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FORAGE & GRAZING LANDS

Estimation of Forage Intake by Ruminants on Pasture

Texas Agric. Exp. Stn., 1619 Garner Field Road, Uvalde, TX 78801-6205

* Corresponding author (h-lippke{at}tamu.edu)

Intake is the most important component of performance by grazing ruminants. A number of techniques have been adopted to estimate intake because it is not practical to measure intake on pasture directly. These techniques all focus on measurement or estimation of fecal output and digestibility and almost all utilize external or internal markers. Variation in marker recovery has been a distinct disadvantage of these techniques. Alkane marker techniques have recently been developed that appear to greatly reduce this variation. Alkanes in plant cuticular wax are predominantly of odd carbon chain length in the range C₂₅ to C₃₇. The rates of recovery from the digestive tract of ruminants appear to be related to chain length, approaching 100% at about C₃₁. Artificial and naturally occurring alkanes in the range C₃₂ to C₃₄ offer unparalleled precision for the estimation of forage digestibility and intake by ruminants on pasture. When rates of recovery of shorter chain length alkanes are known, the variation in alkane patterns among pasture species or plant components may be used to estimate the intake of various dietary components. Other components of plant waxes, for example, alkenes, may also be useful as markers for delineating the composition of grazed diets. Improved techniques for administering exogenous alkanes to grazing animals continue to increase the usefulness of this class of markers. Mathematical treatment of the excretion pattern of pulse-dosed alkanes and other indigestible markers provides additional information concerning ruminal pool sizes and turnover rates that are helpful for interpreting the dynamics of forage intake and digestion.

Abbreviations: CRD, controlled release device • DDM, digestible dry matter • IVDMD, in vitro dry matter disappearance

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