Responses of herbage P, Ca, K and Mg content and Ca/P and K/(Ca plus Mg) ratios to long-term continuous and discontinued cattle grazing on a rough fescue grassland

Herbage minerals affect performance of grazing cattle. We investigated the response of herbage P, K,Ca and Mg contents and Ca/P and K/(Ca + Mg) ratios to long-term stocking rate, continuous vs. discontinued grazing practice, and sampling year. Cattle had been stocked at 2.4 and 4.8 animal unit months ha(-1) since 1949. Exclosures were installed in April 1998. Herbage samples were collected near peak herbage mass in 2001, 2003, 2008 and 2012 and analysed for mineral content. Mineral contents were similar between the two stocking rates, but were lower (P < 0.05) under discontinued than continuous stocking, with the exception of similar P contents. The content of P and K in herbage was higher and the content of Ca and Mg was lower (P < 0.05) in years with greater precipitation and lower temperatures. Herbage mineral content, with the exception of P, exceeded minimum recommended levels for cattle. Given the low P content in herbage (0.74-1.19 g kg(-1)) and high Ca/P ratios during the dry and hot year (of 2001), a dietary P supplement should be considered for cattle grazing rough fescue grassland in drought years. The low K/(Ca + Mg) ratios (< 2.2) suggest there is little risk of grass tetany in cattle grazing on this grassland.