Effect of breed, energy level of diet, and lactation stage on the evolution of milk lipolysis in dairy cow

Spontaneous lipolysis is the result of the activity of a native milk enzyme,the lipoprotein lipase. Lipolysis leads to a release of free fatty acids(FFA) that cause rancid flavor in milk products. A trial was carried outto study the effects of breed, energy levels, and lactation stage on milkspontaneous lipolysis in dairy cows. Sixty-four cows were used through ayear of lactation. Cows were divided into 4 homogenous groups accordingto their breed [Holstein (HO) or Normande (NO)] and to their feedingsystem: the intensive system, with a high energy diet (HED) (in winter,corn silage with 30% concentrate; otherwise, pasture with 4 kg/d ofconcentrate) and the grass system (GS) (in winter, conserved grass withno concentrate; otherwise, pasture with no concentrate). The period ofcalving was synchronized between January and March. Individual milksamples were collected every month from both morning and eveningmilkings. The FFA levels were determined by the difference betweenthe FFA levels after milking and the FFA after 24 h of storage at 4°C.Data were analyzed using SAS mixed procedure. We showed that FFAlevels were higher in the evening milks (0.45 vs. 0.25 mEq/100 g of fat,P < 0.001) and that evening milks were more susceptible to lipolysisvariations. HO cows were more susceptible to lipolysis than NO (0.34vs. 0.20 mEq/100 g of fat, in morning milks; 0.62 vs. 0.33 mEq/100 gof fat in evenings milks, P < 0.001). There was no effect of the feedingsystem on morning milks lipolysis but GS had a tendency to increasemilk lipolysis in evening milks (0.41 vs. 0.55 mEq/100 g of fat, P =0.06). Lipolysis was higher in early and late lactation stage, particularlyin the GS group. Conserved grass may also affect the lipolysis rate. Theenergy status in early and late lactation stage is frequently negative sothe cows may be more susceptible to a low energy diet. No correlationwas found between lipolysis and milk fat content. Poor correlations (r< 0.4, P < 0.001) were found between lipolysis and milk production,fat globule size, proportion of fatty acid and protein composition, bodycondition and weight.