Deliberate use of crossing to generate variation

Pioneered in ornamentals (rose and chrysanthemum) in China. Chrysanthemum mentioned by Confucius (孔夫子) in the fifth century BC and was probably being actively bred there from the early centuries AD

Hybridization in cereals started in the late 1800's:

Wheat 1870s:
In France, Henry de Vilmorin made systematic crosses between English and French wheats, released the variety "Dattel" in 1883.
1900: In Australia, William Farrer released the rust-resistant, high-yielding "Federation"

Maize
1923: In USA, the first commercial maize hybrid Copper Cross produced.
1934: <0.5% of US maize acreage occupied by hybrids; by 1944: 59%, and by 1954: 100%.