UNDERSTANDING WEED SEED IS CRITICAL TO LONG-TERM WEED MANAGEMENT

                                                                             

W. Carroll Johnson, III

Weed Scientist and Agronomist

Tifton, GA

 

Whenever I travel in southwest Georgia in late-summer, my thoughts always turn to one of the factors that separates good farmers from mediocre farmers--clean, weed-free fields.  This is particularly noticeable in late summer when crops are nearing harvest and what needs to be done had already been done.  Every farming operation has access to the same weed management technology and expertise.  Why would one farm have crops heavily infested with mature weeds late season, while an adjacent farm has weed-free crops?  There are numerous reasons for the differences.  A common factor is the farm history of successful weed management.  Successful weed management is in essence a war of attrition, by steadily reducing weed populations with the goals of depleting the number of viable weed seeds in the soil and preventing weed seed production.  In theory, the war of attrition totally depletes numbers of viable seeds for all weed species in the soil, although in practice that goal is like searching for the pot of gold at the end of a rainbow.  Farms that are choked with weeds late season are usually in that condition due to large numbers of weed seeds in the soil caused by poor or sporadic weed control efforts in previous years.  Weeds that escape control produce seed that replenish the weed seed bank. 

 

How does this relate to managing weeds in forage plots for whitetail deer?  Long-term success of managing weeds in forage plots is also dependent in part on depleting the numbers of viable weed seeds in the upper soil layers and preventing weed seed production.  This is not a quick-fix, but part of the long-term commitment necessary for establishing and maintaining quality forages for whitetail deer.

 

Annual weeds are propagated by seeds, with the number and size of seeds varying according to species.   The number of seeds produced will also vary within a species according to size of the weed, population density, and environmental conditions.  One of the smaller weed seeds are produced by pigweed (Amaranthus spp.).  Pigweeds are found throughout cultivated areas of the U. S.  Pigweed seeds are black, shiny, and about 1-mm in diameter, or about the size of a grain of table salt.  Among the largest weed seeds commonly seen are produced by morningglories (Ipomoea spp.).  This group of weeds is also commonly found throughout the U. S.  Morningglory seeds are dark brown, angularly-shaped like an orange slice, and 4 to 5-mm long, or about the size of a BB shot.  Weeds with seeds about the size of a pigweed seed are loosely classified as “small-seeded broadleaf weeds”, while morningglories are “large-seeded broadleaf weeds”.

 

Pigweeds are capable of producing over 200,000 seeds (plants) per acre, whereas morningglories have been reported to produce over 500 seeds (plants) per acre.  What does this mean?  Assume that a forage planting has an infestation of tall morningglory at a density of 1 plant per 10 square feet, and each plant in this scenario produces an average of 500 viable seeds (plants).  With these assumptions, the estimated number of seeds produced is 2,178,000 per acre.  There is the potential of at least 2,178,000 morningglory seedlings per acre to control in the future; and this does not consider the amount of morningglory seeds already in the soil from preceding years.  The mathematics are even more impressive (or sobering) when you consider pigweed, which produces greater numbers of seeds than morningglory.  Change the scenario by increasing the weed density and see what that does to the number of weed seeds added to the soil.  Conversely, reduce the weed density and notice the large reduction in number of seeds produced.  These are very simple examples, and perhaps they are over simplified.  However, the point is that if a manager of forage plots is serious about managing weeds, every effort must be made to deplete numbers of viable weed seeds in the soil and prevent weed seed production.

If a manager is successful in sustaining good weed control for several years, eventually baseline weed densities will be reduced and weed management efforts will become easier.  Does that mean that weeds are gone forever?  In theory - yes, in reality - no.  In a study in Colorado on weed population dynamics in corn over an 8-year period, weeds were intensively controlled for 5 years. The number of pigweed seed was reduced 7 fold, and the number of seeds from other species was reduced 10 fold as a result of intense weed management. Once weed management was halted, it took only 2 years for weed-seed to recover to within 50% of the original baseline density.

When managing weeds in forage plots for whitetail deer, it may take several years of intensive effort using an integration of tactics to reduce weed densities to an acceptable level.  Once weed densities are reduced to that point, adjustments can be made in weed management intensity to reflect the lower weed densities.  However, ignoring weed control will quickly erase the preceeding years of hard work and diligence.

 

How long should weed control intensity be maintained in order to see reduced weed densities?  That depends on weed seed longevity in the soil, which varies among species and environments.  A 17-year research trial conducted at the University of Nebraska on two soils showed that an amazing percentage of common mullein and curly dock seed germination after burial for 17 years.  In addition, velvetleaf and common lambsquarters had significant numbers of viable seed after 17 years of burial.  Conversely, species such as redroot pigweed, yellow foxtail, and large crabgrass had few viable seeds after 17 years of burial.

Weed seed dormancy can be broken by an array of stimuli.  Dormancy is often broken by a combination of tillage practices that bring deeply buried seed to the upper layers of soil and light that trigger seed germination.  This must happen in the presence of adequate soil moisture for seeds to absorb water and within appropriate temperature and oxygen regimes.

 

While weed seed survival in the soil varies according to species, soil type and environmental conditions, the conclusions are generally the same.  Weed seeds in nondisturbed soil can remain dormant for many years and can germinate when stimulated.  Furthermore, efforts to deplete numbers of viable weed seeds in the soil and prevent seed production must be sustained for several years before benefits can be seen.

 

I witnessed a classic example of weed seed survival and dormancy during the dry summer of 1986.  Many reservoirs in the southeastern U. S. were not at full pool level throughout the summer, leaving large amounts of exposed shoreline that was normally underwater.  At Lake Martin in central Alabama, I observed an incredible population of common cocklebur and smallflower morningglory growing on a delta (about five acres in size) formed by a small stream flowing into the lake.  Densities of both species were approximately 2/ft².  The surrounding landscape was in mature timber, with no cultivated land upstream.  What was even more astounding was the fact that Lake Martin was constructed in 1926, and since then the small creek delta had been underwater most years during the summer months.  This may be over simplifying the phenomenon, but it appears that seeds from smallflower morningglory and common cocklebur had remained dormant in the creek delta mud for perhaps as long as 60 years.  During the dry summer of 1986, the mud dried enough for the soil to become aerated, and seed dormancy was broken, resulting in one of the most dense and uniform infestations of weeds I have ever seen.  Remember this is you establish forage plots in seemingly weed-free areas and unexpectedly weeds appear.

Stale seedbed tillage is a logical means to begin the quest of weed seed depletion.  This cultural weed control practice stimulates weed seed germination and emergence, while simultaneously controlling weed seedlings.  When repeated at least twice at 2-week intervals before seeding the forage crop, numbers of viable weed seeds in the upper layers of the soil profile are markedly reduced.  The key to successful weed control with stale seedbed tillage is to thoroughly pulverize the soil 2 to 3 inches deep.  A power-tiller is the implement of choice, although two perpendicular passes with a disk harrow will work.  Use this practice whenever establishing a forage planting, regardless of whether the forage is an annual or perennial.