VARIATION IN SELECTED BLOOD-PLASMA CONSTITUENTS DURING THE POSTPARTUMAND BREEDING PERIODS IN DAIRY-COWS

Concentrations of insulin-like growth factor-I, albumin, beta hydroxyb utyrate, glucose an urea were measured in 12 plasma samples collected over 9 weeks from mid-August from each of 40 cows in four seasonal her ds which calved from mid-July. These herds comprised Jersey or Holstei n-Friesian animals, with each breed grazed at two stocking rates. Dail y dry matter intakes (kg dry matter/cow/day), which were estimated fro m pasture assessments before and after grazing, ranged from 4% (low st ocked Jersey herd) to 38% (high stocked Friesian herd) below recommend ed intakes for each breed of cow in peak lactation. Stocking rate was associated with a 4% reduction in liveweight for cows in the high stoc ked herds and a 20% reduction in daily milk production (as a proportio n of the low stocked herds' liveweight and milk production). Eight cow s produced at least one blood sample with a beta hydroxybutyrate conce ntration of more than 1.0 mmol/l. There were 42% of samples with gluco se concentrations greater than 4.1 mmol/l. These two concentrations we re the maximum values classified as normal for these two metabolites b y the Ruakura Animal Health Laboratory. No concentrations of the other metabolites were outside their respective normal ranges. Breed, age a nd individual animals usually had greater effects on the average metab olite concentrations than did stocking rate. The greatest effect was a ssociated with sample day, especially for insulin-like growth factor-I and urea. A significant breed x stocking rate x sample day interactio n in urea concentrations indicated that changes in the feeding value o f the grazed pastures available to each herd were sufficient to produc e separate fluctuating patterns in the daily average concentrations. T hese results showed that frequent sampling on a herd basis would be ne cessary to identify the significance of factors which may be contribut ing to sample day variation in the concentration of metabolites in the plasma of pasture-fed cows.