<P> In the 1970s, scientists created strains of maize, wheat, and rice that are generally referred to as high - yielding varieties (HYV). HYVs have an increased nitrogen - absorbing potential compared to other varieties . Since cereals that absorbed extra nitrogen would typically lodge (fall over) before harvest, semi-dwarfing genes were bred into their genomes . Norin 10 wheat, a variety developed by Orville Vogel from Japanese dwarf wheat varieties, was instrumental in developing wheat cultivars . IR8, the first widely implemented HYV rice to be developed by the International Rice Research Institute, was created through a cross between an Indonesian variety named "Peta" and a Chinese variety named "Dee Geo Woo Gen ." </P> <P> With the availability of molecular genetics in Arabidopsis and rice the mutant genes responsible (reduced height (rht), gibberellin insensitive (gai1) and slender rice (slr1)) have been cloned and identified as cellular signalling components of gibberellic acid, a phytohormone involved in regulating stem growth via its effect on cell division . Photosynthetic investment in the stem is reduced dramatically as the shorter plants are inherently more mechanically stable . Nutrients become redirected to grain production, amplifying in particular the yield effect of chemical fertilizers . </P> <P> HYVs significantly outperform traditional varieties in the presence of adequate irrigation, pesticides, and fertilizers . In the absence of these inputs, traditional varieties may outperform HYVs . They were developed as F1 hybrids, meaning seeds need to be purchased every season to obtain maximum benefit, thus increasing costs . </P> <P> Crop rotation or crop sequencing is the practice of growing a series of dissimilar types of crops in the same space in sequential seasons for benefits such as avoiding pathogen and pest buildup that occurs when one species is continuously cropped . Crop rotation also seeks to balance the nutrient demands of various crops to avoid soil nutrient depletion . A traditional component of crop rotation is the replenishment of nitrogen through the use of legumes and green manure in sequence with cereals and other crops . Crop rotation can also improve soil structure and fertility by alternating deep - rooted and shallow - rooted plants . A related technique is to plant multi-species cover crops between commercial crops . This combines the advantages of intensive farming with continuous cover and polyculture . </P>

What is intensive subsistence farming write three features of this type of farming