1. Introduction
Mechanical milking is a critical operation in dairy farming where the operating parameters and the milking routine strongly influence milking labor, milk yield and animal welfare [
1]. Thus, it is essential to carefully evaluate the main technological and practical aspects of the mechanical milking process. Moreover, the differences in udder and teat morphology among dairy animals make it necessary to study and evaluate species-specific milking procedures and machine parameters [
2].
Among the various components of a milking system, the milking units have the greatest influence on the efficiency of milking, efficiency being seen as the complete emptying of the udder and a stable vacuum at the teat-end [
3,
4]. Other characteristics of the milking units are also important and influence the pulsation curve. These include the dimensions of the long and short pulse tubes, the volume of the pulsation chamber [
5,
6] and liners characteristics [
7,
8]. In fact, the pressure variance between the pulsation chamber and the liner interior during milking, in addition to the wall thickness, material and tension of the liners, affects the level of compression applied on teat tissues that helps to reduce the congestion or edema that might arise. As reported by Penry et al. [
9], increasing liner compression reduces the effects of teat-end congestion, resulting in an increased milk flow rate and an increased canal cross sectional area at high levels of teat-end vacuum and milk-phase time. As also stated by Mein et al. [
10], a compressive load (12 kPa) would be adequate to relieve congestion, where any additional compression provided by the collapsed liner would provide a slight benefit. Nowadays, dairy farmers have a large variety of liners to select from in order to achieve milking comfort and speed while ensuring the welfare of the animals. The main liners’ design features include several kinds of material and shapes which influence the milking performances. Moreover, Boast et al. [
11] studied the variation in rubber chemistry and dynamic mechanical properties of the milking liner barrel with age and use in dairy cows.
Establishing the correct pulsator rate plays an important role in the well-being of milking animals, contributing to the prevention of edemas or congestion in the teats, reducing the number of new infections and limiting the pain, and thus discomfort, of the animals [
12,
13,
14,
15,
16].
The efficiency of pulsation in ensuring the appropriate extraction of the milk and adequate massage of the teats depends on the dynamics of the pulsation curve, or, to be more precise, the duration of the increasing + milking (a + b) and decreasing + massage (c + d) phases of the curve.
The International Organization for Standardization (ISO) 5707:2007 standard [
17] prescribes that for cows and water buffaloes, phase “b” (maximum vacuum phase) shall be not less than 30% of a pulsation cycle and phase “d” (minimum vacuum phase) shall be not less than 150 ms. The latter is the threshold under which there is a considerable increase in the thickness of the teats, a factor which may result in an increased probability of new infections [
18,
19,
20]. At present there is no information on what should be the minimum times for the various phases of milking for sheep. However, given the greater sensitivity of sheep tissue, the results for cows may also be considered to be valid for sheep [
21]. Normally it is advisable to limit the duration of phases “a” (increasing vacuum phase) and “c” (decreasing vacuum phase) in order to not reduce the phases of milking and massage, without, however, reducing them excessively. Too short intermediate phases do, indeed, cause a rapid fall in the vacuum inside the liners. This creates instability in the vacuum below the milking units, which could be one of the causes of mastitis [
22]. In order to avoid prolonged milking time and prevent health problems, phase “a” should generally be between 15% and 20% of the whole cycle, while phase “c” should be between 12% and 15% [
23].
Awareness of the influence of the milking units on the pulsation curve is of particular importance in sheep milking parlors because these often use different kinds of components when the maintenance is accomplished. Moreover, it is also important in mixed dairy farms, which raise both sheep and goat species, where the same operative parameters and milking parlor, with only one type of milking unit, are adopted.
This study presents the results of laboratory tests on nine milking units, for the mechanical milking of dairy sheep, to evaluate the effects of the volume of the pulsation chamber, the touch point pressure of the liners and the operative parameters of the milking system on the duration of the increasing and decreasing vacuum phases of the pulsation curve.
4. Conclusions
The study allowed us to define the influence of specific physical characteristics of milking units and the operative parameters of the milking system on the pulsation curve. The factors which influenced phases “a” and “c” were the pulsator rate and the volume of the pulsation chamber. Contrary to prior expectations, the touch point pressure of the milking liners did not significantly affect the pulsation, in addition, increasing the vacuum level from 38 kPa to 44 kPa did not affect the two phases considered (“a” and “c”).
Comparison of the different milking units showed that there was variation in the performance of the same pulsation system depending on the milking units used. With a pulsator rate of 120 cycles/min, in the milking units with a pulsation chamber volume of less than 85 mL, the duration of the “a” and “c” phases was too short to guarantee a suitable pulsator rate. In such conditions, there is a risk that the abrupt opening of the liners could result in the milk returning from the short tube into the teats.
Considering the delicate role that the pulsation plays in ensuring animal welfare during milking, it may be important to properly certify the complete configuration and operative characteristics of the milking units. This will ensure that the complex interaction between the pulsation system and the milking unit is taken into consideration when planning and assembling the milking systems.