Main subject - nov/dec 2008
Skips and double seed drops
Luis Osmar Braga Schuch
Silmar Teichert Peske
The level of technology presently applied to agriculture cropping is responsible for the high productivity per unit area recorded in commercial crop fields. Much work has been undertaken to improve agronomic traits and plant breeding has been essential to the creation of high-yielding cultivars, with enhanced resistance or tolerance to different kinds of stresses. Hence, there is a lot of effort put forward on the use of superior cultivars, high quality seeds, the appropriate use of fertilizers, observance of planting dates and weed and pest control, all of which involve considerable costs.
One of the main agronomic aspect to maximize productivity is the number of plants per unit area and its distribution. This factor has been extensively researched and is highly dependent on the interaction between the cultivar being sown and the cropping environment, i.e. soil fertility, rainfall or irrigation, ambient temperature and sowing date, among others. Today, the breeding/seed companies issue specific information on the plant stand necessary to each cultivar, and for the particular region in which it will be sown.
The recommended plant population per unit area, specifically issued for each cultivar and bound to be distributed among the planting rows, will determine the distance between two successive plants. This arrangement, known as plant distribution will best optimize productivity when in any one row the plants remain at the same distance from one another, i.e. evenly distributed along the row. This arrangement will reduce the level of interplant competition, since each individual will be located at the same distance from any of the other two neighboring plants.
Exemplifying the concepts from the previous paragraph, for a maize crop field sown to a row spacing of 80 cm at a population of 60,000 plants/ha, each plant will be at a distance of 20.8 cm from its neighbor plants. Following this example but referring to soybean, a crop field of this species sown to a row spacing of 45 cm to a population of 250,000 plants/ha will have any one plant distanced 8.9 cm from its neighbors.
To achieve the ideal distribution in each crop species, it is necessary for each seed to be placed precisely at the row position it belongs and, of course, that it originates a plant, i.e. 100% of the seeds germinate and produce an established plant.
In a high-producing crop field all plants are subjected to high levels of competition between them, and the closer they are to one another, the higher the level of competition. Furthermore, the more intense this competition, the lower its dry matter yield, the leaf area index and the grain yield per plant. This is exactly what happens when the stand (plant population) of a soybean crop is changed, reflecting on the grain yield per plant.
When the stand is below the standard, each plant is capable of producing higher grain yields, a situation that is reversed when the stand has exceeded that recommended to optimize yields.
In general, all crop species have developed a certain level of plasticity or buffering capacity, being tolerant to changes on their plant population.
This trait varies with the species and rice and soybean are able to withstand considerable variations on the plant population per unit area, whereas the modern maize hybrids have a narrow tolerance for variations on plant stand. Maize can then be ranked as a low-plasticity species, which reflects its low tillering capacity and the potential to produce only one ear per plant.
To illustrate the importance of competition, consider the effect of plant distribution within each row from two different crop fields that have the same number of plants/ha. Under an ideal distribution arrangement, each plant will be equidistant from its neighbors and when considering a soybean crop sown to achieve 250,000 plants/ha at a row spacing of 0.45 m, each plant will be at a distance of 0.089 m from one another. The main advantage of such distribution is that it delivers the lowest possible level of competition between plants, since they're all equidistant.
Based on the available knowledge on plant nutrition, canopy photosynthesis and soil water availability and uptake among other production factors, any other scheme of plant distribution will impact negatively on its productivity.
Competition can be defined as the interaction between the members of the same community with the purpose of obtaining a growth factor which is necessary to all members of the community but only available at a limited supply.
In highly productive crop fields, the plant community is subjected to a high degree of competition for growth factors that are normally in limited supply; hence, they are insufficient to cope with the total demand of the plant population in the field. Plants will compete for growth factors such as water, light, nutrients, oxygen and carbon dioxide. Under the field environment, the position the plant occupies relative to the others will define the degree of competition to which it will be subjected and the supply capacity for the limiting growth factor. Under a closer distribution than the one described previously, the plants will be exposed to a higher degree of competition for growth factors which will impact on lower growth rates through lower leaf area development with the end result of a reduction on the grain yield of those plants.
In a situation of uniform plant distribution in the crop field, the recorded failures could have been the result of low quality seed. Within this context, failures on the plant population or distribution can be the result of poor seed quality and/or improper planting, causing the seed to be deposited at a different position from which it should have been laid into the row.
Regarding this issue, beside the failure of placing all seeds equidistant within the row, another event occurring and compromising yields is the laying of two seeds instead of only one, commonly known as double seed drops.
In a situation like this two plants try to grow in the same space that would allow such event for only one of them, which will inevitably cause a high level of competition between both that will compromise development traits such as culm diameter and leaf area, with the end result of a decrease on grain yield.
Under a situation of double seed drops, the plants will show the behavior of a highly populated crop canopy, with individual reductions on grain yields.
Under the current scenario in which seeds are being traded/purchased on the basis of a quantity sufficient to plant one unit area (i.e. hectare) and not by weight, if double seed drops occur it implies that there is a high probability that failures will happen in some areas of the crop field. In the event of this happening, besides compromising yield through an individual plant reduction on productivity, there'll also be losses associated to the lack of these plants at the failed seed drop positions.
The main inconvenience for two seeds to be placed together is that it equals a double seeding and the end result is as if only one was yielding grain. Considering a maize crop and the occurrence of only one spot with double seed drop for every linear meter of row, in the case of a crop with a population of 60,000 plants/ha it involves 10,000 more seeds per hectare, which at a cost of US$200.00 for 60,000 seeds implies an excess cost of US$15.00/ha, without any profits to be collected. It is worth remembering that success for any crop involves the interaction of several factors, which together will define the level of profit or loss for each growing season. Some of these factors can be controlled by growers, others not; for the former it is sound to try the best to either enhance them or minimize their impact.
Various mechanisms are available to ensure that plant distribution in the field reaches the ideal or that the negative effects of a nonuniform distribution are minimized, e.g. high quality seed lots, planters with high-efficiency seed distribution mechanisms, seed grading by size, as employed with maize and soybean, the use of products that reduce the friction between seeds such as graphite and polymers, so that they flow more uniformly and ensure an appropriate distribution.
When seed lots of low physiological quality are used, besides considering all the processes that will eventually perform poorly, it is necessary to increase the amount of seed to be planted to compensate for those seeds that will not germinate or will do but won't be able to yield an established plant.
This increase in the quantity of seed sown per unit area will not only increase costs but also affect the stand distribution, as two successive seeds within the same row will be placed at a shorter distance.
Going back to the previous example on soybean in which plants were distanced 8.9cm within the row, and considering adjustment factor of 20% more seeds for germination values lower than 100%, the target population would be of 300,000 plants/ha, which would put the distance between to successive seeds at 7.4 cm. In a scenario like this plants would develop at high densities where no planting failures occurred, and several empty spaces at the row locations where the seeds failed to produce an established plant. The overall yield will then reflect the enhanced competition due to the closer plant distribution as well as for those failure areas, devoid of plants.
Despite soybean shows some degree of plasticity, which enables adjacent plants to cover up for the empty spaces at their side, it is impossible to compensate for the overall area devoid of plants, and the end result will be a decrease on the grain yield per unit area.
In the case of maize, due to lower target stands (longer distance between plants) and the inherent low plasticity of this species, the consequences are more intense and yield reductions more pronounced.
In a situation of low seed quality, the bigger the adjustments on the number of seeds to be planted, the shorter the distance between neighboring seeds/plants, with the end result being an enhanced level of competition for all the individuals. Whenever the environment becomes more adverse at planting, the higher the number of seeds that will not yield plants, i.e. empty spaces within the rows. Research world wide has provided more than enough results relating poor seed quality with high rates of failure to produce established plants. Some of this research has indicated that low quality seed promotes the grouping of plants and an increase on failure frequency, i.e. the number of locations within rows with two to up to five consecutive failures is higher. Another characteristic of poor quality seed lot is its nonuniform distribution of live and dead seeds within the population.
It is common place to illustrate the relevance of seed quality emphasizing that planting 100 kg of seed with 100% germination is not the same as planting seed lots with 50% germination at seeding rates of 200 kg/ha. The first one will definitely yield a better stand, reflecting a uniform seed distribution.
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