A large range in the coefficients of variation for the parameters describing the lactation curve of these dairy sheep was observed, ranging from 34.5 for TMY150d to -79.7% for P. Carta et al. (1995) reported coefficients of variation between 22 and 65%, which are in the same range as found in this study. The large coefficients of variations are due to temporal environmental factors such as management system, grazing, confinement, and season, among others (Angeles-Hernandez et al. 2013; Vázquez-Peláez et al. 2014), or due to genetic factors (Cappio-Borlino et al. 1997).
In this study the flock was comprised of crossbred ewes of different parity number, lambing at different seasons of the year with different litter sizes at birth. All these factors created large coefficients of variation in the estimates of the regression coefficients modelling the lactation curves for individual lactations.
Total milk yields observed in this study were lower than 258 kg milk/lactation reported for crossbred ewes in Spain (Abecia et al. 2019). In contrast, Morgan et al. (2006) reported production levels of 107 kg/lactation in 90 days with PY of 1.2 kg/day in crossbred ewes which were the progeny of Ef sires and non-dairy dams.
Several studies have reported that ewes from specialized dairy sheep breeds can sustain longer lactations with higher yields than crossbred ewes (Rhind et al. 1992; Pollott and Gootwine, 2004; Morgan et al. 2006). In this study, Ef and Ot ewes had longer LL (300 and 306 days) and produced ̴111 kg (+37%) more TMY than the average of the flock (190 kg/lactation). In contrast, Awassi ewes produced ̴ 9% less milk than Ef and Ot breed ewes.
In general, second parity ewes had higher total milk production than 1st and +3 parity ewes, which is similar to that reported by Peralta-Lailson et al. (2005) in the Chiapas white (non-dairy) breed where second-lactation ewes had their highest TMY. Lower PY and TMY in first parity ewes most likely was due to the fact that their mammary glands are not totally developed (Abdelsayed et al. 2014). Angeles-Hernandez et al. (2018), reported higher TMY in third lactation ewes grazing irrigated pastures in an organic production system in central Mexico, which are contrary to our results where the highest TMY were achieved by 1st and 2nd parity ewes.
Estimated PY in this study (1.3 kg/day) was lower than that reported by Morgan et al. (2006) of 2.1 kg milk/d in first lactation crossbred ewes out of Merino dams and sires from different breeds grazing in Australia; although milk let-down was enhanced by oxytocin injections.
The effects of lambing season on parameters describing the lactation curve were similar to another study in Mexico (Angeles-Hernandez et al. 2018). Ewes lambing in autumn and winter had 15% higher PY and longer LL, produced 32% more milk than ewes lambing in spring and summer. However, our findings are contrary to a report from Spain (Abecia et al. 2019) where crossbred ewes lambing in spring and summer had higher yields (269 and 260 kg milk/lactation respectively) than those that lambed in autumn or winter (246 and 246 kg milk/lactation respectively).
Morgan et al. (2006) also reported that crossbred ewes lambing in spring-summer yielded 12% more milk than those lambing in autumn-winter under grazing conditions of Australia; although the authors noted a possible effect of age at lambing since ewes lambing in spring-summer were younger (12 months old) than ewes lambing in autumn-winter (19 months old).
Pollot and Gootwine (2004) with data from Assaf ewes in Israel also found higher milk yields for ewes lambing in spring-summer compared with ewes lambing in autumn-winter, when changes in ambient temperature and shorter photoperiod (shorter days towards winter), reduced the length of lactation; while ewes lambing in spring-summer benefitted from longer photoperiods and therefore showed longer lactations.
The results from our study contradicts this trend. The ewes lambing in autumn-winter had higher PY, LL, TMY, TMY305 and TMY150 than the ewes lambing in the spring-summer season. The fact that the flock was managed in total confinement with homogeneous feeding management across seasons and no direct exposition to sun light, and less harsh winter climate, could be a factor that explains our results.
Litter size only had a significant impact on lactation persistency where single lambing resulted in more persistent lactations than twins or triplets, for the rest of the parameters there was no differences which is in contrast with the results reported by Pollot and Gootwine (2004) who observed a significant increase on TMY (6%) in ewes that had two or more lambs. The overall trend is that ewes that start the lactation with twin and triplets have higher TMY than ewes that start the lactation with single lambs (Cardellino and Benson 2002; Morgan et al. 2006; Angeles-Hernandez et al. 2018; Chay-Canul et al. 2020). This is due to the number of lambs born during pregnancy affect the development of the mammary gland, which is stimulated by an increase in the blood concentration of placental lactogen (Lérias et al. 2014; Selvaggi et al. 2017; Knight and Sorenson, 2001; Gootwine, 2004).
The shape of the lactation curve of the flock and the Aw, Ef and Ot breeds is similar to the standard lactation curve reported by Ruiz et al. (2002). However, about day 250 of lactation persistency tended to improve showing increases in milk yields. This effect is more manifest in lactations of ewes of other breeds. The mean shape of the lactation curves in this study are contrary to reports in Mexico that showed lactations without PY and downwards trend from the beginning to the end of lactation. (Angeles-Hernandez et al. 2013; Vázquez-Peláez et al. 2014). Cappio-Borlino et al. (1997) stated that these atypical lactation curves may be due to the low genetic merit of the ewes as well as due to environmental factors that limit the genetic potential of dairy ewes. The lactation curve of Awassi ewes was always below the lactation curve of Ef ewes, which agree with the breed effects presented in Table 2.
The negative correlation between DP and PY indicate that early DP will result on higher PY. The correlations between DP and persistency suggested that ewes that reach peak yield later during lactation would have higher persistency as reported in dairy cows (Tekerli et al. 2000). The negative correlation between PY and persistency (-0.47) indicate that lactation curves with high peaks would be associated with lower persistency as reported also in dairy cows (Tekerli et al. 2000). The strong correlation between TMY and lactation length agrees with reports by Angeles-Hernandez et al. (2018) from a study with dairy breed ewes in central Mexico. This means that the longer the LL the greater the TMY. The correlations between P and TMY and TMY305 indicated that the lower decreased of milk yield after PY (higher persistency) had a significant impact on higher milk yields.
Repeatability has been described as the proportion of total phenotypic variation that is explained by the sum of the permanent environmental effects and the genetic variation. This parameter is useful to indicate if the trait is repeatable through the life of the animals. If a trait has high repeatability, then animals can be selected or culled based on the first records of their life. The estimation of permanent environmental effects for single animals are required to calculate productive values and have been used to develop culling indices for dairy cattle (Kelleher et al. 2015). The lowest repeatability estimates in this study were for PY (5%) and P (28%) which agree with estimates for these traits reported by Pollott and Gootwine (2004) in Assaf sheep. On the contrary, repeatability estimates for LL and TMY were high (86% and 89%, respectively) indicating that animals can selected or culled for these traits based on the first records.
It is concluded that ewes of second parity had longer lactations and were the most productive. Autumn and winter lambing season had an important effect resulting on higher milk productivity; whereas ewes lambing single lambing had higher persistency than ewes lambing twins or triplets. Breed had a significant effect both on peak yield and total milk yield. Awassi breed was the least productive breed in the flock outperformed by Ef and other crossbred ewes.