141 Mineral Requirements of Dairy Cattle: Emphasis on organic minerals

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.

Calcium and phosphorus utilization in growing sheep supplemented with dicalcium phosphate

SUMMARYThe objective of the current study was to evaluate the utilization of calcium (Ca) and phosphorus (P) in growing sheep consuming increasing amounts of dicalcium phosphate. Eighteen growing sheep, aged 8 months, were fed a basal diet supplemented with 0, 12·5 and 25 g of dicalcium phosphate/day. During the experiment, animals were injected intravenously with 7·4 MBq of 45Ca and 32P and samples of plasma, faeces and urine were subsequently taken daily for 1 week after injection. Rumen fluid was sampled on days 4–7 after injection. Specific radioactivity in plasma and in faeces were used to determine true absorption of Ca and P, whereas plasmatic and ruminal specific radio-activities were used to determine endogenous P flow into the rumen and turnover time of rumen P. Increasing dicalcium phosphate intake led to linear increases in faecal excretion of endogenous Ca and P (P<0·05), suggesting that surpluses of ingested Ca and P were voided through secretion to the gut. True absorption coefficients for 0, 12·5 and 25 g of dicalcium phosphate ingested daily were 0·54, 0·41 and 0·38 for Ca, and 0·66, 0·62 and 0·64 for P, respectively. Flows of endogenous P into the rumen increased linearly and ruminal turnover time of P decreased linearly (P<0·01) as P intake was increased. Concentrations of Ca and P in bone were not affected by the increased amounts of these minerals ingested (P<0·05). In conclusion, increasing ingestion of dicalcium phosphate increases faecal excretion of Ca and P, thus decreasing the efficiency of utilization of both minerals. Moreover, increasing levels of dietary P increased endogenous P excretion, contributing to the amount of P disposed of in the environment.

Adaptive responses in men fed low- and high-copper diets

The study of Cu metabolism is hampered by a lack of sensitive and specific biomarkers of status and suitable isotopic labels, but limited information suggests that Cu homeostasis is maintained through changes in absorption and endogenous loss. The aim of the present study was to employ stable-isotope techniques to measure Cu absorption and endogenous losses in adult men adapted to low, moderate and high Cu-supplemented diets. Twelve healthy men, aged 20–59 years, were given diets containing 0·7, 1·6 and 6·0 mg Cu/d for 8 weeks, with at least 4 weeks intervening washout periods. After 6 weeks adaptation, apparent and true absorption of Cu were determined by measuring luminal loss and endogenous excretion of Cu following oral administration of 3 mg highly enriched65Cu stable-isotope label. Apparent and true absorption (41 and 48% respectively) on the low-Cu diet were not significantly different from the high-Cu diet (45 and 48% respectively). Endogenous losses were significantly reduced on the low- (0·45mg/d;P<0·001) and medium- (0·81 mg/d;P=0·001) compared with the high-Cu diet (2·46mg/d). No biochemical changes resulting from the dietary intervention were observed. Cu homeostasis was maintained over a wide range of intake and more rapidly at the lower intake, mainly through changes in endogenous excretion.