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Introduction
As sheep operations intensify in Mexico, it becomes important to enhance the efficiency of meat production. Crossbreeding can be used to improve lamb production efficiency, but the Mexican sheep industry has yet to determine the optimum production system and which specific crosses will result in the highest profitability under intensive management conditions.
The introduction of a number of sheep breeds in Mexico during the last decades has contributed to improve the productivity of indigenous breeds (Bores- Quintero et al., 2002; Cuéllar, 2007). Katahdin, one of these introduced breeds, has been widely disseminated in Mexico because of its easy care, lamb production, tolerance to internal parasites (Burke and Miller, 2004), resistance to harsh environments (Marai et al., 2007), good fertility and capacity to reproduce in spring (González-Godínez et al., 2014; Burke, 2005) and good efficiency of forage utilization (Wildeus et al., 2005, 2007; Brown et al., 2012). However, Katahdin, as most hair sheep, are generally smaller and have lower growth rates than traditional wool sheep (Wildeus et al., 2007).
Thus, crossbreeding between hair-sheep and meat-type sheep with superior growth to increase lamb production seems to be a feasible alternative to increasing sheep productivity, as has been demonstrated by a limited number of studies in Mexico (Partida de la Peña et al., 2009; Vázquez- Soria et al., 2011). The number of lambs marketed per ewe favorably impacts profitability (Braga Lôbo et al., 2011), but liveweight of lambs also constitutes a major factor determining the total Kg of lamb weaned or marketed from a sheep flock. Therefore, improving total weight of lamb weaned is important for Katahdin producers because sale of meat is the primary source of income. This aspect is becoming more important as intensive sheep operations are rapidly increasing in Mexico. Poorly executed crossbreeding programs may bring disappointing results. This research was undertaken to evaluate genetic and non-genetic factors influencing growth traits and mortality rates of crossed lambs from Katahdin females with Charolais, Hampshire, and Texel meat breeds.
Material and methods
Ethical considerations
This experiment was conducted in conformity with the “Guiding principles for research involving animals and human beings” approved by the Council of the American Physiological Society (Published 1 August 2002 Vol. 283 no. 2, R281-R283 DOI: 10.1152/ ajpregu.00279.2002). Ethical considerations did not arise during the course of the study.
Animals
The study was conducted in a commercial sheep farm in central Mexico (20 °N, 550 mm rainfall, 1940 m above sea level; 16 °C annual median temperature) between September and November 2013. One hundred and twenty-eight multiparous Katahdin (K) ewes were separated into 3 groups. Estrus of all ewes was synchronized with intravaginal sponges containing 20 mg fluorogestone acetate (Chronogest , Intervet, Mexico) for 12 days. Ewes in estrus were detected with intact rams wearing an apron to prevent copulation. Ewes in estrus were inseminated in the uterus 12 hours after the beginning of estrus with non-frozen semen (laparoscopy) from Texel (n = 6), Hampshire (n = 6) or Charolais (n = 5) rams. Ninetytwo ewes lambed and 152 crossbred lambs were born (53 Texel, 43 Hampshire and 56 Charolais).
Sheep were kept in shaded open-sided pens with dirt floor. Feed bunks were located at the front of each pen. Late pregnant ewes were kept in individual lambing pens with no bedding for better care during lambing. Lambs were kept together with their dams in these pens. All lambs were weighed within 12 h after birth using a platform scale weighing from 0 to 20 Kg and reading to 0.1 Kg.
Lambs were again weighed individually at 30 and 60 days of age. Average daily gains (ADG) from birth to 60 days were registered. Lambs were ear tagged at birth and birth date, type of birth and gender were recorded. Lamb mortality was recorded as number of lambs born dead or alive dying within 60 days post-lambing.
All lambs were offered a total-mixed diet ad libitum. The diet met the NRC (2007) recommendations for maximum growth rate (2.90 Mcal metabolizable energy (ME)/Kg dry matter (DM) with 20% crude protein (CP) (Lamb Tech, Purina , Salamanca, Mexico) from approximately 14 d of age and until weaning at 60 days of age. Lamb survival was defined in this paper as the number of lambs weaned per 100 lambs born (dead and alive). The lamb survival trait was coded as follows: 0 = dead if any lamb recorded at birth subsequently had no live weight recorded at 60 days, and 1 = alive, if it survived at 60 days so that a binomial analysis of this trait could be used.
Statistical analysis
Growth data were analyzed with the MIXED procedure of SAS (SAS version 9.4, 2013, SAS Inst., Inc., Cary, NC, USA). The model measured fit effects on lambs of breed, gender, litter size and three two-way interactions between variables. The nonsignificant interactions were removed from the final model. Means were compared using the probability of a statistical difference (PDIFF option of SAS). The effect of genotype group, gender, litter size (one to three offspring) and birth weight of lambs (three categories: <3.5, 3.5-4.5, and >4.5 Kg) and their interaction with mortality (yes/no) until weaning was assessed with a logistic model (Proc Genmod of SAS). Comparison of means was carried out using the Least Square/DIFF procedure of SAS. Statistical differences were considered significant at p<0.05.
Results
No interactions were statistically significant for any trait. Therefore, only main effects are described. Breed of sire was not significant for birth weight (Table 1). Likewise, weaning weights were similar (p>0.10) among breed types, but KH and KCH lambs had higher (p<0.05) weight gain from birth to weaning than KT lambs (Table 1). Contrary to expectation, lamb gender was not a significant source of variation for birth and weaning weight, as well as pre-weaning daily gain. Lambs coming from higher litter sizes had lower (p<0.05) birth weights than single-born lambs across all breed groups (Table 1). Breed by type of birth interaction was not significant; therefore, the reduction in birth weight of triple-born lambs compared to single-born lambs was 2.1 Kg for all breeds.
Table 1 Growth performance of progeny sired by Charolais (CH), Hampshire (KH), and Texel (T) breeds on Katahdin ewes in central Mexico.
Values are means ± SD. Means within the same column with different superscripts letters (a, b, c) indicate significant difference (p<0.05).
Overall, 27.6% of the lambs (42/152) died before 60 days of age. Mortality rate was significantly lower for KH lambs compared to KCH and KT lambs (Table 2). There was no significant difference in lamb mortality attributable to litter size and gender (p>0.05). Heavy lambs at parturition exhibited lower (p<0.05) mortality rates than lighter lambs. On the other hand, light-born lambs experienced the highest rate of mortality (p<0.05).
Discussion
Different to other studies with hair sheep (Freking and Leymaster, 2004; Osorio-Avalos et al., 2012) sire breed did not alter birth weight of lambs. Values for this trait, regardless of genetic group, were higher than the 3.3-3.9 Kg reported for purebred Katahdin lambs (Burke et al., 2003; Hinojosa-Cuellar et al., 2009; López-Carlos et al., 2010) or hair x meat sheep crossbred lambs (Cardoso et al., 2013) from well-fed mothers. On the other hand, birth weight of purebred Texel and Charolais lambs observed in other studies (Christley et al., 2003; Yaqoob et al., 2004) is higher than weight found in the present study for Texel and Charolais-sired lambs. Thus, it seems that the use of meat-type sires on Katahdin ewes resulted in an intermediate birth weight of these hybrid lambs, as it has been observed in other studies (do Prado Paim et al., 2013; Ríos-Utrera et al., 2014). In addition, clearly there was not a genetic component accounting for some of the variation in birth weight of these lambs.
Lamb gender did not affect birth or weaning weight, nor pre-weaning daily gain, which is not in accordance with various reports indicating that ram hair lambs grow faster than withers (Cloete et al., 2007; Ríos-Utrera et al., 2014). But a number of reports indicate no differences in weight and daily gain between male and female hair lambs (Hinojosa- Cuéllar et al., 2012; Cardoso et al., 2013).
Breed group did not affect weaning weight, which is in agreement with observations by Hinojosa-Cuellar et al. (2011) on crossbreed Pelibuey x Blacbelly and Katahdin x hair sheep. Likewise, Malhado et al. (2009) found similar weaning weight between Dorper x Morada Nova and Dorper x Rabo Largo sheep. This trait is strongly affected by maternal environment and it seems that the good mother ability of Katahdin ewes caused the similar weaning weight observed among genotypes.
ADG from birth to weaning was higher in KH and KCH lambs. This trait is a common measurement of growth and is an economically important characteristic in the breeding of mutton sheep. In addition, this trait has a high genetic and phenotypic correlation with feed efficiency (Cammack et al., 2005). Pre-weaning ADG was higher than the weight gains reported by VázquezSoria et al. (2011), Macías-Cruz et al. (2012) and Moreno-Cáñez et al. (2013) for pure or crossbred Katahdin lambs in intensive systems in central and northern Mexico.
Similar to findings in multiple previous studies, lambs from higher litter sizes had reduced birth weights compared to single-born lambs across breed groups. Lamb characteristics at birth were in agreement with Godfrey et al. (1997), Osorio-Avalos et al. (2012) and Ríos-Utrera et al. (2014), who reported that single born lambs are heavier at birth and grow faster than twins and triplets.
Mortality of lambs in the present study is much higher than that observed with other well-fed hair lambs (Macías-Cruz et al., 2012; Knights et al., 2012) but close to the findings by Cloete et al. (2007) with Dorper ewes crossbred to meat sheep. The extent of preweaning mortality is a complex issue. It is influenced by the lamb capability for survival, the ewe’s maternal ability, and to management practices at lambing and during the rearing period (Nowak and Poindron, 2006; Sawalha et al., 2007). The fact that birth weight of lambs was similar among genotypes suggests that factors other than birth weight were involved in the deaths of lambs.
In the present study, a many deaths were due to dystocic parturition. This condition is associated with increased risk of lamb loss (Hinch and Brien, 2014; Holmøy et al., 2014). The marked difference in lamb mortality between genotype groups remains unexplained. Heritability for lamb survival is extremely low (Brien et al., 2010; Hatcher et al. 2010), hence the interactions between rearing ability of the mother, lamb viability, and climatic conditions around lambing possibly accounted for the difference in the incidence of lamb losses among genotype groups.
In the present study, male and female lambs had similar chances of surviving to weaning. This is in agreement with results of Matos et al. (1993), who found that males and females have similar survival rates, but it does not agree with previous studies both in wool (Sawalha et al., 2007; Hatcher et al., 2009; Everett-Hincks et al., 2014) and hair sheep (Holmøy et al., 2012; Hinch and Brien, 2014), where male lambs had higher mortality than females. Contrary to the above-mentioned studies, Atashi et al. (2013) found that Iranian male lambs showed higher survival rate than females. It could be that Katahdin dams are not drastically affected by the greater size and weight at birth of male lambs, which leads to longer labor, compared to females (Dwyer, 2003) and eventually tend to have higher mortality than females.
In this study, either no consistent superiority was observed of singles or twins compared with tripletborn lambs in terms of survival to weaning. This result does not agree with previous reports (Gama et al., 1991; Matos et al., 1993) where increased litter size lead to decreased lamb survival. On the other hand, other researchers observed that lamb survival was higher for lambs born as twins compared to singles (Everett-Hincks et al., 2005). The hybrid nature of all lambs in the present study could have reduced the risk of hypothermia or starvation, which is characteristic of smaller lambs from large litters (Kerslake et al., 2010). This study suggests that Katahdin ewes did apparently not display the result and competition between lambs for milk supply was not strong.
Regardless of birth type, higher weaning weights were associated with lower mortality risk, whereas the risk of mortality at weaning was greater for lambs with low weaning weight. These results agree with those by several authors (Sawalha et al., 2007; Hatcher et al., 2009; Boujenane et al., 2013). This study suggests that selection programs for increased birth weight in crossbred animals would reduce lamb mortality.
The present study indicates that under intensive management conditions, farmers with flocks of Katahdin ewes would expect similar weaning weight of lambs from crossbreeding these ewes with either Charolais, Texel or Hampshire sires. However, higher survival to weaning can be expected for Hampshiresired litters compared to Texel and Charolais-sired lambs.
