Dietary anion-cation difference and day length affect milk calcium content

Milk and dairy products are an important source of calcium for humans but recent studies in France have shown a clear decrease in milk calcium content during May and June and with grass diets compared with corn silage diets. The aim of this study was to identify the reasons of this seasonal drop of milk calcium content by testing the effect of 2 levels of dietary anion-cation differences (DCAD; 0 mEq/kg DM for D0 and 400 mEq/kg for D400) and 2 d lengths (8 h of light/d for short days and 16 h/d for long days) on calcium balances of dairy cows. The DCAD treatments were conceive to mimic diets based either on maize silage or on herbage. The cows were only lightened by solarium lights providing UVA and UVB. The trial was carried out according to a Latin squaredesign using 8 dairy cows averaging 103 ± 44 DIM with 4 periods of 14 d. Data were analyzed accordingly using Mixed procedure. The significance threshold was set at P ≤ 0.05. There was no significant interaction between day length and DCAD level. With D400 compared with D0, blood pH increased and plasma ionized calcium content decreased, while the plasma total calcium content was not different between treatments.However, milk calcium content increased, in relation with a decrease of the amount of calcium excreted in urine. DCAD had no significant effect on protein and casein contents and D400 tended to decrease milk yield. This illustrates that the udder did not decrease Ca uptake from the blood at high DCAD even though high DACA decreased the availability of Ca by decreasing the proportion of blood ionized Ca. Milk calcium and casein contents were significantly lower with long compared with short days, whereas day length had no effect on milkyield. This work highlights that long and sunny days can explain a part of the seasonal decrease of milk calcium content in summer and refutes the hypothesis that low milk calcium contents at grazing could be due to the high DCAD of herbage.