Variations in the dietary cation-anion difference and the acid-base balance of dairy cows on a pasture-based diet in south-eastern Australia

The variation in the dietary cation-anion difference (DCAD) and the urinary pH of dairy cows was examined over the year 1996-97 in Victoria, south-eastern Australia. Mineral concentrations in the pasture and dairy cow milk production were also examined. Three farmlets (A, B and C) under different feeding and management systems were used for the purpose of the study. Feeding management was based on grazed grass with stocking rates of 1.4, 2.5 and 4.7 cows ha(-1) for farmlets A, B and C respectively. Cows on farmlets B and C received more supplementary feed than those on the A farmlet. The urine of the cows in each herd was sampled for pH twice monthly, after morning milking. A sample of the feed on offer the previous day was collected and analysed for crude protein, in vitro dry-matter digestibility and macrominerals. Milk yields were recorded on the same day as urine sampling and weather data for the previous day were also collected. Pasture cation-anion difference was not greatly influenced by stocking rate or associated management practices, although mineral concentrations in pasture did vary. Urine pH was unaffected by changes in climate, management strategies (e.g. stocking rate), season and stage of lactation. Moreover, urine pH was also unaffected by changes in DCAD until the DCAD declined below approximately +15 mequiv. 100 g(-1) for two consecutive sampling periods. It is concluded that when this threshold for DCAD (+15 mequiv. 100 g(-1)) is breached, even in late lactation, a decrease in urine pH occurs. In south-eastern Australia, the DCAD offered to non-lactating cows in the last 2 weeks of pregnancy, in spring-calving herds, on a pasture-based diet is nearly always above that regarded as optimum in other feeding systems.