Efeito de três ordenhas diárias na composição do leite de vacas da raça Holandês / Effect of three-times-a-day milking in the composition of the Holstein cows’ milk

ABSTRACT

Background: Milk quality can be assessed in many ways. The solid contents of the milk, more specifically fat, protein and lactose, can determine its quality and the industrial use, as well as the payment made to the producer. The solid contents of the milk can be influenced by several factors, such as genetic, nutritional and management. Knowledge on this variation in the chemical composition is of paramount importance, since it allows the producer to define the necessary strategies to maintain the quality of the product. Variations according to the milking period may occur, thus influencing the results of analyses. The objective of the present study was to evaluate possible variations in the composition of milk extracted in the morning (6 AM), afternoon (2 PM) and evening (10 PM), during the Expointer 2010. Materials, Methods & Results: During the Expointer 2010 held in Esteio, RS, Brazil, samples were collected from thirteen Holstein cows participating in this milk competition organized by Associação de Gado Holandês. Freshly extracted milk samples were obtained from three consecutive milkings (complete milking) during a 24-hour period, at an 8-hour interval. Individual samples were sent to UNIVATES’ Laboratório do Leite, in Lajeado/RS, for determination of fat, raw protein, lactose and total solid contents, using infrared radiation. Variance analysis and Tukey’s Test were carried out to verify differences between the means through the SAS program. There was no significant difference in the protein and lactose contents found in the results of the morning, afternoon and evening milkings. These components presented percentage means of 3.00 ± 0.57 and 4.58 ± 0.27, respectively, for the morning milking; 2.95 ± 0.53 and 4.57 ± 0.27 for the afternoon milking and 3.03 ± 0.68 and 4.55 ± 0.28 for the evening milking. However, the quantity of fat and total solids varied among the different periods. The fat content of the milk was higher in the afternoon and evening milkings, and the results for these periods were not statistically different from each other. The mean percentage of this component was 3.28 ± 0.79 in the morning milking; 3.36 ± 0.64 in the afternoon milking and 3.69 ± 1.02 in the evening milking. The total solid contents were higher in the evening milking, with a mean of 12.28 ± 1.47; however, there were no significant differences in the morning and afternoon milkings, with means of, respectively, 11.89 ± 1.23 and 11.89 ± 1.00. Discussion: As the milk component with less variation, lactose did not have significant variations between the milking periods in this study. On the other hand, milk fat is the component with the highest ability to become altered, with the highest percentage found in the milk extracted in the afternoon and evening. However, such results are associated with the fact that the volume of milk accumulated inside the udder in the morning milking is higher, which dilutes the fat content. This variable was not observed in the present study, since the three milkings were made at 8-hour intervals. The variations in the fat content infl uenced the variations in the total solid contents, since the amount of both milk components was higher in the evening milking. Although the variation ability of proteins is similar to that of fat, in this study the contents of this component did not vary significantly among the different periods