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 (Frances) for its ability to increase Mg intake, Mg utilization and blood Mg concentration in sheep, and so reduce the incidence of hypomagnesaemic tetany. Swards of the two varieties were established and each was grazed by ewes suckling twin lambs from turn-out to weaning and thereafter by dry ewes. In early and late season herbage from a proportion of each award was cut daily and fed fresh to ewes housed indoors in metabolism crates. Under grazing conditions herbage from Ramore swards had, over the 1992 and 1993 grazing seasons, a higher Mg concentration than 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. 0.0074) 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)) whereas 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 the difference was not significant. Indoor feeding studies showed that Mg intake was significantly higher (P < 0.001, s.e.m. 0.026) in ewes offered Ramore (1.98 g d(-1)) herbage compared with those offered Frances (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 periods than those fed Frances, but during the autumn feeding periods the reverse was true. Under indoor feeding conditions, variety had no significant effect on blood Mg concentrations. It was concluded that under grazing conditions a high Mg variety could provide an effective means of reducing the incidence of hypomagnesaemic tetany.