3.1. Analysis of Environmental Effects of Growth Traits
The descriptive statistics of growth traits of Lohi sheep are shown in
Table 1. The LS means for each category of Lohi animals are presented in
Table 2 and
Table 3. The significant (
p < 0.001) variation in the body weights of lambs born in different years could be explained due to differences in environmental conditions, management of the farm and the availability of forages throughout these years. Factors like climatic conditions, availability of nutrients, fodder quality and disease prevalence can affect an animal’s growth performance. The maximum performance of an animal can only be attained if it is provided with good-quality nutrients, comfortable surroundings and disease control. As farm resources did not remain constant during the study period (12-year span), the availability of resources could have influenced the decisions of the farm manager, and, as a result, animal management was affected. Similarly, the availability of grasses to the lambs for grazing also depends on various factors, including rainfall. Hence, collectively all of these factors could have contributed to the significant variation in lambs’ growth performance in different years of study. The significant variation in growth traits of lambs born during different years observed in the current study was similar to previous studies on sheep [
9,
27,
28,
29].
The studied growth traits, except BWT and WT365, were significantly affected by SOB (
Table 2). The insignificant influence of SOB on BWT was also observed in Kajli, Thalli and Dorper crossbred sheep [
30,
31,
32]. Likewise, the non-significant influence of SOB on WT365 agreed with the findings of Zaffer et al. [
31] in Dorper crossbred sheep. Contrary to the current study, Momoh et al. [
33] and Mohammadi and Latifi [
34] reported a significant effect of SOB on WT365 in different sheep breeds. The LS means of lambs born in autumn were slightly higher (3.09 kg) but not significantly different to those born in spring (3.05 kg). The difference in the growth performance of animals born in different seasons might be due to the difference in the availability of nutrients during these seasons. The animals were mainly fed with green forages, and their availability was inconsistent in different seasons of the year. The forages like berseem (
Trifolium alexandrinum) and oats (
Avena sativa) were excessively available during the winter and spring seasons at the farm. Hence, dams which completed their gestation during these seasons had a better chance to avail more fresh fodder, which had an effect on their milk performance and lambs belonging to these dams were found heavier at weaning. The significant influence of SOB on some growth traits agreed with previous findings [
31,
33,
34,
35].
Lamb’s sex significantly influenced all the studied traits (
Table 3). Male lambs had higher LS means compared to females for all the traits. The male lambs were also found heavier at various age points in studies published by Bahreini Behzadi et al. [
36], Rahimi et al. [
9] and Tohidi et al. [
28] on Kermani, Makuie and Iran-Black sheep, respectively. A possible reason for males being heavier at birth is explained by Benyi et al. [
37], who stated that male lambs grow faster in the uterus than females. Similarly, Babar et al. [
38] reported that pregnancy duration for male lambs is slightly longer than for females. The other possible reason for variation in the body weights of males and females might be hormonal differences. The influence of sex hormones becomes more prominent as the animal reaches maturity. In males, testosterone is produced in larger quantities, acting as a growth enhancer [
39]. Whereas in females, the significant hormones are oestrogen and progesterone. Oestrogen has a restricted influence on the growth of long bones [
33].
Dam’s age only significantly affected weights at early ages, i.e., BWT, WT120 and WT180 (
Table 3). Some other studies also reported a significant influence of AOD on pre-weaning growth traits, which is in line with present findings [
9,
32,
34,
40,
41]. The non-significant effect of AOD on post-weaning traits observed herein was also in line with previous studies [
9,
33,
42,
43,
44]. Contrary to this, Rahimi et al. [
9], Naderi [
41] and Bahreini Behzadi et al. [
36] observed a significant effect of AOD on WT180 and WT270 in Makuie and Kermani sheep, respectively. In the present study, we observed that younger ewes produced lighter lambs, indicating their physiological events. As younger dams and their organs are still under development, they might not support larger foetuses. Their energy was not only utilised in the development of foetuses present in their uteruses, but also in their own development [
38]. As mature and older ewes were thoroughly developed, most of their energy was utilised in foetal development, producing heavier lambs at birth, contrary to younger ewes. The ewes which become dams at younger ages produce less milk because their udder is not completely developed at that time. So, less milk is available to the lambs born from these dams compared to those born from aged ewes, and the impact of this fact was clearly observed in the weaning weight of lambs.
We observed a significant influence of TOB on BWT, WT120, WT180, WT270 and WT365 (
Table 3). The single-born lambs were found to be heavier than twins for these traits. Some previous studies also reported single-born as heavier than twins [
33,
35,
37,
45]. One reason for singles being heavier at birth can be explained by the fact of availability of space and nutrients in the uterus [
38]. The single-born lambs do not compete for space and nutrients in the dam’s womb. Therefore, their growth was faster in the uterus, and they attained higher BWT than twins. In the case of twins, they had to struggle to get nutrients and space in the uterus, and due to less accessibility of both nutrients and space, they were lighter at birth than single-born lambs. After birth, twins again had to face competition to access milk from the dam, and as a result, their weights were again lower at weaning. On the other hand, each lamb gets an equal chance to take nutrients from the feedlot during the post-weaning stages. Even though the body weights of twins were lower than that of singles for post-weaning growth traits, which might be due to their compromised growth at early ages.
3.2. Estimation of Variance Components and Heritability
The log-likelihood test revealed Model 3 as the best equation for BWT and WT120 (
Table 4). This model represents direct and maternal additive genetic effects, including covariance between them. In agreement with previous findings, the permanent maternal environment did not significantly affect the preweaning growth traits [
46,
47]. The animal model only including direct additive genetic effect was the best equation for WT180, WT270, and WT365. The estimates of variance components and corresponding heritability obtained by six different equations for all studied growth traits of Lohi sheep are represented in
Table 5.
The estimate of direct heritability attained by model 3 for BWT was 0.15 ± 0.08. These results were close to values of 0.10 and 0.11 reported by Babar et al. [
19] and Javed et al. [
20] in Lohi sheep, respectively. A similar estimated heritability (0.14) was also reported by Qureshi et al. [
32] in Kajli sheep. However, relatively higher estimates of heritability for BWT were also reported by other studies, such as values of 0.28 for Ghezel sheep [
11], 0.32 for Harnai sheep [
48], 0.39 for Mengali sheep [
35] and 0.39 for Djallonke sheep [
49]. Meanwhile, lower heritabilities for BWT were reported by Balasubramanyam et al. [
50] and Boujenane and Diallo [
51] for Madras Red (0.08) and Sardi sheep (0.07), respectively. The estimates of direct heritability for BWT in Pelibuey and Blackbelly sheep were very low (0.01 and 0.05, respectively) [
52]. The estimate of maternal heritability for BWT herein was moderate and agreed with previous findings [
47,
53].
The direct and maternal heritability estimates for WT120 were high and moderate, respectively. The high heritability for WT120 indicates that selection for heavy-weight animals at weaning will improve the growth performance of Lohi sheep. Similar to the present study, higher direct heritability estimates were observed in Madras Red, Menz and Djallonke sheep [
49,
50,
54]. Contrary to this, several workers reported lower estimates ranging from 0.03 to 0.17 in different sheep breeds [
4,
14,
32,
55,
56]. The literature found lower to moderate maternal heritability for weaning weight [
14,
57,
58]. The moderate maternal heritability estimate of weaning weight indicates that this trait is not only dependent on the lamb’s own genetic potential, but also on the mothering ability and milk production of the dam. Since measuring ewes’ milk production is not a common practice in most sheep enterprises in Pakistan, the weaning weight of lambs could be the selection criteria [
59]. MacNeil et al. [
60] reported that selection based on EBVs for maternal pre-weaning gain could be as effective in improving milk yield as direct selection.
The magnitude of the relationship between direct and maternal genetic effects for BWT and WT120 was highly negative, indicating the significant role of maternal genetic effect in the pre-weaning growth traits of Lohi sheep. It also suggests that improving one effect consequently decreases the other one. According to Szabó et al. [
59], by selecting sires only on their direct EBVs without taking into account maternal EBVs, the weaning weights of their grand offspring will decrease, and therefore, no genetic progress will be expected from this selection. Moreover, adding negative additive-maternal covariance in the equation improved these traits’ direct and maternal heritability. A negative correlation between direct and maternal genetic components was also found in Nellore [
56] and Muzaffarnagari sheep [
61].
Post-weaning growth traits of Lohi sheep were not significantly affected by maternal genetic and permanent environmental effects, limiting maternal effects to pre-weaning stages. The direct heritability for WT180 was estimated as 0.20 ± 0.07, similar to the findings of Kumar et al. [
62] in Deccani Sheep and Akhtar et al. [
14] in Buchi sheep. However, very low estimates were also reported by Hussain et al. [
63] in Thalli (0.07), Mandal et al. [
61] in Muzaffarnagari (0.06), Naderi [
64] in Kurdi sheep (0.06) and Senemari et al. [
65] in Zandi sheep (0.047). Some other authors found comparatively higher estimates ranging between 0.24 to 0.51 for the trait [
42,
50,
53,
54,
66,
67,
68,
69,
70].
The heritability estimate (0.21 ± 0.07) for WT270 in the current study was in the range of earlier reports. The estimates for Makouei, Malpura, Muzaffarnagari, Zandi and Nellore sheep ranged between 0.1 to 0.16 [
34,
41,
53,
61]. On the other hand, relatively higher estimates were also observed in different breeds of sheep, such as heritabilities of 0.25, 0.27, 0.28, 0.30, 0.37, 0.45 and 0.49 were reported for Zandi, Nilagiri, Deccani, Madras Red, Mehraban, Ghezel and Santa Ines sheep breeds, respectively [
11,
34,
42,
50,
62,
66,
68].
The estimate of heritability for WT365 was 0.19 ± 0.08, which was in the range from 0.14 to 0.43 reported in other sheep breeds, including Muzaffarnagari, Ghezel, Djallonke, Pak-Awassi, Nilagiri, Harnali, Deccani, Horro and Harnai [
11,
48,
49,
55,
61,
62,
66,
69,
71]. Overall, the growth traits of Lohi sheep had low to moderate heritability. These results indicate that these traits are under some genetic control and can be used for selection purposes. Moreover, the higher heritability estimated for WT120 shows that selection for this trait could improve growth and mothering ability in Lohi sheep.