<P> Many of the maize varieties grown in the United States and Canada are hybrids . Often the varieties have been genetically modified to tolerate glyphosate or to provide protection against natural pests . Glyphosate is an herbicide which kills all plants except those with genetic tolerance . This genetic tolerance is very rarely found in nature . </P> <P> In the midwestern United States, low - till or no - till farming techniques are usually used . In low - till, fields are covered once, maybe twice, with a tillage implement either ahead of crop planting or after the previous harvest . The fields are planted and fertilized . Weeds are controlled through the use of herbicides, and no cultivation tillage is done during the growing season . This technique reduces moisture evaporation from the soil, and thus provides more moisture for the crop . The technologies mentioned in the previous paragraph enable low - till and no - till farming . Weeds compete with the crop for moisture and nutrients, making them undesirable . </P> <P> Before the 20th century, all maize harvesting was by manual labour, by grazing, or by some combination of those . Whether the ears were hand - picked and the stover was grazed, or the whole plant was cut, gathered, and shocked, people and livestock did all the work . Between the 1890s and the 1970s, the technology of maize harvesting expanded greatly . Today, all such technologies, from entirely manual harvesting to entirely mechanized, are still in use to some degree, as appropriate to each farm's needs, although the thoroughly mechanized versions predominate, as they offer the lowest unit costs when scaled to large farm operations . For small farms, their unit cost can be too high, as their higher fixed cost cannot be amortized over as many units . </P> <P> Before World War II, most maize in North America was harvested by hand . This involved a large numbers of workers and associated social events (husking or shucking bees). From the 1890s onward, some machinery became available to partially mechanize the processes, such as one - and two - row mechanical pickers (picking the ear, leaving the stover) and corn binders, which are reaper - binders designed specifically for maize (for example, Video on YouTube). The latter produce sheaves that can be shocked . By hand or mechanical picker, the entire ear is harvested, which then requires a separate operation of a maize sheller to remove the kernels from the ear . Whole ears of maize were often stored in corn cribs, and these whole ears are a sufficient form for some livestock feeding use . Today corn cribs with whole ears, and corn binders, are less common because most modern farms harvest the grain from the field with a combine and store it in bins . The combine with a corn head (with points and snap rolls instead of a reel) does not cut the stalk; it simply pulls the stalk down . The stalk continues downward and is crumpled into a mangled pile on the ground, where it usually is left to become organic matter for the soil . The ear of maize is too large to pass between slots in a plate as the snap rolls pull the stalk away, leaving only the ear and husk to enter the machinery . The combine separates out the husk and the cob, keeping only the kernels . </P>

What is the type of leaf of maize