The effect of ruminal urea infusion on magnesium absorption in sheep : A dissertation submitted in partial fulfilment of the requirements for the degree of Bachelor of Agricultural Science with Honours in the University of Canterbury

Abstract

Four twenty month old rams were offered a chopped meadow hay diet at two hourly intervals, and received continuous intraruminal infusions of urea (treatment 1) - to elevate rumen ammonia concentrations to > 600 mgNH₃₋N/1 or water (treatment 2) as a control, in a cross-over design. There were two runs, each consisting of six to eight day digestibility trials, followed by two day digesta sampling periods. Dry matter digestibility and apparent absorption of magnesium was measured over the gastrointestinal tract, by calculation of the dry matter and magnesium intake and faecal output, and prior to the duodenum by reference to an inert soluble phase marker, CrEDTA. The coefficient of apparent absorption of magnesium across the gastrointestinal tract appeared to decline from 0.20 to 0.04, as mean rumen pH increased from 6.6 to 7.0. However, the apparent absorption of magnesium prior to the duodenum was not significantly correlated with mean rumen pH, but it was suspected, from the abnormal dry matter and magnesium flow rates, that sampling errors occurred in collection of duodenal digesta. Urea infusion had a variable effect on apparent absorption, and a clear statement cannot be made on the mean effect. The extent of change in rumen conditions may be important in determining the response. The coefficient of apparent absorption of magnesium decreased from 0.226 to 0.097 in sheep 1 when rumen ammonia concentration increased from 119 to 884 mgNH₃₋N/1, and from 0.215 to 0.025 in sheep 2 when rumen ammonia concentration increased from 141 to 544 mgNH₃₋N/1 in conjunction with a significant rise in mean rumen pH from 6.6 to 6.98.Apparent absorption of magnesium was not lower in sheep 3 and 4 when rumen ammonia concentrations were elevated to an average of 650 compared to 160 mgNH₃₋N/1. Rumen ammonia concentrations and/or rumen pH were not elevated to the same extent as in sheep 1 and 2. Urinary magnesium excretion was approximately 200% lower when rumen ammonia concentrations were elevated, in sheep 1, 2 and 3, despite the trend of elevated water intake and urinary output. There was considerable variation between animals in the quantity of magnesium excreted in any treatment. Serum magnesium concentrations were not significantly different within animals, between treatments, and ranged from 19.3 to 23.5 mgMg/1, despite apparently negative magnesium balances in both treatments. In conclusion, the effect of intraruminal urea infusion was variable, preventing valid presentation of the mean effect on apparent absorption of magnesium. However, it appears that apparent absorption of magnesium may be impaired if rumen ammonia concentration is high 800 mgNH₃₋N/1, or moderately high ( 600 mgNH₃₋N/1 in conjunction with a rumen pH above 7.0.