Spring Pasture Fertilization

Richard Taylor, Extension Agronomist; rtaylor@udel.edu

While preparing to teach my Soil Fertility and Plant Nutrition course this week, I came upon some information that will be of interest to those who have pastures where they graze livestock. Most grazers are aware of the inherent springtime problem with grass tetany which is caused by low blood magnesium (Mg) levels. The rapid growth of forages in the spring plus cool, cloudy and sometimes rainy weather can restrict both root growth and transpiration in forages to the point that magnesium levels in the grass are too low for animal health.

However, another complicating factor is that we often try to stimulate early growth of pastures with nitrogen (N) fertilizer and with the blending capabilities of many fertilizer companies we can easily add in the extra potassium (K) that the soil test might suggest. Nutrients can actually be what is called antagonistic; where one or more nutrients act together to depress the uptake of another nutrient. This is the case for N and K, which if applied in too large amounts can depress further the uptake of Mg by forages in the spring. The depressed uptake of Mg then puts the grazing animals at risk for grass tetany. To avoid the possibility, hold off K applications until late May or early June since K applied at this time will be much less likely to increase the incidence of grass tetany and will improve the tolerance of the grass to the various stresses encountered during the hot summer months.

Nitrogen application in very early spring is useful in getting enough pasture growth to begin grazing a week or two earlier than you otherwise could. If the soil test indicates that soil Mg levels are low, use only a small amount of N in early spring and wait until mid-May to apply larger amounts. This should help reduce the chance for grass tetany but still provide plenty of early season grazing.

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