141 Mineral Requirements of Dairy Cattle: Emphasis on organic minerals

Abstract

Abstract Requirements for most minerals are expressed on an absorbed basis. The true absorption of minerals can vary widely because of source, presence of antagonists, dietary concentrations, and animal mineral status; therefore, expressing mineral requirements on an absorbed basis is theoretically sound. Unfortunately measuring true absorption of minerals is exceedingly difficult; available data is limited which means that often we are using constants. Requirements for lactation, growth, and conceptus growth are known with reasonable certainty; however establishing maintenance requirements is plagued with methodological difficulties, and the classical definition of maintenance (i.e., replenishment of inevitable fecal and urinary losses) ignores effects on water balance, acid-base balance in the rumen, and the ruminal and intestinal microbiome. For Na, Cl- and K, absorption is essentially 100% regardless of source. Lab methods are available to estimate absorption of P. Source of Ca accounts for the majority of the variation in Ca absorption and constants are available for feedstuffs and supplements. We have good data on absorption of Mg from basal diets and are able to estimate antagonism of Mg absorption caused by K. We are less able to estimate absorption of the various Mg supplements, some of which are highly variable. Except for the electrolytes (which affect water balance), requirements are known with reasonable certainty for macrominerals. Absorption coefficients for trace minerals (TM) are known with much less certainty. Source of supplemental TM (e.g., organic vs sulfates) can affect absorption but the effect depends on the mineral TM (e.g., source affects absorption of Cu more than that of Mn) and on interactions with basal diet. Maintenance requirements are generally poorly defined for TM and because of ‘non-factorial’ requirements such as effects on microbiome, source of TM likely affects requirements. This means that for some TM, the factorial approach to requirements may not be adequate.