high warm and cool germ). When planting shallow, you can see that approximately a 52% stand was achieved at 7 days after planting (during the middle of a cool spell) whereas an ideal stand was achieved by 14 days after planting. When planting deep, hardly any seed emerged by 7 days after planting, and only a partial (but acceptable) stand was achieved by 14 days after planting. It is important to remind you that this was BY FAR the best quality seed and best-case scenario amongst the 15 different seed lots we are evaluating.
Figure 3 illustrates emergence of slightly smaller and slightly lower quality seed, but not poor quality seed by any standard. In fact, Figure 3 illustrates what we most commonly encounter from year to year in terms of seed quality and emergence. In this case, hardly any seed emerged by 14 days after planting when it was planted at 1 inch deep, however planting shallower resulted in an acceptable stand. Point being, plant shallow.