Evolution of milk freezing point depression during the year in Holstein and Normande dairy cows

Milk freezing point depression (FPD) content is regarded as beingrelatively stable in cows’ milk during lactation. Since the 30s, its measurementis a means to determine if water has been added to the milk.However, results from commercial herds have suggested that changesin milk FPD occurred during the year. The reason of those variationsis unclear. However, dairy cow feeding might be an explanation. Theobjective of this experiment was to compare the characteristics of milk(especially milk FPD measured by MIR spectrometry) based on lowinput grass-based system compared with corn silage-based system,across 2 breeds of dairy cows (Holstein vs. Normande). Sixty-four dairycows were observed from calving to drying off. Two feeding systemswere compared. The Intensive system was designed to maximize individualperformance, with a high energy diet (in winter, corn silage with30% concentrate; in spring, summer and autumn periods, pasture with4 kg/d of concentrate supplemented with corn silage from July). TheGrass system was designed to decrease inputs (in winter, conservedgrass with no concentrate; in spring, summer and autumn, pasture withno concentrate). The experimental design was a continuous design. Datawere analyzed by using SAS mixed procedure. There was no significanteffect of breed and feeding system on milk FPD. There was a significanteffect of month of sampling and month of lactation. Milk FPD did notchange from calving to 7th month of lactation. After that date, it largelydecreased. These effects could be related to the high temperature insummer, specific feeding (grazing), or restricted access conditions forwatering. There was also a significant effect of rank of lactation. FPDwas higher for multiparous cows. This difference could be due to theopening of mammary tight junctions and a different repartition of ionsand lactose on both sides of the epithelial barrier. Surprisingly FPD didnot correlate with lactose content the most important osmotic agent inmammary epithelial cells. Some investigations have to be done to explainthe mechanisms of FPD and its physiological variations.