|dc.description.abstract||Magnesium (Mg) status was monitored in a herd of dairy cows in early lactation during spring (August - December). This herd is on seasonal supply with a compact spring calving of the whole herd between August to October. The cows were grazed on pasture (predominantly ryegrass and white clover). One kg of crushed barley/cow/d was offered during milking from 8 September. The cows received three kinds of Mg supplementation. Firstly, the pasture grazed by pregnant cows was dusted with magnesium oxide. Secondly, milking cows received Mg through the drinking water and, thirdly, Mg was added to the barley meal from early October until the end of November.
From the herd total of 345 cows, 51 were selected on the basis of age and previous performance. For age, cows were allocated into contemporary groups of 2, 3, 4-8, 9+ year old animals. From these age categories 9, 9, 24, and 9 cows were selected, respectively. The selection for performance was based on milk volume production of the previous season in order to represent low, medium and high producers in the 3, 4-8 and 9+ year age category. Two year old heifers were picked at random. Individual blood and urine samples, individual live weight measurements and bulk milk samples were collected at fortnightly intervals on eight occasions from the end of August until early December. Pasture samples were taken from paddocks grazed during the 48 hours prior to the other measurements.
Pasture mineral composition for Mg, potassium, sodium varied between 1.8 and 2.3, 24.9 and 38.9, and 1.5 and 4 g/kg DM, respectively. On average, all cows gained weight during the experimental period.
The mean 24 h urinary Mg excretion increased linearly between August and December from 1.1 to 3.2 g Mg/d. This excretion was estimated from urinary Mg and creatinine concentrations, and live weight data. Cows of 9+ and 3 years showed the highest (3.9g/d) and lowest (1.9 g/d) rates of urinary Mg excretion, respectively. Cows seemed to differ in physiological regulation of Mg metabolism, because the increase in urinary Mg excretion observed in all ages from August to December was much stronger in older (4+ years) than in younger cows. The within animal repeatability of urinary Mg excretion was 0.62. An estimate of Mg availability suggests that Mg absorption was enhanced with increasing age.
Mean plasma Mg concentration on each sampling day ranged between 0.76 and 0.87 mM. Blood Mg concentrations below the normal range(<0.75 mM) were most commonly measured amongst the 3 and 9+ year old cows, namely in 34 and 44% of blood samples taken between September and November, respectively. The majority of cows tended to maintain an individual plasma level, while their urinary Mg excretion varied largely. The within animal repeatability of plasma Mg concentration was 0.70.
Mean milk Mg concentration (104 mg/l) was markedly below the value commonly reported in the literature (120 mg/l). The repeatability of 0.68 indicates milk Mg concentration was a highly repeatable individual cow characteristic.
The results indicate that the assessment of Mg status of dairy cows by measuring only urinary Mg excretion could be misleading. This was the case particularly in older cows, which showed low plasma Mg and high urinary Mg excretion levels. In addition, the individuality of plasma Mg concentration might demonstrate an important genetic involvement.||en