The present study provides for the first time, detailed account of the influence of lactation day, vacuum level and milking session on udder morphology, milking time, milk yield and components in the Sicilo-Sarde sheep. Compared to our result (26.4 mm), lower average of teat length was previously founded by Ayadi et al. (2011) in the same breed (18.5 mm), and by Martínez et al. (2011) in Chilota and Suffolk Down breed (22.6 and 22.8 mm, respectively), however, higher teat length (31 ̶ 34 mm) was reported in Barbarine ewes (Ayadi et al., 2014). Small teats can reduce the milking ability of the ewe, although an increase in teat size can be expected as a result of milk yield selection (Martínez et al., 2011). The udder length in our work (78.62 mm) was close to that reported for Suffolk Down ewe (76.9 mm), but smaller that those reported in Chilota (86.3 mm) and Churra (93.0 mm) (Fernández et al., 1995). The distance between teats of Sicilo-Sarde ewes (7.7 cm) was similar to that reported (7.5 cm) for the same breed (Ayadi et al., 2011), but lower than those reported for Najdi ewe (10.4 ̶ 15.1 cm). Similarly, the citerns of Sicilo-Sarde showed lower height (49.3 ̶ 50.6 mm) than those reported by Martínez et al. (2011) for Chilota (86 ̶ 87 mm) or Suffolk sheep (77 ̶ 90 mm).
The significant effect of lactation day on udder measurements and milking time observed for Sicilo-Srade ewes, excepting the teat length, are similar to the results reported in Barbarine ewes by Ayadi et al. (2014). It has been reported that udder size change is compatible with the decrease of milk yield (Martínez et al. 2011). Similarly, milking time was the lowest at 200 ± 7 and 169 ± 7 d compared to that at 141 ± 7 d of lactation, such variation was compatible with the reduction of milk yield potential throughout lactation (Ayadi et al., 2014).
The teat length of Sicilo-Sarde ewes did not vary during the trial. This result is similar to those of Najdi sheep (Ayadi et al., 2014). The increase of distance between teats and height of the cistern as lactation progresses may be due to an increase in the size of udder as per synthesis of mammary tissues, with advancement of lactation stage. However, this was not the case for some traits defining udder size, in particular udder length which diminished at 200 ± 7 d, coinciding with the involution of mammary gland at the end of lactation.
The milking time was influenced by vacuum level, with a reduced value following the use of the highest level (60 kPa) as it has been reported in goat (Zucali et al., 2019). The morning milking time and milk yield were higher than those of evening milking. Similar differences in the amount of partial morning and evening milk yields with the same milking interval were observed in cows (Lee et al., 1995).
The increase of fat and total protein in milk until 169 d and their decreasing at 200 d of lactation were comparable to those reported in Lacaune ewes (Kuchtík et al., 2017) and in Dalmatian Pramenka ewes (Matutinovic et al., 2011). Noting that, the major change in milk components occurred during the first 30 d of lactation which coincide with the highest milk yield in dairy ewes. Excepting the SCC at the 141 d of lactation, our results are in line with the legal limit of SCC designed of bulk tank milk for goats and sheep (should not be lower than 1.5×106c/mL) as suggested by the EU authorities (International Symposium of Somatic Cells and Milk of Small Ruminants, 1994).
A medium vacuum level increases milking time and improves the extraction of alveolar milk richest in fat, and this can explain partially the significant difference of milk fat content (Zucali et al., 2019). Following one only machine milking, the fat content raises with time in cow milk as a part of fat granules is retained and the amount of milk corresponds to the alveolar fraction the most rich in fat compared to the cisternal fraction (Lollivier et al., 2002). Milk somatic cell count did not vary significantly according to vacuum level in Sicilo-Sarde ewe similarly to previous results founded in dairy ewes and goats (Zucali et al., 2019). In contrast, a high somatic cell count has already been reported in a study maintaining a high vacuum level for a longer period in sheep and this increases the risk of the udder infection (Lu et al., 1991).
It was concluded that vacuum level does not affect udder measurements in dairy Sicilo-Sarde sheep but has several effects on machine milking time, milk yield and components; the use of a medium vacuum level of 50 kPa seems to be the suitable choice, since milk MUN, SNF and Fat contents were the highest. This level was less stressful for teat tissue, did not cause any detrimental effects on udder health and appears more appropriate than the extreme vacuum levels (40–60 kPa) regarding milking performances.