High-yielding variety
From Wikipedia, the free encyclopedia
High-yielding varieties (HYVs) are any of a group of genetically enhanced cultivars of crops such as rice, maize and wheat that have an increased growth rate, an increased percentage of usable plant parts or an increased resistance against crop diseases. Those crops formed the basis for the Green Revolution. In general, they require a higher level of agricultural care, such as intensive disease control, higher fertilizer levels and controlled water supply. While those crops have enabled the multiplication of agricultural production, their increased demands of fertilizers, pesticides and water control have drawn criticism from environmentalists. Another name is high-response variety, as the high yield requires high input.
[edit] High-yielding varieties versus traditional cultivars
The difference in gross primary production between traditional and high-yielding varieties is limited, because until 2005 all attempts to improve the photosynthetic efficiency of any crop have failed. The main explanation of the difference in yield between high-yielding varieties and traditional cultivars is the increased rate in which primary plant production is redirected to harvestable products, such as grains. High-yielding varieties of food grains in general have much shorter stems than traditional cultivars, which solves two problems: excessive investment of primary production in the stem and eliminating the balancing problem, which limits the weight of the halm and limits maximum production in traditional cultivars. In addition, HYV cultivars respond favourably to high fertilizer gifts, e.g. nitrogen. This is achieved, because an HYV invests gross photosynthetic production in live cell material rather than in woody structural materials, thus allowing for an increased vegetative growth. As a consequence of their increased growth rate, HYV crops lack an important plant defence mechanism against predatory animals or fungi and as a result are more prone to pests and diseases than traditional cultivars, thus requiring both intensive fertilizer input and high levels of pesticides.
A second characteristic of high-yielding varieties is their homogeneity. In general, HYVs are a cross between two inbred lines which produces a homogenous genotype as offspring, a so-called F1 hybrid, which is genetically homogenous. This makes F1 hybrid HYVs very suitable for mechanical crop care and processing and makes it likier that a certain choice of management parameters will approach the optimum combination for an average plant, thus increasing yield. A drawback of the absence of genetic variation in F1 hybrid HYV crops is the alarming rate in which pests and plant diseases can spread. The high variation in genetic characteristics between plants in a traditional variety make it less likely that a particular pest can attack all plants, but makes it less easy to protect all plants against infestations.
High-yielding varieties often have certain kinds of disease resistance built in, such as resistance against rust, blight or other fungus diseases which reduce the leaf surface and hence hamper the primary production of crops. Because of their homogeneity, disease resistances can be crossed in the population with ease.
