Breeds and lines of sheep suitable for production in challenging environments

Abstract

The resistance of unimproved, indigenous fat-tailed sheep breeds and their composites to ticks and heat make them suitable when challenge conditions are high. Within-breed selection in Merinos resulted in lines resisting challenges from internal parasites, blowfly strike, and sheep lice. Lamb survival benefitted from selection for number of lambs weaned per ewe mated, being able to resist cold stress during a winter lambing season. Variation between breeds as well as between animals within breeds can thereby be exploited to allow the better adaptation of sheep to challenging environments. The adaptability and success of sheep are confirmed by the fact that they are the world’s most diverse mammalian livestock species, contributing some 25% to the global number of farm animal breeds (Cottle, 2010). The latter author showed that the ovine species is indeed globally successful and present in farming landscapes throughout the world, ranging from arid to high rainfall areas and from sea level to the highest mountains. Sheep are often farmed in resource-poor and otherwise marginal regions, constrained by climate and soil type (Cloete and Olivier, 2010). There is also consensus that the global climate is highly likely to become more unstable in future (Rust and Rust, 2013). Droughts are expected to become more intense and persist longer, also leading to periods of prolonged heat stress (Van Wettere et al., 2021). Challenging environmental conditions are often characterized by further constraints, such as a variety of external and internal parasites (Karlsson and Greeff, 2012), as well as extremes in terms of climate, soil, and topography (Cloete and Olivier, 2010). Challenging climatic conditions at lambing also commonly compromise lamb survival (Pollard, 2006). Fighting and controlling endemic diseases and other stressors represent a considerable cost to livestock production, creating the need for more robust and easy-care animals being able to thrive in particularly challenging environments (Friggens et al., 2017). This paper seeks to (1) explore the utilization of divergent genetic resources including unimproved, adapted indigenous resources to contribute to sustainable sheep farming in challenging environments; and (2) determine the role that lines of sheep, derived from directed genetic selection to resist such stressors, must play in such conditions. Gastro-intestinal nematodes are major parasites of economic importance, infesting sheep in all producing areas (Kelly et al., 2010). These nematodes can be divided as hematophagous worms such as Haemonchus contortus, more commonly found in areas with summer rain, or irrigated pastures as opposed to non-hematophagous worms such as Teladorsagia spp., Trichostrongylus spp., and Nematodirus spp. in more temperate winter rainfall areas where irrigated pastures are uncommon (Besier et al., 2010). The acquisition of resistance to the major drenches used for chemical control is a problem in the sustainable management of these parasites (Kelly et al., 2010; Bath, 2014), and farming with resistant breeds or lines is considered as an alternative to relying on chemical drenches alone. In breeding programs, fecal worm egg count (FWEC) is commonly used as a proxy for nematode burdens (Colditz and Le Jambre, 2008). Adapted Red Maasai sheep had lower values for FWEC than Dorpers under conditions of high challenge with H. contortus (Baker et al., 2003, 2004). Red Masaai sheep also outperformed Dorpers in terms of reproduction in the semi-humid region, but not in the semi-arid region (Baker et al., 2004). Good et al. (2006) similarly reported that Texel sheep were more resistant to gastro-intestinal nematodes than Suffolks, although reasons for this breed effect was not evident. Cloete et al. (2016) also reported that Dormer lambs had lower means for FWEC than South African Mutton Merino lambs during 6 of 8 yr studied. No direct selection for a low FWEC was practiced in either breed. Based on FAMACHA© eye scores, Dormer lambs were slightly less anemic than their SAMM contemporaries, with better condition scores. It might be that the improved energy reserves of Dormers relative to SAMM lambs contributed to the apparently greater resistance. Many research groups have successfully bred sheep for lower FWEC (see Morris et al., 2005) within breeds. Karlsson and Greeff (2006) selected Merino sheep for low FWEC in a Mediterranean environment over a 15-yr period. FWEC genetically decreased on average by 2.1% per year. More importantly selection not only reduced FWEC in the selection line, but also significantly reduced the number of intestinal worms (immature and mature) and their egg laying ability in sheep that received a high or a maintenance level of nutrition (Greeff et al., 2019). The mean worm burden in the resistant line was only 7% of that in the control line (Kemper et al., 2010). However, there was a tendency for animals in the resistant line to become more prone to develop dags (Karlsson et al., 2005), particularly in a Mediterranean environment (Karlsson et al., 2004). It is therefore important to also select against dags in breeding programs to increase worm resistance based on FWEC. Adapted indigenous and selected genotypes were better able to withstand the challenge from local parasites, even in more susceptible breeds such as the Merino and even on a low feeding level. It is thus not surprising that FWEC made its way to formal sheep recording schemes in Australia and New Zealand (Brown and Fogarty, 2017). Modelling studies indicated that breeding sheep for resistance to parasites should be more productive (Bishop, 2011). This prediction was supported by Greeff and Karlsson (2020) indicating that Merino sheep that were 21% to 25% more worm resistant than the progeny of the control sires were 1.6 kg heavier at weaning, received less anthelmintics during the experiment, and produced 0.3 µm finer wool at hogget shearing than progeny from control sires. Cutaneous myiasis, primarily caused by larvae of the Australian Sheep blowfly, Lucilia cuprina under humid and warm conditions, is a major source of production loss of wool sheep, while also having a marked impact on animal welfare (Horton et al., 2017). Flystrike of primarily the breech area was largely controlled by chemicals, as well as a surgical procedure referred to as the Mules’ operation, where the folds on either side of the perineal area of lambs are surgically removed (Greeff et al., 2014; Horton et al., 2017). Chemical resistance of blowflies as well as welfare considerations regarding the Mules’ operation necessitate other means of dealing with flystrike such as breeding for a lower incidence of flystrike (Horton et al., 2017). One of the difficulties of breeding for a resistance trait is to provide all sheep with an equal challenge (Scholtz et al., 2010; Greeff et al., 2014, 2017). Moreover, the incidence of flystrike is dependent on a favorable climate, causing it to be transient and unpredictable. Therefore, through favorable genetic correlations with breech strike, several indicator traits were identified to serve as a proxy for the susceptibility of sheep to flystrike (Scholtz et al., 2010, 2011; Greeff et al., 2014, 2017). These traits are wrinkles in the breech area (Figure 1a), fecal soiling or dag formation (Figure 1b), urine stain, and larger bare areas around the breech and crutch. Neck wrinkle score also qualified as a significant indicator trait and is a particularly useful alternative trait where it is difficult to score breech wrinkle (Scholtz et al., 2010; Greeff et al., 2014). It is important to note that these traits are equally effective when an adequate blowfly challenge is absent and are therefore amenable for inclusion in formal evaluation schemes (Brown et al., 2010). However, skin wrinkle and fecal soiling only explains about 40% of the variation in breech strike in a Mediterranean environment (Greeff et al., 2017). In contrast, Smith et al. (2009) found that skin wrinkle was the dominant factor in a summer rainfall region. In a study on crossbred sheep, Scobie et al. (2008) found that dag score was negatively correlated with breech bareness score of sheep. This was in contrast with findings of Greeff et al. (2017) and Smith et al. (2009) who showed that bare breeches are not important in Merino sheep. Although the incidence of breech strike increased with an increase in skin wrinkle and dags (Greeff et al., 2014), a significant proportion of sheep with high skin wrinkle and dag scores were never struck. Large differences in breech strike were found between sire progeny groups, which were not entirely explained by breech wrinkle and dag scores (Greeff et al., 2018). None of the daughters of the two most resistant sires were struck over their lifetime, whereas a high proportion of the daughters of the two most susceptible sires were struck every year over their lifetime. The repeatable nature of breech strike implied that other factors, inherent to sheep, contribute to susceptibility of individual sheep. Further studies on breech moisture failed to conclusively account for the prevalence of breech strike when wrinkle and fecal soiling were considered (Greeff et al., 2022). However, Yan et al. (2019) demonstrated that the semiochemicals, octanal and nonanal, evoked an antennal and behavior response in gravid L. cuprina females, confirming the potential role as of these substances to attract L. cuprina. Greeff et al. (2021) found that odor profiles attracting blowflies are inherent to specific sheep and independent of the microbiome. Further studies on odor profiles of breech strike resistant and susceptible sheep are warranted. Extremes in terms of breech skin wrinkles (a) and soiling by feces and urine (b) are conducive to flystrike under climate conditions favorable for blowfly activity. A variety of tick species across the world are hosted by sheep. The impact of these ectoparasites on their hosts is detailed by Cloete et al. (2016) and include the transmission of disease, severe tissue damage and necrosis, the production of toxins that may cause paralysis, tissue damage on the udders and hooves of animals, blood loss at high infestations and “tick worry”. All these factors combine to cause serious welfare issues in livestock at high tick loads (Porto Neto et al., 2011). Tick control in the past mostly centered on the usage of acaricides to curb the infestation. However, as for gastrointestinal nematodes and blowflies noted above, the major tick species became resistant to this line of defense (Porto Neto et al., 2011) and alternative measures of tick control are seriously needed. As for other pathogens, ticks may also be controlled to an extent by farming with breeds expressing a higher level of resistance to the pathogen. An indigenous, unimproved fat-tailed breed, the Namaqua Afrikaner (Figure 2), had lower tick loads than those of internationally farmed commercial Dorper and SAMM ewes under extensive conditions, particularly in the sensitive loin and udder region (Cloete et al., 2016, 2017). Udder health scores were improved in the Namaqua Afrikaner when compared the other breeds (Cloete et al., 2016), with fewer Namaqua Afrikaner ewes being culled for udder problems than SAMM ewes (Cloete et al., 2016). A similar overall result was found when Namaqua Afrikaner lambs were compared to Dorper lambs (Cloete et al., 2021a). The cross of Namaqua Afrikaner rams with Dorper ewes produced lambs resembling the Namaqua Afrikaner regarding their lower tick loads and the Dorper regarding their higher weaning weights (Cloete et al., 2021a). Favorable hybrid vigor was thus suggested for both tick count and weaning weight, but this was not true for tick counts when crossing the commercial Dorper and SAMM breeds (Kao et al., 2022). It stands to reason that non-additive variation for a fitness trait such as tick load may not be present when two relatively susceptible commercial breeds are considered. Farming with a locally adapted indigenous breed may thus be advisable under conditions of severe tick challenge and general failure of chemical control. When contemplating breed substitution, however, it needs to be considered, though, that the Namaqua Afrikaner was generally inferior to the commercial breeds for carcass traits (Burger et al., 2013), mature live weight (Cloete et al., 2016), and weaning weight (Cloete et al., 2021a). The Namaqua Afrikaner breed carry lower tick loads than commercial breeds, while it also copes better with heat. Limited other studies explored breed effects for tick burdens in sheep. Female Ixodes ricinus ticks were more likely to infest Scottish Blackface than Cheviot ewes (Macleod, 1932). Dorper and Merino sheep were also studied on Karoo shrub pasture (Fourie and Kok, 1996), with a lower burden of the tick species Ixodes rubicundus in Merinos. This result contrasts with the conclusions drawn above, as no differences were found between the Dorper and another wool breed (SAMM) in the study of Kao et al. (2022). In fact, autumn total tick counts were higher in SAMM than in Dorper ewes, whereas this breed difference was not recorded in spring (Cloete et al., 2017). The contrasting results between the South African studies may be associated with different tick species studied. Fourie and Kok (1996) related their results to different grazing habits in the breeds studied. Nevertheless, it is evident that further research is needed to inform industry on the putative benefit of breed substitution to alleviate the impact of ticks on extensive sheep. We could not find evidence on within-breed selection to establish ovine lines exhibiting a higher resistance to ticks. It is worth mentioning that worthwhile genetic variation in tick loads has in South (Cloete et al., 2016, and et al., 2017). These studies the for within-breed should it be of have an important impact on while caused by cause skin in of by the industry et al., 2016). to and Sheep from by in is an important condition of sheep, welfare by causing and behavior and in of wool and condition to an (Van and These that breed differences that it may be to the prevalence of sheep by genetic It has indeed demonstrated that on Dorper sheep are and more difficult to when compared to the as of wool sheep (Fourie et al., The however, that the in with the to sheep in this breed, may contribute to the of the No evidence to successful within-breed selection for a reduced susceptibility to sheep was found in the and no could be The sheep is an important of sheep. It is an with no and survival from the and The chemical load of the Australian wool is by the used to control sheep it an important from an economic with results in to severe thereby sheep welfare while also weight by kg per year et al., 2011). and reported an higher count in ewes compared to ewes over a challenge under conditions. et al. also reported differences within the Merino breed in total that within the more susceptible Merino breed may for their ability to withstand This of genetic variation between animals within a breed was supported by of genetic variation for counts et al., on sheep are as they are mostly farmed under extensive or on where they and of in (Cloete Olivier, 2010). to other sheep are relatively as as heat stress is (Rust and Rust, 2013). Nevertheless, the extensive and general of to most sheep commonly them to both heat and cold to heat stress can impact sheep by ewe and and in lamb and et al., Wettere et al., 2021). et al. showed that heat stress was associated with lower lamb weights and suggested that lamb survival of Merinos was to heat In their on the and of indigenous genetic et al. (2020) to heat stress as of the of unimproved, indigenous breeds such as the Namaqua This was supported by results from Cloete et al. indicating that the of Namaqua increased by only from of to of In the of increased by that of Dorpers by and those of breeds from temperate Merino and by between and These results that the of ewes need to to was to the indigenous of the breeds. breed, the as a commercial temperate sire breed, was to these resources when a study was by (2022). (Figure and means for (Figure are for all breeds for and conditions. that the and lines on these mean values for the temperate breeds, the Dorper a adapted and the and Namaqua Afrikaner as more breeds. The latter two breeds not and were from an average of for the and the Namaqua Afrikaner to for Dorpers and an average of for the temperate breeds from in the SAMM to in the in the were and from in the Merino to in the Dormer and All breeds were generally able to their in a relatively (Figure but breeds from temperate on a higher to their heat stress conditions this energy need could with energy for other production Namaqua ewes also outperformed Dorper and SAMM ewes for number of lambs weaned under marginal extensive conditions (Cloete et al., 2016). This difference was not for total weight of lamb as the commercial breeds weaned heavier The was primarily from Dorper rams to indigenous ewes the The breed copes well with heat when compared with commercial breeds. of sheep breeds during and The breeds on the of the are from temperate regions, the Dorper from a cross of a temperate breed with a heat adapted breed, and the two breeds have a higher indigenous The lines are mean values to the of the sheep breed for and recording from 2022). in per of sheep breeds during and The breeds on the of the are from temperate regions, the Dorper was derived from a cross of a temperate breed with a heat adapted breed and the two breeds have a higher indigenous The lines are mean values to the of the sheep breed for and recording from 2022). The of indigenous sheep relative to temperate breeds was also in and (see by In this et al. (2016) used to determine for the which values for and eye to increase The for ewes of breeds, a of breeds from temperate and local adapted breeds. These were on climate on a to determine suitable for specific breeds. that the of wool breeds be to the and of while a local adapted breed, the could be farmed throughout the The Dorper could also be farmed In a et al. (2020) that the better adaptation of sheep to heat stress conditions was not by as but by adaptation to environments. This is supported by results from et al. that Merino sheep with lower lower and were more likely to lambs in lambing than those with higher In the of for resistance to heat stress in the the for could be considered as a low were reported for (Cloete et al., 2022). showed but were repeatable et al., for rain, to other such as a high low and a condition can result in sheep et al., 2021). could be high on individual with per from to and to (Fourie and to et al., and to et al., 2021). et al. (2009) reported that to in may during the year weaning, with low being particularly at Many of these was not Lamb which may to in is of greater importance, Lamb in extensive in marginal areas may be as high as to 40% (Kao et al., 2022). from its direct impact on it is also considered as a major welfare et al., 2014). The of lamb are associated with and a with the of and stress is the most important source of lamb but may also be associated with lamb for severe (Pollard, et al., It is therefore important to the ability of lambs to resist cold stress caused by a of rain, and a low There is marked variation between breeds in their ability to with cold with Merino lambs generally than lambs of other breeds et al., However, when the of genetically lamb survival was considered, consensus was that genetic was to low that were derived et al., 2014). was the to that it is to lamb survival by genetic results were reported for New Zealand lambs et al., divergent genetic were reported for lamb survival in a Merino selected for and against et al., resulted in a improved lamb survival from to of in these in of the line selected for et al., 2021a). from the line selected in the were better able to their than those selected under high of cold stress et al., 2021a). These results those breed differences for the ability of lambs to resist cold that the better survival in the selected line from a greater adaptability to cold stress conditions. This has supported on the genetic level by the a greater genetic difference in between selection lines when cold stress were high at al., These findings to the inclusion of lamb survival by ewe ability in the recording and evaluation by Sheep Australia et al., 2021). lamb survival is and genetic in the by Sheep Limited in New Zealand et al., 2014). This study the response of sheep to environmental challenges by parasites and the Variation breeds were to allow a of genotypes that are better able to with environmental stressors to a specific to within-breed selection was particularly successful in lines that are better able to environmental stress than the they were selected There is a though, to with research on traits of adaptation to climatic conditions. There is also a need to the need for unimproved, indigenous as they may contribute to the of some fitness of None Cloete from a at the of in Cloete as an at and and and and all from a in and from the of the was by the of and from to the in was to the of in where in in and with specific in and of is on sheep and Greeff Greeff at the of the in and from and and and from the of as a research at the and at South where studies with the highly and sheep breeds. a at the of of Australia in In in breeding sheep for improved wool and in breeding sheep that are resistant to gastro-intestinal and in is a within the of of the of South and in at the of and in research in of farmed animals, mostly sheep. has the of the studies genetic prediction of South African Merino sheep but has also on lamb survival and cold stress are in the genetic prediction of health and welfare and the of to such breeding at the on for a to the the and has at the of In the was in the but became in studies and sheep research on to control in at the of and in at the of the for and is a and has in research in Sheep and with specific in and

Topics & Concepts