Reproductive performance and productivity of Menz and Horro sheep lambing in the wet and dry seasons in the highlands of Ethiopia

Small Ruminant Research 45 (2002) 261–271

Reproductive performance and productivity of Menz and Horro sheep lambing in the wet and dry seasons in the highlands of Ethiopia E. Mukasa-Mugerwa1 , D. Anindo2 , S. Sovani3 , A. Lahlou-Kassi4 , S. Tembely5 , J.E.O. Rege, R.L. Baker∗,6 International Livestock Research Institute (ILRI), P.O. Box 5689, Addis Ababa, Ethiopia Accepted 15 June 2002

Abstract Ewe reproduction and productivity were studied in the highlands of Ethiopia in Horro and Menz ewes mated to lamb in both the wet and dry seasons from June 1992 to May 1997. The data included a total of 4890 mating records of which 2516 were Menz ewes and 2374 Horro ewes, with 2360 matings in the wet season and 2530 in the dry season. Menz sheep had a significantly higher (P < 0.001) weaning rate (lambs weaned per ewe mated) than the Horro ewes (0.73 versus 0.57, respectively) and ewes which lambed in the wet season had a significantly higher (P < 0.001) weaning rate than those that lambed in the dry season (0.76 versus 0.53, respectively). Menz ewes showed their superiority in weaning rate over the Horro ewes more clearly when lambing in the wet season (0.85 versus 0.67, respectively) than when lambing in the dry season (0.59 versus 0.47, respectively). Overall flock productivity was expressed in terms of potential offtake of yearling sheep from flocks of Menz or Horro ewes lambing in either the wet or dry seasons. Both in terms of number of yearling sheep for sale and total live weight for sale, the offtake of a flock of Menz sheep in this environment was approximately three-fold greater than a flock of Horro sheep when they lambed in the wet season and approximately two-fold greater when they lambed in the dry season. Season had no effect on offtake in Horro ewes, but the Menz ewes lambing in the wet season had about a 1.5-fold greater offtake than Menz ewes lambing in the dry season. In the first 3 years of the study (1992–1995), oestrous synchronisation was used at each mating. Oestrus was synchronised with intravaginal sponges containing 40 mg flugestone acetate (FGA). Most of the ewes (83%) came in oestrus after hormonal treatment and out of these 81% were synchronised and mated within 5 days after sponge removal. The percentage of ewes synchronised was 10% higher after treatment in the wet season than in the dry season (P < 0.01) and 7% higher for Menz

∗ Corresponding author. Tel.: +254-2-630743; fax: +254-2-631499. E-mail address: [email protected] (R.L. Baker). 1 Present address: P.O. Box 3987, Kampala, Uganda. 2 Present address: American Breeders Service International, P.O. Box 14821, Nairobi, Kenya. 3 Present address: v. Irpinia 23, 58100 Grosseto, Italy. 4 Present address: Institut Agronomique et V´ et´erinaire, Hassan II, B.P. 6202 Instituts, 10101 Rabat, Morocco. 5 Present address: Central Veterinary Laboratory, BP 2295 Bamako, Mali. 6 ILRI, P.O. Box 30709, Nairobi, Kenya.

0921-4488/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 4 8 8 ( 0 2 ) 0 0 1 5 5 - 4

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sheep than for Horro sheep (P < 0.01). Of the ewes that eventually lambed, 73, 17 and 10%, respectively, conceived at the first, second and third or subsequent oestrus with an average 1.5 services per pregnancy. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Productivity; Reproduction; Season; Oestrous synchronisation; Sheep; Ethiopia

1. Introduction

2. Materials and methods

Sheep in the tropics can breed throughout the year (Gatenby, 1986), but the impact of this natural ability on lamb production is compromised by high lamb mortality and low growth rates during certain periods of the year. Conception peaks are observed in response to feed flushes or when crop residues are available. However, while pregnancy rates can be increased, lambing may subsequently take place when feed supplies to support higher demands during lactation are extremely restricted. Lambs may also be born underweight which further reduces their survivability (Mukasa-Mugerwa et al., 1994a). Lack of synchrony between the supply of, and demand for, nutrients can depress ewe productivity (Wilson and Murayi, 1988). Furthermore, morbidity and mortality are exacerbated by diseases, such as endoparasitism, which are influenced by season. Parasitic gastroenteritis is a wet season-driven phenomenon, the survival of parasites across seasons being through the persistence of existing established worms and hypobiotic larvae (Ogunsusi and Eysker, 1979; Tembely et al., 1997). As a consequence, depending on the environment and production system, malnutrition and infectious or parasitic disease interact to limit animal performance (Knox, 1995). The objective of this study was to assess the reproductive performance and productivity of indigenous ewes mated to lamb in the wet or dry season. This study was part of a larger experiment investigating genetic resistance to gastrointestinal nematode parasites in Menz and Horro sheep (Baker et al., 1998; Tembely et al., 1998; Rege et al., in press). In the first 3 years of the study, oestrous synchronisation was used at each mating and this paper also investigates the success of this technique in tropical sheep. Reproductive technologies, such as oestrous synchronisation, can increase the probability of ewes being bred over limited time periods (Boland et al., 1981; Oyedipe et al., 1989; Mutiga and Mukasa-Mugerwa, 1992).

2.1. Study location, animals and management This study was undertaken at the ILRI Debre Berhan Station located in the Ethiopian highlands at latitude 9◦ 36 N and longitude 39◦ 38 E, 120 km north-east of Addis Ababa at an altitude of 2780 m. The climate is characterised by a long rainy season (June–September) accounting for about 75% of the annual rainfall, a short rainy season (February/March to April/May) and a dry season (October–January). Annual rainfall recorded at the station, averaged 970 mm over the study period. Mean monthly rainfall figures and their relationship to gastrointestinal nematode infections in the ewes lambing in the wet and dry seasons are described in detail by Tembely et al. (1998). Mean minimum and maximum temperatures were 6.2 and 18.8◦ C, respectively, and the mean relative humidity was 60%. The reproductive performance of the indigenous Menz and Horro breeds were compared. The Menz sheep are indigenous to the study area and a flock was already established at the Debre Berhan Station at the beginning of the study. Menz sheep (also called the Shoa or Legagora) are concentrated in the central highlands between 2500 and 3000 m above sea level. It is a fat-tailed breed of relatively small size (mature ewes range from 25 to 35 kg), principally black or dark brown in colour with frequent white spots on the neck, head and legs and with an open fleece consisting of coarse hair and a woolly undercoat (Galal, 1983). The Horro sheep are found in western Ethiopia at an altitude of 1400–2000 m with a dependable annual rainfall averaging 1000–1400 mm. They are a fat-tailed hair breed, mostly uniformly solid tan or light brown in colour, and are a larger sheep than the Menz, with mature ewes ranging from 35 to 45 kg (Galal, 1983). The Horro ewes and rams required to initiate the experiment were purchased from their traditional habitat in western Ethiopia and were quarantined for 2–3 months at Debre Berhan Station before joining the

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experiment. Additional Menz and Horro rams were purchased as and when required over the study period. Ewes were mated so that they lambed either in the dry (October–November) or the wet (June–July) season over the 6 years from June 1992 to May 1997. Pubertal ewe hoggets (12–18 months of age) detected in oestrus by vasectomised rams running with them after weaning and averaging 22 and 24 kg live weight for the Menz and Horro, respectively, were used to replace older animals culled for age, ill health or after mortality. A number of ewes that failed to become pregnant within a particular mating period were available for mating the following season. However, most of the ewes were re-mated at annual intervals. Parity, pre-mating weight and condition score on a scale of 0–5 (Hossamo et al., 1986) were used to block ewes for randomisation into 11 single sire mating groups per breed each season. Each sire was used for mating for at least a year (two mating seasons) at the end of which seven of the rams were replaced by new sires. Oestrus was synchronised with intravaginal sponges containing 40 mg flugestone acetate (FGA; Chronogest® , Intervet International B.V.) to lamb in the wet or dry seasons in the first 6 out of the total 10 matings in the study. Sponges were applied on the same day for all ewes. Intravaginal sponges were removed after 12 days. Pregnant mare serum gonadotrophin (PMSG) was not concurrently administered since sheep at the station cycle year-round (Mukasa-Mugerwa and Lahlou-Kassi, 1995). Ewes were mated over 25–42 days at the ratio of 20–25 ewes per ram of the same breed during the night. Rams ranged from 2 to 5 years of age and were tested for breeding soundness (evaluation of both testicular measurements and semen characteristics) as described by Toe et al. (2000) and Rege et al. (2000) and given a 5 min libido test using ovariectomised oestrogen treated adult ewes) prior to the start of each mating period. Oestrus was monitored during daytime herding with vasectomised rams and mating marks read every morning when animals left the night mating pens. All ewes grazed together on pastures of Pennisetum, Festuca and Andropogon grass species mixed with Trifolium semense legume and supplied with watering troughs. Ewes were housed at night and had access to hay, water and mineral licks were available ad libitum. Ewes were supplemented with 200–400 g per day of a concentrate mix (33% nough cake

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(Guizotia abyssinica), 65.5% wheat bran, 1% limestone and 0.5% salt) depending on their pregnancy status or during the peak of the dry season. Lambs were kept with their dams all the time. Lambs also had no access to feed other than that offered to the dams before weaning, but after weaning lambs grazed in separate sex groups and were offered 50–150 g per day of the same concentrate the first 2 months and during the extreme part of the dry season. Sheep were strategically drenched against flukes based on the results of epidemiological studies in the area (Tembely et al., 1998). Between mating and weaning, individual ewes and lambs were also drenched against roundworms when their faecal egg counts reached 4000 eggs per gram. All ewes were drenched after weaning to prepare them for their next mating. All animals were vaccinated for pasteurellosis and clostridial infections twice a year. Sick animals were treated and health data, along with necropsy findings in case of mortality, were recorded. Pregnant ewes that died before lambing were regarded as infertile. 2.2. Measurements A ewe was considered to have been synchronised if it came in oestrus within 5 days of FGA intravaginal sponge removal (Spitzer and Carpenter, 1981). The interval from sponge removal to conception and durations of mating and lambing periods were also recorded. Date of conception in ewes returning to service was considered as the oestrus date nearest to 150 days before lambing. Ewe reproductive traits analysed included fertility rate (ewes mated which lambed), services per conception and oestrous cycle length based on detected oestrus, litter size or prolificacy (lambs born per ewe lambing), lambing rate or fecundity (lambs born per ewe mated), lamb survival as a trait of the ewe (lambs weaned per lambs born) and weaning rate (lambs weaned per ewe mated). 2.3. Statistical analysis A fixed effects least squares means model (SAS, 1987) was used to analyse the effects of ewe breed (Menz and Horro), ewe age (five levels: 1 to 5+ years of age at mating), year (five levels) and season (wet and dry) on reproductive performance and ewe live weight. The same effects were included in the model

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when analysing ewe synchronisation response, except that ewe age was replaced with ewe parity (three levels: 1 to 3+ years of age at mating) and just the first 3 years were analysed. Significant first order interactions (P < 0.05) in preliminary analyses were also included in the final models.

3. Results 3.1. Reproductive performance and ewe live weight Menz sheep had a significantly higher (P < 0.001) weaning rate than the Horro ewes (0.73 versus 0.57, respectively) and also had a significantly higher (P < 0.01) reproductive rate for all the other traits except litter size which was very similar (1.14) in the two breeds (Table 1). Season also had a significant effect (at least P < 0.01) on all reproductive traits except litter size

(Table 1) with ewes that were mated in the dry season and lambing in the wet season having a higher reproductive performance than those ewes that were mated in the wet season and which lambed in the dry season (0.76 versus 0.53, respectively, for weaning rate). There was a significant breed–season interaction for all reproductive traits except litter size (Table 1) due to the magnitude of the breed differences varying in the different seasons. In terms of the overall reproductive performance as assessed by weaning rate, this interaction (P < 0.05) was due to the Menz ewes showing their superiority in reproductive performance over the Horro ewes more clearly when lambing in the wet season (0.85 versus 0.67, respectively) than when lambing in the dry season (0.59 versus 0.47, respectively). Although year had a significant effect on all the component reproductive traits (i.e. fertility rate, litter size and lamb survival) these effects tended to cancel out so that the effect of year on weaning rate (P < 0.05)

Table 1 Least squares means by breed, season, the breed–season interaction and year of mating for ewe mating weight (EWT, kg), fertility (EL/EM), lambing rate (LB/EM), weaning rate (LW/EM), litter size (LB/EL) and lamb survival (LW/LB)a Effect

Number of EM

EWT

EL/EM

LB/EM

LW/EM

Number of EL

LB/EL

LW/LB

Total/mean R.S.D.b

4890

25.1 3.5

0.71 0.40

0.84 0.58

0.65 0.55

3670

1.14 0.37

0.75 0.36

Breed Menz Horro

∗∗∗

∗∗∗

∗∗

∗∗∗

2516 2374

23.6 26.5

0.76 0.67

0.87 0.81

0.73 0.57

2004 1666

NSc 1.13 1.14

0.83 0.68

Mating (lambing) season Wet (dry) Dry (wet)

∗∗∗

∗∗∗

∗∗

∗∗∗

2360 2530

24.6 25.6

0.69 0.74

0.79 0.82

0.53 0.76

1667 1993

NSc 1.13 1.14

0.67 0.84

∗∗∗

∗∗∗

∗∗

∗

0.70 0.81 0.67 0.67

0.80 0.94 0.79 0.83

0.59 0.85 0.47 0.67

905 1099 772 894

NSc 1.12 1.14 1.14 1.14 ∗

∗∗∗

549 668 752 866 835

1.16 1.16 1.10 1.12 1.14

0.95 0.75 0.71 0.69 0.67

Breed–mating season Menz–wet Menz–dry Horro–wet Horro–dry

1225 1291 1135 1239

23.5 23.7 25.6 27.4

Year (1) (2) (3) (4) (5)

∗∗∗

∗∗∗

∗∗∗

∗

998 880 975 1035 1002

23.2 25.4 25.4 26.3 25.1

0.57 0.72 0.75 0.76 0.76

0.67 0.85 0.86 0.91 0.91

0.65 0.65 0.66 0.67 0.60

a

1992–1993 1993–1994 1994–1995 1995–1996 1996–1997

EM, ewes mated; EL, ewes lambing; LB, lambs born; LW, lambs weaned. R.S.D., residual standard deviation. c NS, not significant (P > 0.05). ∗ P < 0.05. ∗∗ P < 0.01. ∗∗∗ P < 0.001. b

∗∗∗

∗∗∗

∗∗∗

0.78 0.89 0.57 0.78

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was relatively small with a range among years from 0.60 to 0.67. Ewe age was significant (P < 0.001) for all reproductive traits with the older ewes having a higher reproductive rate than the younger ewes. A number of other interactions were significant for the reproductive traits which are not tabulated. Year–season was significant (P < 0.001) for all reproductive traits except fertility. In all cases, this was due to the magnitude of the difference in performance between the wet and dry lambing seasons varying among years, but in all years there was a superior reproductive performance for ewes lambing in the wet season compared to those lambing in the dry season. Breed–ewe age was significant (P < 0.001) for fertility, lamb survival and weaning rate and was due to the particularly poor performance of young Horro ewes lambing at 2 and 3 years of age. For example, for weaning rate the average performance of 2- and 3-year-old ewes was 0.59 for Menz ewes compared to 0.30 for Horro ewes while the weaning rate of 5 years and older ewes was 0.85 and 0.80, respectively. Year–breed was significant just for litter size (P < 0.01) and lamb survival (P < 0.001). For litter size, the breed effect was not consistent among years. For example, in the first year Menz ewes had a significantly higher litter size than Horro ewes (1.24 versus 1.14, respectively) while in year 5, Horro ewes had a significantly higher litter size than Menz ewes (1.21 versus 1.13, respectively) and in the other 3 years there was no significant difference between the breeds in litter size. Menz ewes had a significantly higher lamb survival than Horro ewes in all years except in year 1, where there was no significant difference (0.96 versus 0.94, respectively). The effects of breed, season, breed–season and year were all significant (P < 0.001) for ewe mating weight (Table 1). The Menz ewes were consistently heavier than the Horro ewes in both the wet and dry seasons, but season had no significant effect on ewe mating weight in Menz ewes while the Horro ewes were significantly heavier (P < 0.001) when mated in the dry season than in the wet season (27.4 versus 25.6, respectively). The other interactions which were significant (P < 0.001) for ewe mating weight were breed–ewe age, year–ewe age, ewe age–season, year–season and year–breed. The year–season interaction was due to ewes being heavier when mated in the dry season compared to the wet season in years

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3–5, with the reverse being true in year 2 and no significant difference in year 1. The year–breed interaction was due to the Horro ewes being significantly heavier than the Menz ewes in all years, except year 1. The breed–ewe age interaction was due to the Horro ewes showing their superior weight more clearly in the older ewes than in the younger ewes. 3.2. Response to synchronisation The flock mating and reproductive performance for those ewes synchronised is shown in Table 2. Out of the 2909 ewes treated with FGA intravaginal sponges, 83% came in oestrus. Out of the ewes that came in heat, 81% did so within 5 days of sponge removal and were considered ‘successfully synchronised’. The rest came in heat after the 5-day period with two smaller peaks at 8 and 18 days irrespective of the season of hormonal treatment. The proportion of ewes synchronised was higher when ewes were treated with FGA sponges so that they lambed in the wet (88%) compared to that for the dry (78%) season. More Menz sheep responded to treatment than Horro (87% versus 80%, respectively) and multiparous ewes responded better than maiden animals mated for the first time (≥83% versus 80%, respectively). The breed–season interaction for the percentage of ewes synchronised was significant (P < 0.001). The difference for ewes synchronised to lamb in the wet season relative to the dry season was larger for Horro (89% versus 70%, respectively) than Menz (87% versus 86%, respectively). The average interval from FGA intravaginal sponge removal to conception was 6.6 days. This interval was shorter when ewes were treated in the wet than dry season. The interval was longest for maiden animals but was not influenced by breed or pre-mating ewe body weight. Over the 3 years, the mating period lasted from 25 to 42 days. Gestation length was estimated only for ewes which conceived at the first service. The mean gestation length was 151 ± 0.1 days and it was not significantly affected by any of the factors in the model. Over the 3 years, the lambing period lasted 30–52 days with a difference of only 5–10 days between the length of mating and lambing periods. Cumulatively, 7.1, 25.6, 53.3 and 97.3% of the ewes had lambed within 10, 20, 30 and 40 days from the start of lambing, indicating a reasonably ‘tight’ lambing season.

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Table 2 Effect of breed, season and year on the percentage of ewes successfully synchronised and the subsequent fertility rate and weaning rate for these ewesa Effect

Number of treated ewes

Ewes successfully synchronised (%)

Fertility (EL/EM)

Weaning rate (LW/EM)

Total/mean R.S.D.b

2909

83.3 35.0

0.69 0.43

0.62 0.54

Breed Menz Horro

∗∗∗

∗∗∗

∗∗∗

1477 1432

86.8 79.8

0.73 0.65

0.70 0.53

Mating (lambing) season Wet (dry) Dry (wet)

∗∗∗

∗∗∗

∗∗∗

1533 1376

78.4 88.2

0.66 0.72

0.55 0.69

Year (1) 1992–1993 (2) 1993–1994 (3) 1994–1995

∗∗∗

∗∗∗

1024 894 991

85.2 90.0 74.7

0.58 0.73 0.76

NSc 0.63 0.61 0.62

a

EM, ewes mated; EL, ewes lambing; LW, lambs weaned. R.S.D., residual standard deviation. c NS, not significant (P > 0.05). ∗∗∗ P < 0.001. b

Fertility rate and weaning rate for the synchronised ewes averaged 0.69 and 0.62, respectively (Table 2), which was not significantly different from the values for the total data set (Table 1). Similarly, the effects of breed, season and year on fertility rate and weaning rate for the synchronised ewes was quite similar to those found for the total data set (Table 1). The fertility rate of sires mated to the synchronised ewes and used for at least a year (i.e. in both the wet and dry season matings) ranged from 51 to 91% for Menz and from 42 to 91% for Horro. Of the ewes that lambed, 73, 17 and 10%, respectively conceived at the first, second and third or subsequent oestrus. Most (86%) of the pregnancies were from ewes which were successfully synchronised and these animals also accounted for 64 and 15% of the pregnancies to first and second services, respectively. There was a decline in the rate of conception to first service after the first 5 days until 19 days later, the time of the next expected natural oestrus. The number of services per pregnancy was 1.5 and was not significantly affected by breed, season or ewe age. Ewes which returned to service had a mean oestrous cycle length of 14.8 days but this inter-service interval was widely variable. Many of the oestrous cycles (28%) were of ‘short’ duration from 5 to 13 days, 48% were of the ‘normal’ length of 14–20 days,

5% were of ‘long’ duration (20–26 days), 10% were ‘multiples’ of the normal range (28–40 days) while 9% were regarded as ‘extended’ (≥41 days). Most (70%) of the ‘short’ oestrous cycles were recorded after the induced first oestrus and their frequency declined to 22 and 8% after the second and third service. Table 3 presents the distribution, by breed, season, parity and ewe pre-mating weight, of ewes which failed to come in oestrus after intravaginal sponges removal and those ewes which failed to get pregnant out of those treated or mated, respectively. The majority of animals that failed to come into oestrus after FGA sponge treatment were maiden ewes or animals weighing <20 kg at the time of hormonal treatment. There was an almost equal seasonal distribution of animals failing to respond to progestogen treatment, but proportionately more Menz ewes failed to respond after wet season treatment while the opposite was true for the Horro breed. 3.3. Overall productivity of Menz and Horro sheep The productivity of Menz and Horro ewes lambing in the wet and dry seasons is shown in Table 4. The lamb survival from weaning to 12 months of age and the yearling live weights are least squares means

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Table 3 Percentage distribution of ewes by failure to respond or conceive after sponge treatment as affected by breed, season, parity and pre-mating body weight Effect

Ewes failing to show oestrus after FGA intravaginal sponge removal

Ewes mated that failed to conceive

Number of ewes treated

Number of ewes mated

Percent failing to respond

Percent failing to conceive

Breed–mating season Menz–dry 760 Menz–wet 717 Horro–dry 773 Horro–wet 659

6.2 15.1 17.1 13.8

713 609 633 568

11.8 20.9 22.4 28.3

Parity 1 2 3+

1527 829 553

18.8 7.7 4.9

1234 763 526

34.1 10.5 2.5

Weight at mating (kg) ≤20 661 20–27 1358 ≥27 687

32.7 6.9 6.3

493 1061 807

32.9 16.0 17.3

Mating season Dry Wet

14.5 11.7

1349 1174

21.3 19.3

1376 1533

extracted from Rege et al. (in press). Overall flock productivity is expressed in terms of potential offtake from flocks of Menz or Horro ewes lambing in either the wet or dry seasons. To calculate the offtake, flocks of 100 ewes were assumed with a 20% female replacement rate. All male progeny and non-replacement females alive at 1 year of age then make up the potential offtake, i.e. animals available for sale. Both in terms of number of yearling sheep for sale and total live weight for sale the offtake of a flock of Menz sheep in

this environment was approximately three-fold greater than a flock of Horro sheep when they lambed in the wet season and approximately two-fold greater when they lambed in the dry season. The very low weaning rates of Horro ewes and the low survival rates of Horro lambs from weaning to 12 months of age meant that this breed was non-sustainable in this environment with only 12 yearling ewes being alive at 1 year of age compared with the required 20 replacement ewes. Season had no effect on offtake in Horro ewes, but

Table 4 Productivity of Menz and Horro ewes lambing in the wet and dry seasons Trait Ewe weight (EWT, kg) Weaning rate (%) Lamb survival (3–12 months; %) Yearling live weight (kg) Offtake (1 year)a Number of sheep Total live weight (kg)

Menz–wet 23.7 85 69 16.7 39 651

Menz–dry 23.5 59 79 16.5 27 446

Horro–wet 27.4 67 35 18.6 12 223

Horro–dry 25.6 47 51 18.6 12 223

a Offtake based on a 100 ewes flock of each breed mated in each season with a 20% female replacement rate and all male progeny and non-replacement females alive at 1 year of age making up the potential offtake.

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the Menz ewes lambing in the wet season had about a 1.5-fold greater offtake than Menz ewes lambing in the dry season. In addition to offtake there are a number of other indexes which have been developed to assess overall sheep productivity (Gatenby, 1986) and many of these express productivity per unit of body weight of ewes in the flock in an attempt to account for the higher feed costs of heavier animals. We investigated this by analysing weaning rate with all the significant fixed effects and interactions identified in Table 1, but also including ewe mating live weight as a linear covariate in the analysis. Mating weight was a significant effect in the model (P < 0.001) as were the interactions of mating weight–breed (P < 0.05) and mating weight–ewe age (P < 0.01). The overall regression of weaning rate on mating weight was 0.028±0.003 (i.e. weaning rate increased by 2.8% for each 1 kg increase in mating weight), but this relationship was stronger in Menz ewes (0.033±0.004) than Horro ewes (0.023±0.003) and stronger in 2-year-old ewes (0.052 ± 0.009) than older ewes (0.022 ± 0.003). When weaning rate is adjusted for ewe mating weight using these regression coefficients the smaller Menz ewes now show a greater advantage in weaning rate (P < 0.001) over the Horro ewes (0.85 versus 0.59, respectively) compared to when weaning rate is not adjusted for ewe mating weight (0.73 versus 0.57, respectively).

4. Discussion Reproduction is particularly important in indigenous hair sheep which dominate the tropics because they are largely kept for meat production. As the product of fertility and prolificacy, lambing rate is a good measure of ewe reproduction (Foote, 1991) and the present values fall within the 65–200% range reported for hair sheep (Foote, 1991; Abassa, 1995). Mating of ewes in the dry season led to higher fertility than those mated in the wet season, probably because ewes came from the previous wet season with enough body reserves. Lambing in the subsequent wet season further enhanced their weaning rates and productivity due to better grazing during lactation. The positive effects of improved nutrition, weight and condition on ovulation rate, litter size, fertility and lamb survival are well documented (Khalaf et al., 1979; Robinson, 1990).

The mean litter size in this study was in the 1.01–1.60 range for tropical sheep (Foote, 1991; Zere, 1992; Tekelye et al., 1993; Abassa, 1995) but was not significantly affected by either breed or season. The overall weaning rate of 0.75 compared well with the range of 0.55–0.90 reported in other investigations (Hohenboken et al., 1976; Iman and Slyter, 1996). The lower weaning rate in the Horro breed resulted from the negative impact of lamb mortality from birth to weaning which was about twice as high in the Horro as in the Menz breed (32% versus 17%, respectively). The poor performance of Horro ewes despite reasonable initial reproduction probably reflected the breed’s lack of proper adaptation to the local environment. The Horro was introduced to the station from an altitude of about 2000 m relative to the altitude of 2800 m at the Debre Berhan Station. Poor adaptation of the Horro sheep could have been due to susceptibility to various diseases (Mukasa-Mugerwa et al., 2000) and the lack of adequate nutrition to meet ewe lactation needs and progeny growth requirements. In the first 3 years of the study, most of the Horro ewes evaluated were those that had been purchased and brought to the Debre Berhan Station. These ewes might be expected to be less adapted to the station conditions than those that were born in the experiment and entered the breeding flock as replacement ewes and these were most of the Horro ewes evaluated in the last 2 years of the study. The mean weaning rate for the last 2 years of the study (0.63) was very similar to that in the first 3 years (0.65) and the breed–year interaction was not significant (P > 0.05). This suggests that if indeed the Horro breed can eventually adapt to this environment it is going to take more than one generation. It also suggests that a hair breed like the Horro might be an entirely inappropriate breed for this relatively cool to cold high altitude environment. The two–three-fold higher offtake of the Menz sheep compared to the Horro sheep is very similar to the three-fold higher offtake of Red Maasai sheep compared to Dorper sheep in sub-humid coastal Kenya (Baker et al., 1999). The higher offtake of the Red Maasai sheep was largely due to their higher resistance to gastrointestinal nematode parasites. However, there was no evidence from the present study that Menz sheep are more resistant to gastrointestinal parasites than Horro sheep (Tembely et al., 1998; Rege et al., in press). It is also important to recognise the potential

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importance of a breed–environment interaction. For example, the poor performance of the Dorper sheep in the hot humid conditions of the Kenyan coast was associated mainly with low reproductive rates, very high lamb mortality rates and low growth rates. When Dorper sheep were compared with Red Maasai sheep in a semi-arid environment their reproductive rate, mortality rate and growth rate improved dramatically (Baker et al., 2002). While there was no breed–environment interaction for resistance to endoparasites (i.e. the Red Maasai was more resistant and resilient than the Dorper in both the humid and semi-arid environments), there was for the production traits. The net result was that the Dorper sheep had a slightly higher offtake than the Red Maasai sheep in the semi-arid environment (Baker et al., 2002). We are not aware of any comparison of Menz and Horro sheep in a lower altitude environment in Ethiopia in which we could expect the Horro sheep to perform better because they would be better adapted. However, there is performance data for Horro sheep which originated from a 19-year study (1979–1997) at the Bako Agricultural Research Centre in the region of Ethiopia from which Horro sheep originated (Solomon and Duguma, 2002; Solomon et al., 2002). In this study, the reproductive performance of Horro ewes, based on 2250 mating records, was higher than that found in the present study, e.g. an average conception rate of 0.77 compared to 0.67 and an average litter size of 1.34 compared to 1.14, respectively. In addition, lamb mortality rates in the Bako study were lower than those in the present study, e.g. average pre-weaning mortality of 0.20 compared to 0.25 and average mortality from birth to 1 year of age of 0.45 compared to 0.65, respectively. The synchronisation of oestrus in tropical sheep through hormone treatment or the ‘ram effect’ has been reported previously (Greyling et al., 1988; Mukasa-Mugerwa et al., 1994b), but subsequent ewe reproduction and productivity under field conditions is sparsely documented (Van der Westhuysen et al., 1981). The proportion of ewes that were successfully synchronised within 5 days of FGA intravaginal sponge removal (83%) was low relative to 96% reported by Spitzer and Carpenter (1981) in Rambouillet ewes but the 90% achieved in 1993–1994 was considered as encouraging. The increase from 78% in the dry to 88% in the wet season, and interactions which showed better performance after ewes of both

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breeds were treated with sponges in the wet season, indicated possible increased ovarian inactivity during the dry season. The fact that a larger proportion of non-responding females were maiden also indicated that pubertal oestrus had not coincided with sexual maturity in all the pubertal ewe lambs, or that effects of environmental nature were more adverse on the younger ewes. It is, therefore, possible that ewe lambs and older animals treated in the dry season, might benefit from PMSG administration to enhance ovulation (Mutiga and Mukasa-Mugerwa, 1992). Time to oestrus after FGA intravaginal sponge removal agreed with studies in ewes given progestogens (Oyedipe et al., 1989; Mutiga and Mukasa-Mugerwa, 1992). However, while the fertility rate of 69% (Table 2) was in the range of 48–100% reported elsewhere (Spitzer and Carpenter, 1981; Zere, 1992; Iman and Slyter, 1996), it was low for tropical sheep which breed year-round. Low levels of fertility reflect the depressing effects of low levels of nutrition, management and ram fertility. The conception rate of 73% at first service was similar to the 78% recorded in naturally-cycling station ewes (Mukasa-Mugerwa and Lahlou-Kassi, 1995). The mean cycle length of 15 days among ewes that returned to service was at the low end of the 14–20 days normal range (Gatenby, 1986) perhaps because of the higher level (28%) of ‘short’ cycles than in non-induced ewes (Mukasa-Mugerwa et al., 1990). Because most ‘short’ cycles were observed after the induced oestrus, this may have been due to conception failure from induced corpus luteum (CL) incompetence followed by early CL regression. Oestrus synchronisation was used in the first 3 years of this study to ensure that the wet and dry lambing seasons were achieved. It was not used in the last 2 years of the study because by then most of the Menz and Horro ewes being evaluated for reproductive performance were those born in the experiment and the design of the experiment required that these ewes be evaluated under natural mating conditions. However, it is interesting to note that all the reproductive parameters assessed were very similar in the non-synchronised ewes compared to the synchronised ewes including the proportion of ewes lambing after a 30–40-day mating period. In retrospect synchronisation was not essential in this study for ensuring that the ewes were successfully bred over a short mating

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period, but it did offer a unique opportunity to successfully test this procedure in indigenous tropical sheep.

5. Conclusions Menz sheep had a higher reproductive rate and overall flock productivity than Horro sheep in this high altitude tropical environment of Ethiopia. Oestrous synchronisation was successfully used in this study resulting in similar fertility and weaning rates to non-synchronised ewes. However, wide implementation of the oestrous synchronisation technique would require resources which are not readily available to most traditional smallholder farmers. The common management practice adopted by most smallholder farmers in the tropics is to continuously mate their ewes and it can be shown that continuously-mated ewes with a lambing interval of about 8–9 months would out-perform animals managed to lamb seasonally (Gatenby, 1986). However, even with continuously-mated ewes it is possible to mate a higher proportion to lamb in more favourable seasons and in this study this was the wet lambing season.

Acknowledgements The participation of all staff at the ILRI Debre Berhan Station and the Animal Genetic Resources Project in Addis Ababa is gratefully recognised for support in animal management, data collection, computer entry, verification and analysis. This paper is ILRI Publication no. 200159.

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