Evolution of milk calcium content during the year

Calcium content is regarded as being relatively stable in cows’ milk during lactation. However, results from commercial herds, have suggested that changes in milk calcium (Ca) content can occur. The objective of this experiment was to compare the characteristics of milk (especially milk Ca content) based on low input grass based systems compared with corn silage based systems, across 2 breeds of dairy cows (Holstein, HO vs. Normande, NO).The experiment took place on the INRA experimental farm of Le Pin-au-Haras. Sixty dairy cows were observed from calving to drying off. Two feeding systems were compared. The Intensive system (IS) was designed to maximize individual performance, with a high energy diet (in winter, corn silage with 30% concentrate; in spring, summer and autumn periods, pasture with 4 kg/d of concentrate supplemented with corn silage from July). The Grass system (GS) was designed to decrease inputs (in winter, conserved grass with no concentrate; in spring, summer and autumn, pasture with no concentrate). The experimental design was a continuous design. No significant interaction was detected between feeding system and breed for milk yield and composition. During the whole year, GS treatment reduced milk yield (−5 kg/d), with no significant effect on protein and total Ca contents. The NO cows produced less milk, but with higher protein and total Ca contents. During winter, GS treatment reduced milk yield, protein and total Ca contents. Milk Ca content decreased from January to June and rapidly increased after July irrespective of breed. During May and June, HO milk had a Ca concentration below the French limit of 1.2 g/L of the European health allegation of milk for consumption. There was a significant effect of stage of lactation and month on Ca content. Month included numerous significant factors such as maximum daily temperature, day length and radiance duration. This study clearly showed that both cow genotype and lactation stage affects milk Ca content. However, the lactation stage did not explain all the seasonal variations of milk calcium content observed in this trial.