Urolithiasis – a review of the causal factors in feedlot sheep and goats

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Urolithiasis is most commonly seen in feedlot wether (castrate male) lambs. While uroliths (urinary calculi) are deemed to form equally in males and females, blockages occur in wethers as a result of them having a narrow urinary tract outlet.

Uroliths form as a result of mineral precipitates in the urine, with the addition of mucoprotein, which acts as a cementing agent. In feedlot animals, struvite crystals / calculi (magnesium ammonium phosphate) are most common. However, calcium phosphate (calcium apatite) or calcium oxalate calculi may also occur.

Diet plays an important role, and in feedlot animals diets are typically concentrate (grain) based and are also frequently pelleted. Grain based diets are typically relatively high in phosphorus (P) and low in calcium (Ca). Pelleting of diets decreases saliva, which results in increased urinary P and probably increases the urinary mucoprotein content.

Urinary P levels are very important in determining the presence of calculi. The single most important dietary factor associated with preventing urinary calculi in feedlot sheep appears to be to maintain Ca: P ratios of at least 2:1. High P diets result in urinary calculi, and decreased growth rates. Addition of calcium decreases the risk of urolithiasis, and negates growth rate effect of high P.

High dietary P levels have been associated with increased levels of calculi (and urolithiasis) despite adequate Ca: P ratios, although in most experiments it appears that increasing Ca levels in the diet negates most if not all the impact of the high P diet.

Both urine output and urine pH are important factors. Increased urine output dilutes the P in the urine and decreases likelihood of calculi forming. The role of dietary magnesium in urolithiasis is not clear. Logically, given the main calculi in feedlot lambs are struvite which contain magnesium, it would seem obvious that higher dietary magnesium (Mg) levels would be associated with increased calculi formation. However, increased Mg also increases urine output, which would help decrease risk.

The following should be considered when formulating a pelleted feedlot ration for lambs to minimise the risk of urolithiasis:
• The Ca: P ratio should be 2:1 or greater – this is critical
• The P level is recommended to be less than 0.6% ie 6 mg/kg, as increased P levels have been shown to increase risk of uroliths even when the Ca: P ratio is 2:1.
• Increasing urine output by the addition of 1% salt would appear prudent
• The addition of ammonium chloride at a maximum of 1% should be considered, although given the possible decrease in growth rates it is reasonable to reserve the inclusion of ammonium chloride until any problems are detected.
Access to clean drinking water with low mineral status is also essential.
Original languageEnglish
Publication statusPublished - 2017
Event9th International Sheep Veterinary Congress - Harrogate Convention Centre, Harrogate, United Kingdom
Duration: 22 May 201726 May 2017
http://www.sheepvetsoc.org.uk/isvc2017 (Conference website)


Conference9th International Sheep Veterinary Congress
Country/TerritoryUnited Kingdom
OtherThe Ninth International Sheep Veterinary Congress will be held in Harrogate, England over a period of five days between 22nd and 26th May 2017, forming the basis for enduring longer- term collaboration between colleagues with complementary interests in small ruminant health and production. The aim is to provide a platform for the translation of applied research findings in the fields of genetics, animal husbandry and disease management into economically and environmentally sustainable utilisation of natural resources by small ruminants in their target environments.
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