The efficacy of intraruminal pellets composed of elemental selenium and iron in sheep

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Abstract
Commercial selenium pellets, manufactured after CSIRO workers drew attention to the significance of grain size on the rate of release of selenium, were tested in 27 sheep grazing a low-selenium New Zealand pasture. The pellets were shown by microscopy to contain mainly 10–20 µm particles of selenium, often agglomerated into larger lumps. There was a considerable variation in the length of time pellets maintained blood selenium levels above the deficiency level of 250 nmol/ℓ (20 µg/ℓ). Whereas four animals given pellets had blood levels below 250 nmol/ℓ after only 343 days, two animals had levels of 375 and 400 nmol/ℓ after 651 days when the level in control sheep was 125±32 nmol/ℓ. The pellets were recovered from all but one animal and had varying degrees of surface coating which was assumed to be mainly calcium phosphate. Two pellets recovered from sheep at 386 and 484 days, when blood selenium levels were 175 and 1813 nmol/ℓ respectively, were sectioned and examined by light and electron microscopy. Both pellets still contained unreacted selenium but differed in the degree of surface coating. The pellet recovered at 386 days had a solid and continuous coating whereas the coating on the pellet recovered at 484 days was not continuous and consisted of an open lattice of interlocking needles. It appears that it is the extent of this coating which limits the effective life of the pellet in sheep.