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Arkansas Animal Science Department Report 2001
Four cages were placed in each field to prevent grazing
and to allow forage growth to be evaluated. Available forage
was measured inside and outside the cages to estimate forage
removal by cattle. Cages were moved monthly and forage
samples were clipped from the pastures and analyzed for
nutrient content. Macromineral concentrations of the forage
were compared to the nutrient requirements of beef cattle
(Davis, 1996). Requirements for cattle that were gestating
and in early lactation were used.
Results and Discussion
In general, forage concentrations of K, Ca, and P were
in excess of the recommended nutrient requirements for beef
cattle throughout most of the grazing season from all three
farms. Potassium concentrations were extremely high for all
farms in early spring (at least five times the required amount)
and continued to stay above the requirement for beef cattle
until January (Fig. 1). The lowest concentration of K during
the grazing season occurred in June at Farm 1. This concentration
was still nearly twice the amount required by early
lactating cows. Forage K concentrations fell below animal
requirements during February on Farm 3 and during January
and February on Farm 1. Calcium concentrations were considerably
lower than K concentrations, but most Ca concentrations
exceeded requirements for gestating and lactating
beef cattle (Fig. 2). Calcium concentrations started high in the
spring and slowly decreased to the required amount by winter.
The exception to this was forage gathered from Farm 2 in
early May that met the Ca requirement for gestating cows but
was insufficient for lactating cows. Forage magnesium (Mg)
concentrations remained fairly constant throughout much of
the grazing season with the exception of Mg concentrations
from Farm 1, that decreased and became deficient from late
May throughout the remainder of the year (Fig. 3). Forage
Mg concentrations remained below the requirements for lactating
beef cows during the winter months. Phosphorus levels
exceeded cattle requirements at all three farms throughout the
grazing season and declined to levels approximating cattle
requirements during the winter months (Fig. 4).
High K values coupled with low Ca and Mg values
could lead to grass tetany problems. Kemp et al. (1956) found
that when the K/(Ca + Mg) ratio was above 2.2 the occurrence
of tetany cases increased substantially. This ratio was
plotted for each farm based on the concentrations of K, Ca,
and Mg in the forage sampled each month (Fig. 5). For all
farms, the ratio exceeded the threshold value of 2.2 throughout
most of the grazing season and particularly during the
spring grass tetany season. The grass tetany ratio fell below
this threshold during the winter months when hay would typically
be fed. Although the forages contained 0.2 percent Mg
or more, caution should be exercised and supplemental magnesium
should be fed when the K/(Ca + Mg) ratio exceeds the
2.2 threshold value.
Implications
Forages fertilized with broiler litter may contain
macromineral concentrations in excess of the requirements of
beef cattle. These concentrations could reduce the need for
supplemental minerals, particularly Ca and P. However,
excessive concentrations of K in the forage could lead to
problems such as grass tetany. When high levels of K are
present and Mg and Ca levels are relatively low, supplements
of Mg may need to be given to prevent tetany problems, especially
in the spring. Prior to using a commercial fertilizer, a
soil analysis should be obtained so that only the nutrients
deficient in the soil are added. This should lead to more efficient
use of fertilizers and help limit problems associated
with cattle performance and environmental issues.
Literature Cited
Kemp, A. and Hart, M.L. 1956. Neth. J. Agric. Sci. 5:4-17.
Davis, G.V., Jr. 1996. Cooperative Extension Service.
MP391.
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