THE EFFECT OF A HIGH-MAGNESIUM PERENNIAL RYEGRASS VARIETY ON THE MAGNESIUM STATUS OF SHEEP

An early-heading variety of perennial ryegrass (Ramore) bred for high magnesium (Mg) content was compared with an early control variety (Fra nces) for its ability to increase Mg intake, Mg utilization and blood Mg concentration in sheep, and so reduce the incidence of hypomagnesae mic tetany. Swards of the two varieties were established and each was grazed by ewes suckling twin lambs from turn-out to weaning and therea fter by dry ewes. In early and late season herbage from a proportion o f each award was cut daily and fed fresh to ewes housed indoors in met abolism crates. Under grazing conditions herbage from Ramore swards ha d, over the 1992 and 1993 grazing seasons, a higher Mg concentration t han Frances (2.35 g kg(-1) vs. 2.18 g kg(-1); s.e.m. 0.019, P < 0.001) . During the spring period there was a higher (P < 0.001, s.e.m. 0.007 4) Mg concentration in the blood of ewes grazing Ramore (0.854 m mol 1 (-1)) swards compared with those grazing Frances (0.793 m mol 1(-1)) w hereas during the autumn period blood Mg concentrations were higher in ewes grazing Frances. Ewe and lamb liveweight gains, recorded in 1992 only, were higher on Ramore swards than on Frances swards, although t he difference was not significant. Indoor feeding studies showed that Mg intake was significantly higher (P < 0.001, s.e.m. 0.026) in ewes o ffered Ramore (1.98 g d(-1)) herbage compared with those offered Franc es (1.74 g d(-1)) herbage with no significant difference in dry-matter intake between the two varieties. Apparent availability and retention of Mg were higher for animals fed Ramore during the spring feeding pe riods than those fed Frances, but during the autumn feeding periods th e reverse was true. Under indoor feeding conditions, variety had no si gnificant effect on blood Mg concentrations. It was concluded that und er grazing conditions a high Mg variety could provide an effective mea ns of reducing the incidence of hypomagnesaemic tetany.