Influence of milking units and working vacuum level on the mechanical milking of goats

Abstract The objectives of this study were to evaluate and compare the effect of working vacuum levels (35 and 44 kPa) and liners dimensions (mouthpiece lip diameter and overall length, 20–185 and 22–170 mm) on the main milking characteristics of goats. The results highlight that both the working vacuum level and the liner dimension have influenced the milk flow curve parameters. The maximum variations were found for peak flow rate, which increased significantly with liner dimensions of 20–185 mm at a working vacuum level of 44 kPa as well as average milk flow rate and for plateau phase duration. The incorrect adoption of operative parameters and unsuitable milking machine components, might affect the performance of the mechanical milking and negatively affecting animal productions and welfare.

The technical and scientific knowledge on milking units, with specific reference to the physical properties of the liner design, influence the milking of the different species, as has been shown by specific studies carried out on dairy cows (Mein et al., 1983;Zucali et al., 2009) and on dairy sheep and goats (Alejandro et al., 2014;Le Du, 1982, 1985. The liners available on the market have different shapes and dimensions for the same dairy species, in accordance with the morphological differences of the mammary gland (Strapák et al., 2018;Tančin et al., 2006). Milking liners with a narrow opening can generate a compression at the base of the teat. On the contrary, a wide opening can cause the liner climbing in high working vacuum settings. The liner length also affects the number of intramammary infections and compromises the benefits of the pulsation on the teat (Mein et al., 1983).
On the whole, incorrect management techniques constitute important stressful causes for animals, producing negative effects on the health status, on the quality of the productions and on their milkability (Labussière, 1984;Zucali et al., 2019).
To investigate this problem, the present study aims to highlight the results of comparative tests carried out to evaluate the effect of the working vacuum levels and the dimensions of the liners on the main milking characteristics of the goats.

| MATERIALS AND METHODS
The study was carried out on a goat dairy farm located in Italy over a 35 kPa and 20-185 mm. Specifically, the 42 goats received the same treatment per two consecutive days (four milkings). Among the 672 recorded milk flow curves, 12 (1.8%) were discarded as they did not achieve the minimum necessary flow to return the main parameters. Thus, at the end of the test, a total of 660 flow curves were analyzed, and as many qualitative-quantitative milk samples, to evaluate the effect of four different combinations of working vacuum and liner dimensions on milk fractioning, milk flow, milking time and milk quality.

| Milk flow curves
The milk flow curves were recorded during the morning and evening milking sessions using two electronic mobile milk flow meters (LactoCorder ® ; WMB, Balgach, Switzerland), as used in previous studies with goats measurement program (Baši c et al., 2009;Caria et al., 2013;Šlyžiene et al., 2016;Zucali et al., 2019). Variables measured per milking were the following:

| Milk samples
At each milking a milk sample representative of the whole milking was automatically collected by Lactocorder for each animal to be analyzed for milk quality parameters. After collection, milk samples were refrigerated (5 C) and transported to the milk laboratory of Istituto Zooprofilattico Sperimentale del Lazio e Toscana (Rome, Italy). Content of fat (%), protein (%), lactose (%), casein (%), and milk urea nitrogen (MUN) (mg/dl) were detected by MilkoScan™ FT6000 (FOSS Electric, Hillerød, Denmark) using FTIR spectrophotometry calibrated with appropriate goat standards. Somatic cell count (SCC) (n/ml) were detected by Fossomatic FC (FOSS Electric, Hillerød, Denmark) using a fluoro-optometric method. To achieve a normal distribution, a logarithmic transformation (log10) was applied to SCC (Ali & Shook, 1980).

| Statistical analysis
To evaluate the effect of the working vacuum levels, liner dimensions, and their interaction on the main milking characteristics of the goats, the data were analyzed by the PROC MIXED procedure of SAS version 9.2 (SAS Institute Inc., Cary, NC, USA), with the following mixed linear model:

| RESULTS
In general, both the working vacuum level and the liner dimensions have influenced, though to different degrees, the milk flow curve parameters ( Table 1).
The milk yield, as well as the quantity of milk produced in the first minutes, was greater using the lower vacuum level, except for the milk obtained in the first minute of milking. As expected, the reduction in the working vacuum level led to an increase in almost all the four milking time variables. Instead, the lag time to 0.25 kg did not show significant differences between the two working vacuum levels considered. The variables related to milk flow rates were higher with a working vacuum of 44 kPa, specifically the peak and average flow rate.
In reference to the qualitative analyses performed on the milk samples, the fat content was higher with a working vacuum of 44 kPa (3.62%-3.43%), a similar result was found for the protein content  (Table 2).

| DISCUSSION
The study showed that the effective milking time was less than 15% using the highest working vacuum level (44 kPa T A B L E 1 Effect of vacuum level (kPa) and liner dimension (mm) on milk and milking parameters (LS-mean AE standard error)