Mjölkningsfrekvensens inverkan på mjölkfettets kvalitet
The milk production has developed rapidly the last years. The milk production per cow is increasing and the production units are larger with an increasing number of animals and a decreasing number of workers. Many milk producers invest in automatic milking systems (AM) to meet this change. In these systems the cows can voluntary decide milking frequency and usually the milking frequency are higher than two times per day. But how does the milking frequency effect the composition of the milk?
The aim of the present study was to study the effect of milking frequency on the milk composition and the milk fat quality. The experiment was carried out at Kungsängen Research Centre, Department of Animal Nutrition and Management at The Swedish University of Agricultural Sciences (SLU) in Uppsala. Eleven cows from the experimental herd of the Swedish Red and White Breed were used in the study. They were kept in a tied-up system and were milked with a specially designed milking machine for milking and sampling at each separate udder quarter. The experiment included two periods. Each period lasted for 5 days. During the first period the cows were milked two times a day at each udder half. In the second period the cows were milked two times a day on one udder half and the opposite udder half four times a day. The milk yield was measured and milk samples for analyses of fat, protein and lactose content were taken at each milking in both periods. Samples for analyses of free fatty acids (FFA) were taken at day 5 in the morning and evening milking in both periods. Fatty acid composition, fat globule size and ?-glutamyl transpeptidase were taken at day 3 and 5 in the morning and evening milking in both periods. FFA content was analysed both in fresh milk and milk cold stored for 24 hours. Three times under the total experiment period, samples for estimation of the cell count in the milk were collected at each separate udder quarter.
The milk and the lactose yield were significantly higher in the more frequently milked udder half while the percent of protein decreased. No differences were observed in the fat- and protein yield between two and four times daily milking. The FFA levels increased significantly in the stored milk, collected from the more frequent milked udder half, while no differences were observed in the fresh milk. The proportion of polyunsaturated fatty acids was lower in milk from the udder half milked four times compared with the twice daily milked udder half. The de novo synthesis of milk fat was not affected by milking frequencies, since the proportion of C4-C14 in milk was invariant between two and four times daily milking. Milk from the half udder milked four times per day contained fat globules with a significantly larger average diameter compared with milk from the udder half milked two times daily. The amount of membrane material is an important factor for the resistance of milk fat globule to lipolysis. To quantify the amount of membrane material in milk, activity of ?-glutamyl transpeptidase was used as a marker. No significant effect on the activity of ?-glutamyl transpeptidase upon milking frequencies could be found, neither in total activity in whole milk nor when adjusted for fat content.
The results from the present study indicate that the milking frequency has an effect on the milk quality and is coursed by local mechanisms in the udder. Increased milking frequency stimulates to an increased level of FFA in the stored milk and could also have an impact on the milk product.