Using Magnesium and Phosphorus Fertilization to Improve the Macronutrient Quality of Stockpiled Tall Fescue During Winter Months.

Much of Missouri’s 13+ million acres of tall fescue pasture is grown on soils with low plant available P (Bray I).  Stockpiling tall fescue allows fall growth to accumulate for grazing in late fall and winter.  We have shown that leaf P and Mg concentrations of stockpiled tall fescue decrease to below the 0.2% levels required for lactating beef cows and that Mg fertilization alone did not increase leaf Mg concentration on these low P soils.  P fertilization, however, did increase leaf Mg concentration, albeit levels still fell below the cow’s requirement.  This study focuses on maintaining leaf Mg and P concentrations of stockpiled tall fescue above the required levels for lactating beef cows through the use of both P and Mg fertilization.  An established tall fescue pasture in southwest Missouri on a low plant available P soil (5 lbs/acre Bray I) was selected.  Treatment combinations of 0, 50, 100, or 200 lbs P/acre and 0 or 50 lbs Mg/acre were applied in September 2006.  Twenty of the most recently collared leaves in each plot were harvested monthly from November 2006-April 2007 and October 2007-April 2008 for macronutrient analysis.  P fertilization increased leaf P concentrations incrementally during both stockpiling periods.  All P fertilized plots had increased leaf P concentrations above the 0.2% requirement only throughout the first stockpiling period.  Mg fertilization did not affect leaf P concentrations and only increased leaf Mg concentrations when applied with P.  Leaf Mg stayed near or above 0.2% in late winter and early spring during both years when Mg was applied with P.  This study suggests leaf Mg concentrations may be suitably increased in stockpiled tall fescue when Mg fertilizer is applied in addition to P.