A meta-analysis of the effect of the diet fed to Holstein cows on their milk calcium content
Résumé
Milk Ca is an important determinant of milk coagulation and cheese-making abilities. Milk and dairy products
in westernized diets also represent an optimum source of Ca. This meta-analysis aimed to determine the
effect of the diet fed to the cows on the calcium content of their milk. The 611 measurements of milk calcium
contents and diet composition, obtained during 31 experiments realized at the INRA experimental farm of
Méjussaume, were gathered in a data basis. Each data represents an average of measurements obtained on
a same individual during a period of 3 to 5 days. The experiments aimed to determine whether milk quality
was affected by the nature of the forage (148 data), the nature and the amount of dietary N (108 data) or
the nature and the amount of dietary energy (223 data). Milk calcium content was on average 1.19 g/kg
(±0.13). It was not affected by the dietary calcium balance, i.e. the difference between calcium intake and
calcium requirement, as long as the balance remained lower than 20 g/d. Above this limit, milk calcium
contents declined with dietary calcium balance. In experiments comparing natures of forage, total, soluble
and colloidal calcium contents of milk were lower when diets were based on pasture rather than maize silage
while milk calcium contents obtained with diets based on grass silage or hay were intermediary. The dietary
calcium and phosphorus balances were also higher with the diet based on pasture. Both the dietary PDI and
net energy balance had slight positive effect on colloidal calcium content of milk (P<0.05) but they did not
affect soluble or total calcium content. It could not be concluded from this study if the lower milk calcium
content of grazing cows was explained by high dietary calcium and phosphorus balances or by other intrinsic
differences between maize silage and grazed pasture.