ABSTRACT
During a four-year period, mature ewes were either shorn in mid-pregnancy or post-partum to study impacts on lamb live weight and survival. Furthermore, placental measurements were collected post-partum from a subset of single-bearing ewes and yearly fleece weights and wool samples were collected from a subset of single- and twin-bearing ewes. Lambs born to ewes shorn in mid-pregnancy were heavier (p < .05) and displayed greater (p < .05) survival than those born from post-partum shorn ewes regardless of litter size. Shearing treatment had no effect (p > .05) on the weight of the placenta, and diameter or the weight of the cotyledons. However, the number of cotyledons was greater (p < .05) in mid-pregnancy than post-partum shorn ewes. Ewes shorn in mid-pregnancy produced lighter (p < .05) fleeces with greater (p < .05) yields. The fleece displayed shorter (p < .05) staple length, stronger staple strength (p < .05), and was less (p < .05) bright with poorer (p < .05) colour than those shorn in post-partum.
Introduction
Mid-pregnancy shearing of mature ewes under pastoral conditions has consistently increased lamb birth weight (Dýrmundsson, Citation1991; Kenyon et al., Citation2003; Gootwine, Citation2013) although the effect on lamb weaning weight has been less consistent (Dýrmundsson, Citation1991; Kenyon et al., Citation2003). It has been theorised that to improve lamb survival by mid-pregnancy shearing the increase in birth weight must be of a magnitude to move a significant proportion of otherwise lightweight lambs into a higher survival rate range (Kenyon et al., Citation2002). However, few studies have had sufficient lamb numbers to determine whether mid-pregnancy shearing can increase lamb survival (Kenyon et al., Citation2006).
A potential mechanism suggested for the increase in birth weight from mid-pregnancy shearing is enhanced placental development (Morris and McCutcheon, Citation1997). It has been reported (Banchero et al., Citation2010) that shearing mature ewes in mid-pregnancy increased placental weight, but not in all studies (Revell et al., Citation2002). Therefore, more research is needed to unravel the mechanism that stimulates increased survival.
Wool growth is slower in winter (Sumner et al., Citation1994; Champion and Robards, Citation1995; Dick and Sumner, Citation1997), which often coincides with mid-pregnancy. Therefore, it is unsurprising that in Romney-based breeds, shearing during pregnancy has been reported to have no effect on wool growth or annual wool production (Kenyon et al., Citation2003). However, advantages of pregnancy shearing ewes in winter on fleece characteristics have been suggested (Hawker and Litllejohn, Citation1989). If shearing occurs when the thinnest and weakest part of the fibre is at the base of the staple, this technique could be used to manipulate fibre strength. Although, with Romney-based ewes, no effect on fibre strength (Morris et al., Citation2000) or diameter (Dabiri et al., Citation1996) has been observed. There is a lack of information on the effect of mid-pregnancy on fleece characteristics in medium fibre diameter breeds such as the Corriedale.
The present study tested the hypothesis that mid-pregnancy shearing of mature Corriedale ewes under commercial Uruguayan conditions would increase lamb weight and survival to weaning, and that this effect is driven by changes in the placenta characteristics. It was also hypothesised that mid-pregnancy shearing would improve ewe fleece quality, especially staple strength.
Materials and methods
Animals, treatments, and experimental design
The procedures for this experiment were carried out according to the recommendations set by the Uruguayan Honorary Animal Ethics Committee. The study was undertaken in Uruguay at Glencoe Station (latitude 32° 01′ 32″ S, longitude 57° 00′ 39″ O, 124 m altitude, shortest day 21st June), over four years (2000–2003) utilising 1141 mature Corriedale ewes (three to nine years of age). Ewes were either shorn in mid-pregnancy or post-partum. In the first year, ewes were allocated to the shearing treatments balanced for number of foetuses carried, days of gestation, and ewe live weight. In the subsequent years, additional ewes were added into each shearing treatment. These additional ewes were allocated to their treatments as hoggets, but only data from their second parturition onwards was considered in this experiment. Once animals were allocated to a given treatment, they remained in that same treatment for subsequent years.
Shearing was undertaken in mid-pregnancy using winter combs, which left a stubble depth of at least 7.5 mm (Sunbeam Corporation Ltd., Australia or Uruguayan Wool Secretariat, Uruguay), in contrast, shearing post-partum utilised a standard comb leaving a stubble depth of at least 3.5 mm (Heiniger AG, Switzerland). Mid-pregnancy shearing occurred between 27th June and 18th July, while ewes shorn post-partum were shorn from 24th October to 3rd December. The exact time of shearing differed between years due to weather conditions.
Ewe breeding was in autumn, starting between 28th of March and 9th of April across the four years. Between years, the exact timing of pregnancy scanning, shearing, and weaning varied slightly (). In all years, ewes from both treatments remained together all year around and during both pregnancy and lactation, ewes were managed under commercial grazing conditions (Montossi et al., Citation2000) on native grasslands at 3.1–3.6 ewes/ha and on improved grasslands at 12 ewes/ha (twin-bearing ewes in peri-partum).
Table 1. Calendar of events in each year.
Ewe live weight and condition score
Each year, all ewes were weighed unfasted just prior to mid-pregnancy shearing, in late pregnancy (17th to 22nd of August) and at weaning. Ewes were also scored for body condition (Russel et al., Citation1969) at live weight measurement. During the lambing period, ewes, which had not yet lambed, were re-weighed weekly. Therefore, late pregnancy weight used in the analysis of live weight was the live weight measurement closest to lamb delivery (lamb birth date). The live weight of ewes in mid-pregnancy, late pregnancy, and at weaning were adjusted using later fleece weights as follows: adjusted live weight = live weight – (fleece weight/days between shearings)*days between previous shearing and live weight. Furthermore, live weight of ewes during pregnancy were adjusted in order to remove the predicted weight of the conceptus from the measurement, by following the procedure described by Corner-Thomas et al. (Citation2014), with the gestation length being assumed as 150 days based on data from this flock when using artificial insemination at mating.
Placental measurements
Placentas from a random sample of single-bearing ewes were collected fresh in the field during twice-daily lambing inspections. Sixty-eight placentas were collected from mid-pregnancy shorn ewes and 65 from post-partum shorn ewes over three years (2001–2003). These were refrigerated until dissection. The placenta was weighed and individual cotyledons excised, and counted. Twenty per cent of cotyledons per placenta were recorded for their diameter at the widest point and weighed.
Ewe wool characteristics
In all years of the experiment and in the following year, fleece weights from ewes shorn in either mid-pregnancy or post-partum were recorded. In addition, wool mid-side samples of 458 adult ewes were collected on three years (2001–2003) during shearing and analysed for fibre diameter, yield, coefficient of variation of fibre diameter, and staple length. A subset of wool samples from 134 ewes were analysed for staple strength, and 102 for colour (yellowness) and brightness. Wool samples were conditioned (20 ± 2°C and 65 ± 2% relative humidity) for 24 h and then weighed (greasy weight). Greasy wool samples were washed and assessed by a third-party laboratory (Wool Laboratory, Uruguayan Wool Secretariat, Uruguay) with Interwoollabs endorsement. Washing yield (%) was calculated as clean weight (g)/greasy weight (g) × 100.
Lamb live weight and survival
Lambs were weighed (birth weight), their sex determined, dam, and birth rank identified and date of birth recorded during twice daily (morning and afternoon) lambing inspections. Lambs were weighed again at weaning. Lamb survival to 72 h and weaning was based on recorded lamb deaths and date, during the lambing inspections which were performed until weaning.
Statistical methods
Ewe and lamb data were analysed using mixed models in a factorial randomised design using the software SAS (Version 9.1. SAS Institute, Cary, NC, USA). ‘Shearing treatment’, ‘litter size’ (single vs. twin), their interaction, and ‘year’ were included in the model as fixed effects in the analysis of the data of ewes (live weight, body condition score, and wool measurements). For the analysis of placenta traits, only single-bearing ewes were considered, where ‘shearing treatment’ and ‘year’ were the fixed effects included in the model. In the case of lambs, ‘shearing treatment’, ‘litter size’, their interaction, ‘year’, and ‘sex of the lamb’ were included in the models as fixed effects. Lamb survival to 72 h and weaning were analysed as binomial traits using the CATMOD procedure in SAS. The model included the fixed effects of ‘shearing treatment’, ‘litter size’, their interaction, ‘year’, and ‘sex of the lamb’. Least square means, standard errors, and level of significances are presented for the estimated variables.
Results
Ewe live weight and body condition score
There was no interaction (p > .05) between shearing treatment and litter size for any ewe live weight or body condition score measurements. Shearing treatment had no effect on ewe live weight at mid-pregnancy, pre-lambing or weaning (p > .05, ). However, those ewes shorn in mid-pregnancy presented a lower body condition score at pre-lambing (p < .05) than the ewes shorn at post-partum. Twin-bearing ewes were consistently heavier than single-bearing ewes from mid-pregnancy to weaning (p < .05), although body condition score was not affected by litter size (p > .05).
Table 2. Effect of shearing treatment (Mid, mid-pregnancy shearing; Post, post-partum shearing) and litter size (Single, ewes bearing one lamb; Twin, ewes bearing two lambs) on ewe live weight and body condition score (mean ± SE).
Single-bearing ewe placental measurements
Shearing treatment had no effect (p > .05) on the weight of the placenta (p = 0.10, 404 ± 23 vs. 374 ± 24 g for mid-pregnancy versus post-partum shearing, respectively), diameter (19.5 ± 0.34 vs. 19.2 ± 0.36 mm) or weight of cotyledons (1.17 ± 0.049 vs. 1.21 ± 0.051 g). However, the number of cotyledons was greater in mid-pregnancy than post-partum shearing treatment ewes (86 ± 1.8 vs 80 ± 1.9, p < .05).
Ewe wool traits
No interactions were found between shearing treatment and litter size on wool measurements (p > .05). Ewes shorn in mid-pregnancy produced (p < .05) lighter and stronger fleeces with greater yields, shorter staple lengths, less bright, and poorer colour than those shorn post-partum. Litter size had no effect on wool parameters (p > .05) ().
Table 3. Effect of shearing treatment (Mid, mid-pregnancy shearing; Post, post-partum shearing) and litter size (Single, ewes bearing one lamb; Twin, ewes bearing two lambs) on ewe wool traits (mean ± SE).
Lamb live weight and survival
There was no interaction between litter size and shearing treatment for lamb weight measures or survival to 72 h or weaning (p > .05). At birth, lambs born from ewes shorn in mid-pregnancy were 9% heavier (p < .05, ) than those born from ewes shorn post-partum and twin lambs were approximately 25% lighter (p < .05) than single lambs. These differences were still present (p < .05) at weaning.
Table 4. Effect of shearing treatment (Mid, mid-pregnancy shearing; Post, post-partum shearing) and litter size (Single, ewes bearing one lamb; Twin, ewes bearing two lambs) on lamb live weight at birth and weaning and survival to 72 h and to weaning (mean ± SE).
Shearing treatment affected (p < .05) survival of lambs to both 72 h after birth and weaning, being greater in mid-pregnancy shearing in comparison with post-partum. Survival of singleton born lambs was greater (p < .05) than twin lambs at 72 h of age and weaning.
Discussion
The hypothesis that mid-pregnancy shearing of mature Corriedale ewes would increase lamb weight and survival at birth and weaning was supported. Moreover, it was associated with a decrease in ewe body condition score and a tendency for an increase in the weight of the placenta and an increase in the number of cotyledons. These suggest that the metabolism of the ewe and the placenta play important roles in the underlying mechanism. Furthermore, the hypothesis that mid-pregnancy shearing would increase staple strength was also supported, although this improvement on one parameter of wool quality was accompanied with an undesired consequence on wool production, staple length, brightness, and yellowness.
This effect of mid-pregnancy shearing on lamb weight at birth and weaning is supported by previous research (Cam and Kuran, Citation2004; Kenyon et al., Citation2006; Banchero et al., Citation2010; Sphor et al., Citation2011). However, previous studies concluded that birth weight response was not driven by differences in ewe live weight or body condition score (Dabiri et al., Citation1996; Kenyon et al., Citation2002a) in late pregnancy. By contrast, our results show that mid-pregnancy shorn ewes had a lower body condition score, than unshorn ewes. It has been shown that shorn ewes change their metabolism in response to cold stress, which is associated with a change in lipid mobilisation, resulting in an increase in plasma NEFA, glucose and GH compared with unshorn ewes (Symonds et al., Citation1988). It has been suggested that ewes need to have adequate maternal reserves (2.5 condition score units or above) for an increase in birth weight to occur from mid-pregnancy shearing (Kenyon et al., Citation2003, Citation2002b), to allow fat to be available for mobilisation, to promote the desired effects on the foetus. The condition score of mid-pregnancy shearing ewes in this study was superior than 3 units, both at mid-pregnancy and pre-lambing, allowing for fat to be mobilised in response to shearing.
An enhanced placental development (Morris and McCutcheon, Citation1997) including increased placental weight (Banchero et al., Citation2010) has been suggested to explain an increase in birth weight of lambs born from mid-pregnancy versus post-partum shorn ewes. Using a different experimental approach, it has been shown that the changes in the weight of the placenta in response to nutrition depend upon the body condition score of the ewes; if the body condition score was 2 then there was a positive effect, but if the body condition of the ewe was 3.5 no changes were observed (Osgerby et al., Citation2003). This differential response may explain the small (tendency) effect on the weight of the placenta in our study, since ewes were generally in body condition scores slightly below 3.5 units. In this study, we report an increase in the number of cotyledons in the placenta from shorn compared with unshorn ewes, not previously reported (Revell et al., Citation2002; Banchero et al., Citation2010). This result may be relevant, since the cotyledons are the units of feto-maternal exchange of gases and nutrients (Latshaw, Citation1987). It has been suggested that the ovine placenta attains its maximum weight by day 75 − 80 of gestation, while placentomes reach a fixed number by day 60 of gestation (Alexander, Citation1964; Ehrhardt and Bell, Citation1995; Heasman et al., Citation1999). In our experiment, pregnancy shearing occurs between days 42 and 108 of gestation when a proportion of the placentas were still under active growth and development, and thus potentially affected by the intervention of shearing. Previous research, evaluating interventions, like altitude (natural hypobaric hypoxia) or nutrition, during first and second trimester of gestation have demonstrated that placental weight, number of cotyledons, cotyledon diameter, contact surface between cotyledons, type of cotyledons, and cotyledon surface area occupied by vascular lumina can be modified (Heasman et al., Citation1999; Parraguez et al., Citation2006). While, lamb birth weight has a poor correlation with number of cotyledons, it is more strongly correlated with placenta weight, and weight of cotyledons (Alexander, Citation1964). The poor correlation with number of cotyledons has been explained due compensation in the size of individual cotyledons. In our study, the number of cotyledons and the weight of the placenta were greater in mid-pregnancy shorn ewes but the weight and diameter of cotyledons did not differ. Combined these results would suggest the amount of cotyledonal tissue would have been greater in mid-pregnancy shorn ewes potentially allowing for improved nutrient transfer between the ewe and foetus. Therefore, these changes could be the mechanism by which mid-pregnancy shearing increases foetal growth, vigour (Banchero et al., Citation2010) and survival at birth. Further research would be necessary to clearly establish if pregnancy shearing can modify macroscopic and microscopic placenta characteristics and function. Management strategies applied during foetal life can have a long-term effects on the performance of the offspring (Gunn et al., Citation1995; Kenyon and Blair, Citation2014) but further work is required if this is the case of mid-pregnancy shearing. However, it has been established that mid-pregnancy shearing also increases milk production (Cam and Kuran, Citation2004; Sphor et al., Citation2011) which would be expected to improve lamb weight and survival to weaning.
Survival of lambs at 72 h post-partum and at weaning was increased by shearing ewes in mid-pregnancy. Survival can be improved through mid-pregnancy shearing by an increase in birth weight of otherwise lightweight lambs (Kenyon et al., Citation2006, Citation2002). Lamb survival has been reported to be optimal in the range of 4–6 kg with an increase in birth weight up to 4 kg having positive effects (Dalton et al., Citation1980; Thompson et al., Citation2004a). Lambs born to unshorn ewes weighed 4.05 kg at birth, with 41% of those lambs being lighter than the optimal recommendation, which likely explains their lesser survival. While only 21% of the lambs from mid-pregnancy shearing ewes were lighter than 4 kg at birth.
Ewes shorn in mid-pregnancy produced lighter fleeces with greater yields. Although, this effect has not been reported in Romney-based breeds where no effect on wool production has been found (Kenyon et al., Citation2003), it has been indicated (Thompson et al., Citation2004b) in Merinos, a parent breed of the Corriedale, that wool growth decreases through pregnancy and foetus requirements are prioritised compared to maternal requirements. Therefore, the ‘trade-off’ between wool and foetal growth could be greater in mid-pregnancy shorn ewes, based on the heavier birth weight through enhanced foetal growth.
The value of the fleece is primarily affected by its micron, strength, length, and colour (Nolan et al., Citation2014). There was no difference in fibre diameter between shearing treatments in agreement with previous findings (Dabiri et al., Citation1996). In contrary with research in New Zealand (Morris et al., Citation2000) and in agreement with Mueller et al. (Citation2013), the staple strength was affected by treatments, supporting the potential advantage (Hawker and Litllejohn, Citation1989) suggested for shearing pregnant ewes in winter when the weakest part of the fibre is situated at the base of the staple. The wool of mid-pregnancy shorn ewes was found to be yellower than that from ewes shorn at post-partum, which has not been previously reported. The staple length of ewes shorn in mid-pregnancy would be longer in spring/summer than ewes shorn post-partum, that were recently shorn. Longer wool in humid conditions is more likely to be yellow (Henderson, Citation1968; Reid, Citation1998). Overall the lighter, shorter, and yellower fleeces of ewes shorn in mid-pregnancy will negatively affect the value of wool produced. This influence on wool quantity and quality would need to be compared with the beneficial effects of this practice in terms of lamb weight and survival when farmers decide the time of shearing.
In conclusion, under extensive pastoral conditions of the north of Uruguay, where a proportion of lambs normally have a birth weight lower than 4 kg, shearing single- and twin-bearing ewes in mid-pregnancy can positively influence lamb live weight and survival. The changes in body condition and metabolism of the ewe and an increase in the number of cotyledons and placental weight could be potential mechanisms to explain the effect of shearing on lamb birth weight. The negative effects observed by shearing ewes in mid-pregnancy on some wool traits need to be considered when analysing the potential positive impacts on lamb performance to weaning.
Acknowledgements
The authors thank the staff of Glencoe Research Unit (INIA Tacuarembó Research Station, Uruguay) for their technical assistance, the Uruguayan Wool Secretariat for the analyses of wool samples, and Dr. Gabriel Ciappesoni for his advice on the statistical analyses.
Disclosure statement
No potential conflict of interest was reported by the authors.
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