Partition of Mg in Forages as Affected by Fertilization, Species, Growth Stage, and Sample Preparation1

Abstract

AbstractA 2‐year study was conducted in order to study the chemical form of Mg in different grass and legume species that are commonly used for livestock feeding in the Northeastern United States. Orchardgrass (Dactylis glomerata L.), tall fescue (Festuca arundinacea Schreb.), and alfalfa (Medicago sativa L.) were treated with magnesium sulfate (MgSO4. 7H2O) and dolomitic limestone (CaCO3.MgCO3) at 280 kg Mg ha−1, or with no Mg fertilizer. The soil was a fine, loamy, mixed, mesic Hapludalf. Grass species received 34 or 224 kg N ha−1 as NH4NO3. In the 1st year samples were taken at four growth stages, prepared as fresh, oven‐dried or freeze‐dried herbage and partitioned into acetone soluble, water soluble, and acetone and water insoluble fractions. In the 2nd year samples were taken at two growth stages and prepared as fresh material only. Fertilization with MgSO4. 7H2O increased (P<0.01) total Mg concentrations in herbage compared to CaCO3. MgCO3 and no Mg treatments. High N fertilization of the grass species increased (P<0.01) total Mg levels compared to low N treatments. Magnesium as percent total Mg in each chemical fraction was affected (P<0.05) by forage species, stage of maturity and method of sample preparation and ranged from 0.7 to 24.8% (x̄ = 6.7%) for acetone soluble, 42.8 to 82.3% (x̄ = 65.7%) for water soluble, and 15.5 to 42.9% (x̄ = 27.6%) for acetone and water insoluble Mg fractions over all treatments. The proportion of total Mg in the water soluble fraction was higher (P<0.05) at mature growth stages for alfalfa and tall fescue compared to orchardgrass and the proportion of Mg extractable with acetone was less (P<0.0001) for dried compared to fresh sample preparations. Although fertilizer treatments affected total herbage Mg concentrations, there was no apparent effect of fertilization upon the chemical form of Mg in the plant.