Urinary pH and mineral excretion of cows fed four different forages supplemented with increasing levels of an anionic compound feed

Effects of progressive supplementation of four different basal forage diets with a compound feed having a high negative cation/anion difference (about -750 meq./kg dry matter (DM)) on urinary pH and urinary mineral excretion was evaluated in dry cows. Four diets made up of 6 kg DM/day forage (Italian ryegrass (R), meadow hay (M), mixture of corn silage (C) and R or alfalfa (A) and R) and 3 kg DM/d of a compound feed, were offered in equal portions at 8.00 and 16.00 h to four dry, non-pregnant Friesian cows. Diets and cows were arranged in a 4 × 4 Latin Square design having experimental periods of 21 days. In each period, starting from the afternoon meal of day 11, the compound feed was gradually substituted in three 1 kg increments with the same compound feed supplemented with anionic salts. Interactions between type of forage and concentrate were significant for urinary pH values. Urine pH decreased at an increasing rate after anionic feed supplementation at different rates for the different of basal forage diets, with the lowest value for R and the highest for A diet. The 1 and 2 kg levels of substitutions elevated urinary C1 and urea concentrations, but not Na and K concentrations or urine volume: thus the osmolality increased from 266 to 334 mOsm/kg. The highest level of anionic feed supplementation (3 kg) sharply changed urine output patterns with increased urine volumes from 3.14 to 4.63 1/6 h and lowered urinary Cl and urea concentrations resulting in reduced urine osmolality from 334 to 277 mOsm/kg. The anionic compound feed caused a marked increase of the Ca concentration in urine, from 0.96 to 4.30 mmol/l, while P levels were not affected. Overall the anionic supplementation caused a progressive increase in urinary output of Cl and Ca (577-1085 and 2.50-15.53 mmol/6 h, respectively), while only the highest level of anionic compound feed resulted in significant increases in urea, Na and K outputs. Given the different effect of dietary forages on the pH decline of urine after anionic supplementation, the metabolic acidotic condition in close up dry cows should be monitored by the urinary pH to calibrate the dosage of anionic compound feed.