Peterborough. The first use within the breed of artificial insemination through frozen semen was with several Australian mares in 1997. Between the 1920s and 1930s and today, the Shire has changed in conformation. The Clydesdale was used for cross-breeding in the 1950s and 1960s, which changed the conformation of the Shire and most notably changed the feathering on the lower legs from a mass of coarse hair into the silky feathering associated with modern Shires. At the peak of their population, Shires numbered over a million. In the 1950s and 1960s, this number declined to a few thousand. In the United States, the Shire population dropped significantly in the early part of the twentieth century, and continued to decline in the 1940s and 1950s. Between 1950 and 1959, only 25 horses were registered in the United States. However, numbers began to increase, and 121 horses were registered in the US by 1985. The National Shire Horse Spring Show is held annually and is the largest Shire show in Great Britain. The conservation status of the Shire is listed by the Rare Breeds Survival Trust as "at risk", meaning that population numbers are estimated to be under 1500 head. In the United States, the Livestock Conservancy lists it as "critical", while the Equus Survival Trust calls it "vulnerable". Characteristics Shire stallions may be black, bay, brown or grey. They may not be roan or have large amounts of white markings. Mares and geldings may be black, bay, brown, grey or roan. In the UK stallions may not be chestnut, but the colour is allowed by the US association. The average height at the withers of grown stallions is about , with a minimum of ; geldings should stand at least , and mares no less than . Weight ranges from for geldings and stallions, with no set standard for mares. The head of a Shire is long and lean, with large eyes, set on a neck that is slightly arched and long in proportion to the body. The shoulder is deep and wide, the chest wide, the back muscular and short and the hindquarters long and wide. Not too much feathering is to occur on the legs, and the hair is fine, straight, and silky. The Shire is known for its easy-going temperament. Shires have been identified to be at risk for chronic progressive lymphedema, a chronic progressive disease that includes