Some factors affecting urinary calculus formation in weaned lambs

Abstract

The objective of undertaking this study was to examine the relationship between various dietary factors (physical form of the diet, dietary nitrogen level and source, different levels and combinations of calcium, magnesium and phosphorus) and urinary calculus formation in lambs weaned at 4 weeks of age.The first experiment was an incomplete factorial utilising 64 lambs. The results obtained showed that neither pelleting the diet, dietary nitrogen level nor source initiated calculus formation. Pelleting the diet increased mineral digestibility and subsequently urinary mineral excretion was increased. Supplementation of the diet with urea or protein reduced the concentration of urinary minerals although phosphorus values for the urea-fed animals were much lower than those for the protein-fed animals.The second experiment was a complete factorial utilising 64 lambs. The results indicated that supplements of both calcium and magnesium were likely to reduce calculus formation whereas phosphorus supplements enhanced urinary excretion of phosphorus. Calculus formation was not associated with any particular dietary combination of minerals but was more a function of time postweaning. There was a distinct time effect in relation to urinary mineral concentration post¬ weaning: urinary mineral concentrations were very high (e.g. phosphorus, 95,6mg/lOOml) in the immediate postweaning period (2 to 4 weeks) but decreased to comparatively low levels (e.g. phosphorus, 40.9mg/100ml) at 7 to 9 weeks postweaning. Calculi occurred when urinary mineral con¬ centrations were at their highest in the immediate post¬ weaning period. Autopsy findings indicated calculi were of renal origin and composed largely of magnesium phosphate.The results of Experiment II indicated that the transitional period at weaning was important in relation to calculus formation. The water economy at this time was measured in Experiment III and it was found that water intake and urine output were at least halved postweaning. Also, during the preweaning period, water retention was a function of water intake whereas postweaning, it was under renal control. This control was imprecise over the immediate 10 day postweaning period and it appeared that this might be the time when calculus formation was initiated.An attempt to supplement water intake via feeding tubes (Experiment IV) during the postweaning period was unsuccess¬ ful as lambs regurgitated the tubes. Another experiment (Experiment V) was undertaken in which additional water was supplied to lambs via cannulae inserted in the reticulorumen. The water economy of these lambs was compared with those weaned normally. The results obtained were similar to those in Experiment III although there was no evidence of water restriction, in the normally weaned lambs, causing renal deposits associated with calculus formation.The results obtained and their implications in the aetiology and prevention of urolithiasis are discussed.