The number of rhinos killed for their horns in South Africa decreased in the first six months of the year as authorities increased efforts to tackle poaching of the endangered animals. South Africa and neighbouring Namibia and Botswana have for decades struggled to control the poaching of rhinos whose horns are prized for jewellery and traditional medicine in some east Asian countries. Between Jan. 1 and June 30, 231 rhinos were killed in South Africa for their horns - made primarily of keratin, a protein also found in human hair and fingernails - 28 fewer than the same period last year, South Africa's environment ministry said.