The effect of different methods of controlling urolithiasis on ovine mineral metabolism.
The widespread use of high-energy, low roughage diets among feedlot sheep has lead to the development of several production diseases (Bide et al., 1973). One of the least easily identifiable is urolithiasis, yet it is an important cause of death among feedlot sheep (Emerick, 1988). The primary causative factors of urolithiasis are an alkaline urine and a high urinary P level (Bushman et al., 1965a, 1965b, 1968). The prevention of this disease therefore involves the use of an anionic salt such as NH4Cl in the diet, to acidify the urine, or the use of a high Ca:P ratio in order to decrease urinary P levels (Bushman et al., 1965a; Robbins et al., 1965). At present NH4Cl is included in sheep rations with the express purpose of preventing urolithiasis. However, this method has a disadvantage as anionic salts have been shown to cause metabolic acidosis (Harmon & Britton, 1983) and therefore the second means of prevention, that of a high Ca:P ratio, may be the more suitable method. For this reason, an experiment was designed in order to determine whether NH4Cl or a high Ca:P ratio was the better method of urolithiasis prevention with respect to the animal's performance, mineral metabolism and acid-base status. Furthermore, the effect of Ca and NH4Cl on Se metabolism was studied as very little work has previously been done on this subject. With this objective in mind, a growth trial and digestibility study were conducted. For the growth trial, a 3 x 2 x 2 factorial experiment was designed with three levels of NH4Cl (0, 0.75 and 1.5%) at a high (4: 1) and medium (2.5: 1) Ca:P ratio. Se was included in the diet at a level of 0 and 0.3mg/kg. The trial extended over a period of 74 days, and during this time weight and feed intake were measured, and blood, urine and faecal samples were collected for mineral and acid-base status analysis. At slaughter, the liver, kidney, heart, pancreas and a portion of the Longissimus dorsi muscle were removed for mineral analysis. Fluid from various sections of the digestive tract was sampled for digesta pH determination. The digestibility trial was designed as a 4 x 4 latin square change-over design which was based upon a ten day preliminary period and a five day collection period. Urine volume and pH were measured, and faecal mass and feed intake recorded to allow for the determination of the digestibility of the treatment feeds. NH4Cl was found to affect most criteria considered. Increasing levels of NH4Cl caused performance criteria (mass and feed intake) to decrease, as did blood pH, HC03 and BE values. Liver and kidney dry mass, and the urinary excretion of Ca, P and Mg increased. Urine pH and faecal mineral excretion decreased. The effect of 0.75 % NH4Cl on the animal was not significantly different to that of the 0% NH4Cl diet. However, 1.5% NH4Cl had a significantly adverse effect on the animal. The high Ca: P ratio was found to improve mineral retention although absorption decreased as evidenced by an increased faecal mineral excretion. Blood acid-base status was adversely affected by the higher limestone level as blood pC02 levels increased causing blood pH to decrease. Thus, a high limestone level was symptomatic of respiratory acidosis, although blood pC02 levels were not sufficiently high to allow for this classification. The NH4Cl x Se interaction significantly affected blood acid-base status, urine pH and urinary P excretion. The addition of Se to the diet was found to have a slight alkalizing effect on the animal, as it raised blood acid-base status and urine pH above that of the diet containing no additional Se. The NH4Cl x Se interaction also caused urinary P excretion to increase, especially at an NH4Cl level of 1.5%. The NH4Cl x Ca interaction produced varied results, as the high Ca x 1.5% NH4Cl diet had the most detrimental effect on mass and feed criteria and blood BE values, while the most acidic combination according to abomasal and duodenal pH, blood pH, urine volume and urinary mineral excretion was the medium Ca x 1.5% NH4Cl diet. From the results of the current investigation, it was concluded that the best method of preventing urolithiasis was through the addition of 0.75% NH4Cl to the diet, as this resulted in an acidic urine and yet had no significantly adverse effect on the performance, mineral metabolism or acid-base status of the animal.