MACROMINERAL CONCENTRATIONS OF GRAZED FORAGE FERTILIZED WITH BROILER LITTER

Authors: HUMPHRY, BRYON - UNIV OF AR; COFFEY, KEN - UNIV OF AR; Sauer, Thomas; GOODWIN, HAROLD - UNIV OF AR

Submitted to: Arkansas Experiment Station Research Series
Publication Type: Experiment Station
Publication Acceptance Date: 12/1/2001
Publication Date: 12/1/2001
Citation: Humphry, B., Coffey, K., Sauer, T.J., Goodwin, H. 2001. Macromineral concentrations of grazed forage fertilized with broiler litter. Animal Science Department Report - 2001. Arkansas Experiment Station Research Series 488.

Interpretive Summary: The nutritional value of grasses depends on the amount of individual nutrients in the soil. Grass grown on soils with too much or too little of certain nutrients can be damaging to the health of cattle. This study measured the nutrient content of grass grown on four farms that used poultry litter (chicken manure with bedding material) as a fertilizer. The results show that poultry litter provides enough potassium, calcium, and phosphorus to grow good quality grass for pregnant cows and cows with young calves. However, in the spring, there may not be enough magnesium in the grass to prevent a disorder called grass tetany. It is recommended that ranchers provide a mineral supplement in the spring that contains magnesium to prevent grass tetany in their cows. This research is important in assisting ranchers who graze beef cattle on pastures where animal manure rich in potassium is spread. It shows that feeding a magnesium supplement in the spring may be necessary to prevent grass tetany in their cows.

Technical Abstract: Three farms in Northwest Arkansas and Northeast Oklahoma that utilized broiler litter for fertilizer were monitored for nutrient cycling from April 2000 to February 2001. Forage samples were taken monthly and analyzed for K, Ca, P, and Mg. Concentrations of these minerals in the forages were compared to the requirements of gestating and early lactating beef cattle. The grass tetany ratio (K/[Ca +/ Mg]) was calculated for each farm at each sampling date to determine the likelihood of grass tetany problems. Calcium and P concentrations for all farms met or exceeded requirements for lactating cows from April through December. Potassium concentrations from all three farms exceeded the requirements for lactating beef cows from March through early January. Magnesium concentrations on Farm 1 were deficient for cattle in early lactation throughout much of the year except for April and May and were deficient for Farm 3 from early November through February. Forages from Farms 2 and 3 had sufficient quantities of Mg throughout most of the grazing season to meet the requirements of lactating beef cows. However, the grass tetany ratio was above the desirable threshold for all farms throughout the typical spring grass tetany season (February to April) indicating a strong potential for grass tetany on these pastures. Therefore, pastures fertilized with broiler litter may meet the macro-mineral requirement of lactating beef cows, but the grass tetany ratio may be sufficiently high to warrant supplementation of cattle diets with Mg to prevent tetany problems.