Over a decade ago, funding by this Lime and Fertilizer Committee allowed my group to begin research program on finding an answer to the grass tetany problem faced by Missouri cattlemen. Using this initial funding and then continuous support by PPI/FAR, we found that the level of free phosphorus (Bray 1) in the soil solution is the key for increasing magnesium uptake and leaf concentrations of tall fescue in Missouri pastures. We started this research with hydroponic studies in growth chambers, moved to Perlite pots in the greenhouse, then to tall fescue plots at our Southwest Center near Mt. Vernon, and now we are in the third year of grass tetany project with lactating beef cows on tall fescue pastures at Mt. Vernon. In our studies, as little as 12.5 lbs P/acre applied in late February or early March increased leaf Mg concentrations in late March and throughout April, significantly. Currently, in growth chamber, hydroponic studies, we are finding that boron deficiency causes magnesium uptake problems in squash, alfalfa and soybean. Consistently, under boron deficient conditions, magnesium uptake is rapidly impaired. Based on these results, we would like to move to field studies at the SW Center near Mt. Vernon, where many of the soils are rather low in boron, and are typically low in plant available phosphorus with a high rate of phosphorus fixation. We would like to test phosphorus and boron applications, separately and in combination, to determine if the combination will bust the magnesium levels in leaves above those that we find with phosphorus fertilization alone.

Methods:

Tall fescue plots will be selected at Mt. Vernon in an area where untreated soil Bray I P levels are often around 10 lbs P/acre on the Creldon silty clay loam soil. The first week in March, we will add phosphorus fertilizer to treated plots at the rate of 12.5 or 25 lbs P/acre, as triple super phosphate, and boron will be added to certain plants at 0.5 and 1.0 lbs B/acre as boric acid. The key treatments will be all combinations of these phosphorus and boron treatments with the following treatments in total: 0, 12.5 P, 25.0 P, 0.5 B, 1.0 B, 12.5 P + 0.5 B, 12.5 P + 1.0 B, 25 P + 0.5 B and 25 P + 1.0 B. These nine treatments will be replicated six times for a total of 54 plots and plots with be 10 ft x 25 ft. At the time of P and B application, K and N will also be applied to all plots at the levels of 100 lbs K/acre as KCl and N will be added as NH4NO3 at the level of 100 lbs N/acre. Leaf samples will be collected from each plot on the third week in March, the first and third weeks of April. Also, leaf samples will be taken the third week in May immediately before hay is cut. Grab samples of the hay also will be taken. Leaf samples will be taken from each plot in September and in November. All leaf and hay samples will be wet ashed with nitric/perchloric acids and digested samples will be used for flame ionization, atomic absorption analyses of K, Mg and Ca. Phosphorus concentrations in the samples will be determined colorimetrically. Data will be analyzed using an analysis of variance.