One for the Snow, One for the Crow and One to Grow

Edição VIII | 03 - Mai . 2004

James Delouche -

    For the title of the second part of my essay Seed Quality Is Not Always the Problem I chose an old-time maxim of farmers in the U.S. Midwest, especially maize farmers. It reminded them to plant three seeds for every plant desired because there was a substantial probability that one would succumb to an adverse climatic condition, i.e., snow, another would succumb to a biological hazard, i.e., a crow (bird), while, hopefully, the third seed would survive, germinate, emerge and develop into a plant. This old-time maxim recognized that many factors other than poor quality seed can contribute to crop emergence failures, that seed quality is not always the problem. Indeed, the multiple hazards encountered by seeds in fulfillment of their propagative and disseminative functions were taken into account from the beginning in the evolutionary equation for seed bearing plants that factored in an abundant seed production.
   The abundant seed production habit of the higher plants greatly favored humankind's early gathering of seed/grains for food and, subsequently, the domestication of food crops. Just consider how meager and different the human diet would be if our main food grain crops, i.e., wheat, rice, maize, soybean, etc., produced only a few rather than many seeds/grains per plant! There is, of course, a downside to the abundant seed production habit: it contributes importantly to the persistence and rapid spread of undesirable plants (weeds) in cultivated fields.               
   Three of the main hazards encountered in establishing economical crop stands or populations were discussed in the first part of this essay: unfavorable soil temperatures, deficient or excessive soil moisture and mechanical impedance of soil crusts. There are several other hazards, one of major importance and several of less general importance but which can be locally and periodically severe. These are discussed in the following sections.     
    Biotic Hazard               
   The seedbed is not sterile. It has a large, diverse and important biological component that varies among climates, soil types, seasons, and cropping systems. This biological component consists of fungi, bacteria, nematodes, insects and assorted other bioforms that under certain conditions can and do attack, infest, infect, rot, or consume seeds introduced into the seedbed and/or emerging seedlings.                
    Deleterious activities of the biotic component against seeds are favored by Unfavorable temperatures and moisture levels that delay germination and emergence and by poor physical and physiological quality of the seeds planted.               
    Chemical seed protectants are the most commonly use defense against microorganisms in the soil that attack seeds. They are usually applied as a coating on the seed but in some cases are applied as a planter box treatment, i.e., the chemical is introduced into the seed zone in the soil at the time of planting. Many seed protectants are available representing a variety of chemistries with general or specific activity against the most troublesome microorganisms. During my teaching days I gave much attention to the role of mechanical injury to seeds in their susceptibility to attack by microorganisms. It was emphasized that cracks, scratches and abrasions of the seed coat (covering of the seed unit) resulted in a loss of its critical protective function and allowed easy entry of microorganisms. In an important sense, therefore, chemical seed protectants replace the physical protection of the seed coat lost through injury with a chemical barrier.               
    I realized then, of course, and concede here that this line of teaching was a huge oversimplification. But, it was used for a purpose that was considered worthy, viz., to imprint in students as deeply as possible the importance of minimizing mechanical abuse to seeds. Damaged seeds are indeed more susceptible to deleterious attack from soil microorganisms but so are low vigor, deteriorated seeds. And, as already pointed out even high quality seeds can succumb to microorganisms in seedbeds that are too cold or too hot, too dry or too wet. Crop establishment problems are most frequently associated with the interactions of several levels of factors, physical, biotic, physiological, that can be very complex.               
    Planted seeds can be attacked and consumed by insects and grubs in the soil, birds and even animals. Chemical and biological insecticides are applied to seeds to protect them from soil insects. Various chemicals have been and are used with variable success to protect seeds against birds and other varmints.               
    Recently developed and adopted conservation production systems involving minimum tillage and no tillage appearto exacerbate the crop establishment problem in several ways. First, conservation production systems do not produce as favorable conditions for seed germination and emergence as does conventional tillage systems; most importantly, perhaps, relatively poor contact between the seeds and soil particles delays germination.               
    Second, there are usually rather wide differences in the biotic components of the seedbed environment between minimum or no tillage and full tillage production systems, e.g. microorganisms and insects involved in decay of organic matter are much more abundant and active in the minimum and no tillage systems. For these reasons, the use of seed protectant chemicals is considered to be critically important in conservation production systems for some crops. For example, the use of chemical seed protectants for soybean seeds has increased in line with the increasing adoption of conservation production systems.    
    Chemical Residues               
    A variety of chemicals are introduced on or into the microenvironment of the seedbed to provide nutrients (fertilizers), control weeds (herbicides), control insects and nematodes (insecticides, nematocides), improve aggregation of soil particles, alter soil acidity, reduce salinity, and a variety of other purposes. Some of these chemicals can adversely affect germination when placed in the seed zone in contact with the seeds. More importantly, however, are the direct and residual effects of herbicides. Improper use of herbicides in terms of the timing of application and planting can have disastrous results. The rotation of crops can expose usceptible crop seeds to the residual activity of a selective herbicide used for the previous crop.              
    It is important, therefore, to maintain good records of the type, rate and time of herbicide applications and the applications of all other chemicals to the different crop lands as well as the sequence of crops over time and seasons.
    Miscellaneous Causes               
   There are other causes of crop establishment problems that I term miscellaneous not because they are not serious but because they are infrequent and most commonly the result of poor farming practices. In a previous essay I related one of my early professional embarrassments when after a superficial examination I diagnosed a crop establishment complaint as due to poor quality seed when subsequent digging in the seed bed revealed that the seeds had simply been planted too deeply and were struggling to emerge. So, poor emergence can result from planting too deep or too shallow, from delaying irrigation too long when moisture is deficient, from inadequate preparation of the seedbed, and so on. Good and attentive management is the main defense against these sorts of stand establishment problems.  
    When I was mentally outlining this two-part essay on crop establishment problems I envisaged an informative and innovative final section on strategies for minimizing or evading the multiple causal factors. Now that I am in the final stage, my vision has about deserted me. The obvious strategy to avoid the adverse effects of unfavorable soil temperatures is not to plant too early or too late based on the long-term mean temperatures, but temperatures fluctuate widely from the mean. The solution for deficient moisture is irrigation if irrigation is available and economical, while the solution for excessive moisture is improved drainage that might or might not be feasible and economically viable.                  
    Apart from good farming management, the most technically and cost effective defense against crop establishment problems is the use of good quality seed treated with a recommended chemical seed protectant at the proper dosage. Thus, while seed quality is not always the problem, it is almost always a part of the solution.



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