Reproductive performance and productivity of Menz sheep in the Ethiopian highlands

Small Ruminant Research Small Ruminant Research 17 (1995) 167-177

Reproductive performance and productivity of Menz sheep in the Ethiopian highlands E. Mukasa-Mugerwa”, Animal Reproduction

A. Lahlou-Kassi

and Health Section, International Livestock Centrefor

Africa (ILCA), P.O. Box 5689, Addis Ababa, Ethiopia

Accepted 20 August 1994

Abstract Ewe lambs attain puberty at 10 months and 16.9 f 0.1 kg (56% of mature weight) and first lamb at 15 months. Postpartum anoestrus is 76 days, lambing interval is 8.4 months and 65% of ewes lamb three times in 2 years with a gestation period of 149 days. Ewes cycle year-round but oestrus activity declines from June to September due to increased silent ovulations. Fertilisation rates are high ( >90%) but lambing rate is 72%, suggesting moderate embryonic mortality. Conception rates to first, second and third oestrus postpartum are 78%, 18% and 4%, respectively. Litter size (prolificacy) is 1.12 but can be increased by lo40% through premating ewe nutrition management or treatment with gonadotropins. Ewe oestrus activity can be synchronised by ram introduction after a period of isolation and with progesterone sponges or prostaglandin F,, therapy. Lambs average 2.0 f 0.1 kg at birth but preweaning mortality ( 15%) and slow growth, resulting in 8.6 kg weaning weight, still limit overall productivity index to 11 .O& 0.87 kg per ewe per year, and the annual reproductive rate to 1.4 lambs per ewe. In the traditiqnal management system, where most animals are raised, further studies are required into better nutrition and control of diseases (endoparasitism) in young animals to increase reproductive efficiency, flock productivity and number of animals for finishing and slaughter. This could serve as a model for interventions in other ruminants in the agroecological zone. Kqwords:

Reproduction management; Sheep; Tropics; Growth; Nutrition

1. introduction Through their contribution to food, fibre and manure production, Ethiopian sheep, estimated at 23 million (Tekelye et al., 1993)) make significant contributions to the country’s mixed crop-livestock agriculture. About 75% of the animals are raised on small-scale peasant farms in the highlands (areas B 1500 m above sea level), but profitability from sheep farming is limited by low performance in market weight, overall reproductive efficiency, wool yield and high mortality. This arises from seasonal fluctuations in feed resources, * Corresponding author. 0921~4488/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved sSDI0921-4488(95)00663-X

poor management and diseases, especially endoparasitism and pneumopathy (Mosi and Butterworth, 1985; Njau et al., 1988; Tekelye et al., 1993). In terms of food production, sheep productivity represents the combined effect of reproductive efficiency (including young and adult mortality), growth rate, yield and quality of desired final products. Therefore, high reproductive output is a basic requirement of efficient systems of sheep production. This paper summarizes data on reproductive physiology and performance, how this could be manipulated in Menz sheep, and highlights levels of production and areas in need of more research.

168

E. Mukasa-Mugenva, A Luhlou-Kassi /Stall

2. Menz sheep and their management

Wide variations in Ethiopia’s climate and terrain have made it a good reservoir of sheep germplasm. The Menz, also called Shoa or Legagora, and numbering over 1.5 million, is one of the primary Ethiopian sheep breeds. The breed is concentrated in the central highlands between 2500 and 3000 m above sea level, 3940”E longitude and lO-11”N latitude. The sheep are raised mainly on small peasant farms in flocks of 11 (range l-32) animals. The Menz is a fat-tailed hair breed of medium size; ewes range from 25 to 35 kg body weight (SW), rams 35 to 45 kg. Average shoulder height is 64 f 1.O cm in rams and 58 + 0.9 cm in ewes. Animals have a semi-open fleece of conical locks, coarse hair that may be 15-20 cm long and a wooly undercoat of 5-8 cm especially in the colder highlands (Galal, 1983). Animals are sometimes sheared, the annual fleece yield being 0.5 kg. The common color is brown or black, rarely white. Ewes have short vestigial horns but which become long and twisted in rams. Sheep are sold for slaughter all year but sales and prices peak in festival periods. Meat and fibre of Menz sheep are in high demand. Sheep represent a source of ready cash for their owners and a potential for export to Middle East markets. Sheep contribute about 45% of the cash income of highland smallholder farmers. Data were collected from 1986 to 1992 in studies at the ILCA Debre Birhan station, 120 km north-east of Addis Ababa at 2780 m above sea level. Annual station rainfall for 1979-1993 averaged 995 mm; 75% comes in the long rainy season from June to September, the rest as short showers from February to May with a dry spell from October to January. Mean minimum and maximum temperatures were 5.3” and 2 1.4”C, respectively. Relative humidity averages 68.2% and daylength variation between summer and winter solstice is only 1 h. During the day animals grazed natural pastures dominated by Andropogon, Festuca and Pennisetum spp. grasses mixed with native Trifolium semense legume. They were enclosed overnight in clean sheltered pens with free access to hay, water and mineral licks. Animals were fed concentrate supplementation depending on the demands of specific experiments.

Ruminant Research I7 (1995) 167-I 77

3. Aspects of reproductive

physiology

3.1. Puberty Menz ewe lambs attain puberty (first oestrus) at 350+ 12 days, within the range of 212 and 615 days reported for tropical and subtropical sheep (Gatenby, 1986; Foote, 1991). Puberty is reached at 16.9 kO.1 kg mean weight or 56% of mature BW; considered the critical body size for pubertal onset in the breed. Progesterone assays revealed, however, that 21% of first oestruses are anovulatory (Mukasa-Mugerwa and Mutiga, 1993). From 117 weaners raised on four levels of nutrition, half of which were also drenched for endoparasites, puberty onset was found to be enhanced by weaning weight averaging 8.6 f 0.1 kg. Weaning weight was itself correlated to birth weight, and influenced by pas; weaning level of nutrition through better BW gains (r= -0.82, P 0.05). But among possible two-way interactions, drenching X season-of-birth proved important for postweaning BW gain (P < 0.01) and approached significance on weight at puberty (P = 0.084). Specifically, drenching improved the growth of lambs born during the short rains (43.8 vs. 33.8 g day ~ ‘, P = 0.032) but not that of lambs born during the heavy rains (26.0 vs. 38.5 g day-‘, P=O.44) and was non-significant for lambs born during the dry period (41.3 vs. 44.9 g day- ‘, P = 0.342). Grazing alone also resulted in only 75% of lambs reaching puberty in the first year of life, lambs likely to have reached the ‘critical weight’ of 16.9 kg. This is important because tropical sheep can cycle year-round. In subtropical and temperate breeds, on the other hand, season of birth can limit puberty

E. Mukasa-Mugenva, A. L.ahlou-Kassi/Small Ruminant Research 17 (1995) 167-177

169

Table I Least squares means ( f standard error) of the effect of season of birth, litter size, nutrition level and drenching against endoparasites onset in Menz ewe lambs Variable

Unadjusted

Observed

mean

117

Weaning weight (kg)

Postweaning daily gain

Pubertal

(g day-‘)

Age (days)

8.6*0.1

37.6 f 2.0

350f

12

on puberty

Weight

Heart girth

(kg)

(cm)

Condition scorea

16.9fO.l

62.050.1

2.2*0.1

Birth season Heavy rain Dry season Short rain

33 39 45

9.3 f 0.4 8.6 f 0.5 7.3 f 0.5

32.3 +4.1 38.8 k2.9 43.1 k2.8

359 f 21 360 * 19 358*18

16.4&0.9 17.5 kO.6 18.4kO.6

61.5f 1.3 62.3f0.8 63.1 f0.9

2.2rtO.l 2.2rtO.l 2.3kO.l

Litter Single Twins

90 21

9.0 + 0.4 7.8 f 0.5

33.6k2.0 42.6f3.2

361514 357f22

16.7 f 0.6 18.3*0.7

61.6*0.7 63.0 f 1.O

2.2fO.l 2.3rtO.l

29 29 30 29

24.7 k 4.7 32.153.4 44.1 rt2.7 51.4*2.9

454*31 383 +23 301 f 18 299 k 19

16.7kO.9 17.2*0.6 17.5 f0.5 18.7kO.6

60.8 62.5 62.4 63.7

f f f f

1.850.1 2.2*0.1 2.450.1 2.5 *O.l

58 59

38.4 k4.3 44.1 f3.6

375 * 19 335 * 19

17.6f0.5 17.8 *0.5

62.5 f0.8 62.4~tO.7

Postweaning Poor Low Medium High

nutrition

Drenching for endoparasites Drenched Undrenched “Condition score: 1 = emaciated,

5 = obese. Source: Mukasa-Mugerwa

onset (Lahlou-Kassi et al., 1989) in that failure to reach a given weight by the first breeding season (short-day photoperiodic stimulus) results in puberty being postponed until the next breeding season. Growth of Menz lambs on medium and high nutrition was not different (P > 0. lo), suggesting that, at high growth rates, there is a threshold beyond which increased nutrition does not appear to enhance puberty. 3.2. Postpartum

anoestrus interval

Interval from lambing to conception largely determines lambing interval and consists of two major phases: the postpartum anoestrus interval (PPI) from lambing to first oestrus, and the service period from the onset of cycles to conception. PPI is influenced by nutrition, lactation, endocrine status, uterine involution and lambing season (Wettemann, 1980), probably through an impaired or inactivated hypothalamohypophyseal system (Gonzalez-Reyna et al., 1987). Postpartum interval lasts 76.2 + 4.6 days (range 13167) but simultaneous progesterone assays showed

1.5 0.9 0.8 0.8

2.3~bO.l 2.2rtO.l

et al. ( 199 1)

that the oestrus recorded 13 days postpartum was false. Earliest ovulation was at 26 days, in accord with previous submissions that true oestrus occurs after complete uterine involution (Novoa, 1984). Two-thirds (67%) of the ewes experience silent ovulation 66.4 + 10.5 days postpartum, and 30% of them experience it twice before first observed oestrus (MukasaMugerwa and Zere, 1991). Initial progesterone rises (silent ovulation) have been reported (Bortha and Morgenthal, 1980) even with higher frequency than was observed by Mukasa-Mugerwa and Zere ( 1991) . For example, among Mexican Pelibuey ewes, Gonzalez-Reyna et al. ( 1987) noted this frequency to be 93100%. These initial progesterone rises are suspected to prime the neuroendocrine system for subsequent behavioral oestrus and continued cyclicity (Wettemann, 1980; Novoa, 1984). However, as our data from Menz sheep showed, the increases may not be an absolute requirement as 37% of ewes cycled and conceived without manifesting them. Postpartum interval was shorter (P 2 0.10) following delivery of twins or male lambs, lambing during

170

E. Mukasa-Mugerwa, A. Lahlou-Kassi/Small Ruminant Research I7 (199.5) 167-177

the rains or after lamb mortality. PPI was also unaffected by birth or weaning weight, dam parity and the correlation between dam weight or condition score at lambing to PPI was only - 0.05 and 0.23, respectively. Mukasa-Mugerwa and Zere ( 1991) found that 65% of the ewes conceived within 90 days of lambing (before lamb weaning) and went on to achieve an 8 month lambing interval, suggesting that postpartum anoestrus was little affected by suckling. Lamb mortality, which eliminates the suckling effect, did not significantly reduce the anoestrous interval of ewes whose lambs died soon after birth (69.3 vs. 77.4 days, P > 0.lo), contrary to results from temperate breeds (Gordon et al., 1987). Genotype and lactation could be major contributing factors to this observation, but the role of prolactin, oxytocin and endogenous opioids on gonadotropin release and the possible influence of placenta and ovarian steroids in postpartum Menz sheep have not been studied to formulate hormone treatment packages to increase the number of ewes conceiving before weaning. In addition, the role of body reserves in regulating ovulation and pregnancy maintenance needs further study. Do ewe lambs start to cycle only after accumulating a level of reserves relative to lean body mass? Also, will mature ewes cease to cycle, delay pregnancy or even abort until they have accumulated enough reserves in the face of nutrition shortages? 3.3. Oestrus q&city

and seasonality

Ethiopian sheep graze in flocks of mixed sexes allowing for adequate exposure of ewes in oestrus to fertile rams. For 48 ewes, half of which were maintained on hay alone or hay plus supplement, bi-weekly progesterone assays revealed that vasectomised ram presence in half the animals did not influence their feed intake, weight gain or oestrus activity. The percentage of ewes showing oestrus at least once a month was very high (95%). Ewes came into oestrus 18-23 times a year with a mean of 21 heats and an oestrus cycle length of 17.9 f 8.7 days within the range of 16-19 days for tropical sheep (Gaillard, 1979; Yenikoye et al., 1981; Foote, 1991) and temperate ewes in the breeding season (Quirke, 1978). Twenty-two per cent of cycles were short ( < 13 days), 56% normal ( 14-19 days), 11% long (20-26 days), 8% silent or missed (27-40 days) and 3% represented anoestrus ( > 40 days) with

,”

,

DJFMAMJJASONOJ

-

Month Ewee.~atlng

Ewmerua

Rell

Fig. 1. Seasonal variation in estrous and ovarian activities in Menz sheep (n =48 ewes).

no major difference due to nutrition level. A period of marked reduction in sexual activity was observed from June to September (the wet season; Fig. 1) at both nutrition levels. Only 79% of ewes cycled in August, the number of heats per ewe per month declining to 1.3 in the wet season in contrast to 1.9 heats for the rest of the year (Mukasa-Mugerwa et al., 1993). The lack of seasonality in oestrus behaviour of tropical sheep has been reported (Mittal and Ghosh, 1980) in which seasonal variation has often been associated with changes in food supply (Molokwu and Umunna, 1980). Progesterone assays in the present studies showed that failure to show oestrus resulted from increased silent ovulations. The pituitary-hypothalamus-gonadal inter-relationships need further investigation to appreciate better the control and response mechanisms regulating ovarian function in Menz sheep and the role of factors like nutrition, endoparasitism and climatic stress. The effect of reduced sexual activity from June to September on breeding and flock production also has not been assessed. It is vital to ascertain if seasonal decline in ewe oestrus activity is associated with reduced ram libido and fertility, which could also affect flock reproduction efficiency.

4. Aspects of reproductive

performance

4.1. Age atfirst lambing Puberty affects production through age at first lambing, and, as Ethiopian sheep are raised for meat, initial growth, puberty onset and the total number of lambs ewes produced in a lifetime are important traits. Age at

E. M&ma-Mugewa,

A. Lahlou-Kassi /Small Ruminant Research I7 (199.5) 167-I 77

first lambing was 523 f 13 days ( Mukasa-Mugerwa et al., 1991), within the wide range of 431-713 days for African sheep (Wilson, 1989). Age at first lambing varied with weaning season and postweaning nutrition (P < 0.05), factors which also affected puberty onset resulting in a strong relationship between the two characters (r = 0.90, P < 0.001). At first lambing, lamb mortality was higher (33%) in ewe lambs that only grazed, in contrast to supplemented animals (7%)) stressing the double advantage of adequate early nutrition to enhance pubertal onset and to minimise perinatal losses in the primiparous ewes. There is a need to establish levels of supplementation for optimum pubertal development in Menz sheep so as to increase the percentage of ewe lambs that can be mated the first year. This would further help to verify the extent to which rapid prepubertal weight gain, which has been found to impair mammary gland development and milk production in temperate lambs (McCann et al., 1989), might influence the lifetime productivity of Menz sheep. In terms of sheep meat production, and given the prospect of market prices that may encourage commercial sheep reproduction, it may be useful to evaluate the effects of manipulating growth and body composition with agents like growth hormone to improve feed conversion and protein deposition in finishing animals. 4.2. Conception andfertility

rates

By fitting rams with mating harnesses and recording service dates that were also matched to subsequent lambing in grazing animals, conception rate to first, second and third mating was estimated at 69%, 29% and 2%, respectively, in pubertal ewe lambs. This increased to 78%, 18% and 4% in mature ewes. In the young group, first service conception rate was just 59% for animals on poor nutrition relative to 68-76% for those receiving some supplement, underlining again the need for adequate nutrition on reproduction. First service conception rates of 65-70% are regarded as acceptable under proper management. The above data further indicate that, when breeding activity of Menz sheep is controlled, around 96% of ewes can be expected to conceive in 40 days. Blood samples taken 17-18 days after mating confirmed that the fertilisation rate in Menz sheep is high ( >90%) and in the range of 80-95% for temperate breeds (Hancock, 1962). However, the lambing (fertility) rate was

171

72%, suggesting moderate embryo mortality. Studies with Pelibuey sheep in Mexico showed that fertility rates were higher in the rainy season (94%) than in the dry (80%) season, because of a higher incidence of unfertilised ova in the dry season associated with poor body condition (Gonzalez-Reyna et al., 1991). Better knowledge of embryo-uterine environment-ovarian interactions in Menz sheep is needed to help improve the management of embryo, foetal and newborn losses, and increase the impact technologies such as artificial insemination and embryo transfer can have on genetic improvement and reducing the cost of research by the production of identical animals. Good fertility rates are important since, aside from reduced flock productivity, low fertility involves the nutritionally expensive maintenance of unproductive ewes and lessens the effectiveness of any selection. 4.3. Gestation length Mean gestation length was as long in primiparous as in mature ewes (149f2 and 149 f 8 days, respectively) within the 147-153 day range reported by others (Gatenby, 1986). There was no significant effect of lamb sex, pregnancy nutrition, mating season or litter size on gestation length (P > 0.10) although twins were carried for 2 days less than singles. About 5% of ewes exhibited an oestrus during pregnancy which was not ovulatory. 4.4. Litter size (prolijcacy) Litter size averages 1.07 in primiparous ewes but rises to 1.12 in adult animals, the rate of single, twin and triplet births being 88.5%, 11.4% and 0.04%, respectively. This performance is in accord with 1.051.15 in other tropical breeds (Wilson, 1975; Wilson and Durkin, 1988). Litter size is largely influenced by ovulation rate and is a major determinant of ewe reproductive efficiency. Ovulation rate is substantially controlled by genotype and improvements could be achieved by selection. Only a few African breeds, like the Moroccan D’Man whose litter size ranges from 1.77 to 2.32 (Lahlou-Kassi et al., 1988), have been selected for prolificacy. In the meantime the parameter is influenced by such environmental factors as age, season and, to a large extent, ewe BW at mating which is itself modulated by nutrition. In the present studies, twinning

172

E. Mukasa-Mugenva, A. Lahlou-Kassi/Small Ruminant Research 17 (1995) 167-177

rate, which largely reflects on ovulation rate, and embryo mortality steadily increased with live weight of ewes at mating from zero at < 15 kg to 3 1% at > 30 kg (Mukasa-Mugerwa et al., 1994a). This agreed with data from ewes presented for slaughter at the Addis Ababa abattoir where mean litter size in utero was 1.24 (Mukasa-Mugerwa and Tekelye, 1988)) but this could have been higher as only visually discernible pregnancies were counted. The disparity between the in utero and present field litter size would therefore suggest prenatal losses of at least 12%. Causes of these losses require further study, including the extent to which uterine capacity might be limiting increased twinning. In addition, the relation between nutritional status to progesterone levels and pregnancy maintenance, and production of uterine proteins including those related to pregnancy recognition deserve further investigation. 4.5. Lambing interval Ewe lambs (n = 115) interacting with fertile rams for two consecutive lambings had a lambing interval of 253 f 5 days (8.4 months; Mukasa-Mugerwa et al., 199413). This was similar to 262 days obtained on-farm in the area by Tekelye et al. ( 1993) and indicates that Menz sheep are therefore capable of three lambings in 2 years except where specific management systems so dictate. Lambing interval varied with previous lambing season (P < 0.05), with ewes lambing in the rains having shorter intervals (252 f 14 and 237 f 15 days) than those lambing in the dry season (28 1 + 16 days), and to a lesser extent by lamb weaning weight (P = 0.06)) possibly an indication of the stress of lactation on postpartum anoestrus. The lambing intervals reported here fell between the estimates of 230-437 days elsewhere in Africa (Wilson, 1989)) estimates whose variation largely appears to reflect the effects of nutrition, lactation and controlled reproduction management on possibilities for prompt reconception after lambing. Inadequate nutrition is a major limiting factor to sheep production in Africa, especially in the arid and semiarid zones. Even when the quantity of vegetation seems adequate, its nutrient content with respect to metabolisable energy, digestible protein and minerals could be poor. More studies are needed into possible responses to nutrition manipulations, throughout the year or strategically only during the dry season, using locally available resources, for animals at different stages of the

reproductive cycle. In this respect, the effect of a particular plane of nutrition or dietary ingredients needs to be better defined. Altered luteal function may be one way these and other variables could affect Menz sheep reproduction efficiency. An intact corpus luteum secreting adequate amounts of progesterone is required for early embryo survival and pregnancy (Asworth et al., 1989). 4.6. Lamb mortality In terms of reproductive wastage, lamb losses represent a serious problem because all investments made for ewes to conceive and maintain pregnancy are wasted. Preweaning lamb mortality averaged 15% over the period of these studies, in the range of lO-30% for tropical flocks (Gatenby, 1986). By raising four groups of ewes on different levels of nutrition, it was observed that, in contrast to supplemented animals, grazing alone increased postweaning mortality by 20-30% (MukasaMugerwa et al., 1991). Losses are particularly high in lambs of f 2.0 kg at birth, most of which die within 24 h of delivery due to starvation/mismothering and exposure (the SME syndrome) (Mukasa-Mugerwa et al., 1994a). Ewe nutrition and gestation weight change influenced lamb birth weight (Fig. 2), which in turn 3.6

T

141

I

-7.6 -6

:

:

:

I

I

:

-26

0

26

6

7.6

10

I

:

12.6 16

:

:

17.6

Ewe liveweight loss or gain (kg) Fig. 2. Effect of ewe liveweight gain during pregnancy on lamb birthweight in Menz sheep (n = 288 ewes) (mean birth weight, 2.0 kg).

E. Mukasa-Mugenva,

A. Lahlou-Kassi/Small

influenced lamb viability as detailed in the next section. Fasciolosis, pneumopathy and intestinal parasites are the serious health problems of Ethiopian highland sheep (Njau et al., 1988; Ngategize et al., 1993) but, although control measures are known for many of the diseases, they are rarely practised or farmers do not implement them correctly. The risk factors associated with these health problems need to be investigated if sheep mortality is to be minimised. 4.7. Pregnant ewe nutrition and lamb survival From 288 ewes that grazed only or, in addition, received extra supplement in the first, second and third trimester or 50 day interval (Mukasa-Mugerwa et al., 1994a), it was found that 6% of ewes lost weight during pregnancy, 31% gained under 3.5 kg (the mean lamb birth and placenta weight) while 63% gained more. Perinatal mortality was 19.8% (62/321), mortality rate declining from 63% below 1 kg birth weight to 3 1.2% between 1 and 2 kg and 1.4% between 2 and 3 kg, average lamb birth weight being 2.0 f 0.1 kg (Fig. 3). Almost 60% of preweaning mortality occurred before 7 days. Single lambs were heavier than twins but litter size had no effect on survival above that on birth weight. There was no advantage to supplement ewes in the first trimester but extra feed in the second and third trimester increased dam weight gain (P < 0.001) ; in particular, dams fed extra in the last 50 days delivered

Ruminant Research I7 (1995) 167-177

heavier lambs with better survival and were themselves heavier postpartum, which would have also improved their maternal ability. The relationship between probability of survival (P) and birth weight ( W,) was given as Zogit(P) = 3.292W, - 4.057. The effect of specific dietary components on foetal growth and lamb neonatal survival is still not sufficiently clear to be able to formulate economic feeding regimes. 4.8. Placenta characteristics and survival

f -

c

-*

60

-

-*< ‘\ I’

_’

% 40-

20 -

’

Birth weight, kg

Fig. 3. Effect of birth weight on lamb mortality in Menz sheep (288 lambings, 320 lambs; mortality < 7 days, 21%; 7 day mortality, 58% losses).

and lamb birth weight

Placenta weight in Menz sheep is 2 10 + 80 g and is not affected by lamb sex or stage of dam supplementation during pregnancy (P > 0.05)) probably because much of the placenta growth occurs in early pregnancy. Total placentomes average 56 f 14 with 39 f 13 large ones ( > 1.5 cm diameter). Although total placentomes were not affected by stage of supplementation, ewes fed extra in the mid- and last trimester have larger cotyledons and ewes gaining weight haGe a heavier placenta with more placentomes. There was, therefore, a positive relationship between placenta weight, number of placentomes and lamb birth weight, consistent with the fact that placenta weight, total number and number of large placentomes were less for ewes with lambs that died (Mukasa-Mugerwa et al., 1994a).

5. Manipulation

60

173

of reproduction

function

5.1. Nutrition and litter size In a study in which groups of ewes either only grazed or received supplementary feed only in the first, second or third trimester, Mukasa-Mugerwa et al. (1994a) found no clear trend in the incidence of multiple births across the four nutrition treatment groups: 16.2%, 4.2%, 14.1% and 1l.l%, respectively. However, ewes that gave birth to twins were significantly heavier and in better body condition (P < 0.001) at mating (mean&standard error 26.5 +0.7 kg and 2.8 +O.l score) than single-bearing animals (22.3 f 0.3 kg and 2.4 + 0.1 score, respectively, P < 0.001) . This was consistent with the steady increase in twinning rate with rising ewe weight at mating, from zero at < 15 kg to 3 1.3% at 30 kg or more, confirming that litter size

174

E. Mukasa-Mugerwa,

(prolificacy) can definitely be increased mating nutrition management of ewes.

A. Lahlou-Kassi /Small Ruminant Research I7 (1995) 167-I 77

by the pre-

6. Measured estimates of productivity 6.1. Annual reproductive

5.2. Hormone treatments

Besides being able to increase twinning through ewe live weight at mating, the trait rose from 11% in control ewes to 23-50% in animals (Mutiga and MukasaMugerwa, 1992) treated with 200 and 300 IU pregnant mare serum gonadotropin (Folligon; Intervet International) irrespective of whether oestrus was synchronised by prostaglandin Fz, (Lutalyse; Upjohn, Kalamazoo, MI) or progestogen sponges (Chronogest; Intervet International, Boxmeer, The Netherlands). Non-genetic methods are therefore available through which prolificacy in the breed can be improved possibly with as much success as in temperate breeds. There is, however, a need to define dose-responses: first, to minimise cost and, second, to define the optimum twinning rates because a large increase in ovulation rate and litter size is likely to reduce lamb birth weight and compromise their survival.

5.3. Ram effect

When rams were introduced to ewes that had previously been isolated from or kept exposed to vasectomised rams for a year, oestrus behaviour was synchronised in isolated ewes. Lambing activity was also compacted except for ewes conceiving on second oestrus (Mukasa-Mugerwa et al., 1994~). As many twins were born to isolated ewes as to exposed ewes, indicating that, although ram introduction will not increase ovulation rate in Menz sheep, it is a natural tool that could be used, for example, to match reproductive cycle events with feed resources. It was not possible, however, to gauge how ewes isolated for less than a year would have responded in order to estimate the minimum separation period to optimise the ram effect. In temperate sheep, an isolation period of 1734 days has been suggested by Oldham ( 1980). A major advantage of Menz sheep being able to cycle year-round, is that oestrus cycles do not have to be induced. Artificial induction of breeding activity is associated with poor luteal function (Lishman and Inskeep, 1991).

rate

Annual reproductive rate ( ARR) is another parameter that can be used to gauge flock fertility, and reflects the total number of lambs weaned per ewe of reproductive age per year. It was generated as ARR = S( 1 -M) /I, where S is litter size, M is lamb preweaning mortality and I is lambing interval in years. The mean ARR of 1.4 per ewe was regarded as favourable in contrast to estimates elsewhere in Africa that are generally below 1.2 (Gatenby, 1986; Wilson, 1989). The moderate performance was attributed to short interlambing interval and minimal mortality rate during the study period. 6.2. Annual production

indices

Data on sheep production in Africa have rarely been constructed into production indices to facilitate easy assessment of the impact of natural and management factors on flock productivity, for which three types of indices are available (Wilson and Murayi, 1988) : Index I (kg per ewe) : (Total live weight of litter at 90 days or weaning X 365) /Subsequent lambing interval (days). Index II (g) : Index I/Postpartum dam weight (kg). Index III (g): Index I/Postpartum dam weight0.73 (kg). Indices for ewes whose offspring die before weaning are zero, the zeros being incorporated in the means and standard errors for the variables under consideration. The importance of lamb growth before weaning, which reflects maternal ability, and dam nutrition status and lactation ability, and the impact of preweaning mortality are obvious. From ewe lambs followed through two lambings by Mukasa-Mugerwa et al. ( 1994b), mean values for Indexes I, II and III were 11 .Of 0.9 kg, 520 f 41 g and 1.18 f 0.1 kg, respectively. Lambing season and postpartum dam nutrition level affected productivity (Table 2). Ewes lambing in the dry period from October to January were most disadvantaged, particularly as a result of reduced weaning rates, slow growth rates, which reduced weaning weight in survivors, and extended lambing intervals in poorly fed dams. Ewes raised on low and medium feeding were, with some

E. Mukasa-Mugenva,

A. L.ahlou-Kassi/Small

Ruminant Research 17 (1995) 167-177

Table 2 Least squares means ( f standard error) of the effects of lambing season and level of nutrition on subsequent productivity ClaSS

Daily weight gain (kg)

Weaning weight

Index I

175

indices of Menz sheep

Index II mean (kg)

Index III

mean (kg)

mean (kg)

(kg) Overall mean

0.05 f 0.01

6.91 f0.51

10.99 * 0.87

0.52 f 0.04

1.18kO.09

Lambing season June-Sep. Oct.-Jan. Feb.-May

0.03 * 0.01 0.03 * 0.01 0.08 + 0.01

7.55 f 1.09 4.58fl.19 9.18* 1.23

9.48 * 2.15 4.92 5 2.35 14.24 f 2.38

0.43*0.10 0.19*0.11 0.62+0.11

0.99 f 0.22 0.47 f 0.25 1.45 f 0.25

Post lambing nutrition Poor Low Medium

0.04*0.01 0.06 f 0.01 0.07 f 0.01

5.33 k 1.23 7.84 + 0.09 8.17*0.22

6.13+2.51 10.25 + 2.22 12.20 f 2.08

0.25*0.11 0.44*0.10 0.55 + 0.09

0.60 f 0.26 1.03 * 0.22 1.28f0.22

Source: Mukasa-Mugerwa

et al. ( 1994b).

supplementation, 1.67 and 1.99, respectively, times as productive as controls that grazed only. This was largely through reduced reproductive wastage and better growth rates among surviving animals. Supplementary feed at the highest level was only 35% of the market value of the 6.07 kg BW increase realised which was regarded as favourable considering feed was the most expensive input. Because rural farmers are less likely to purchase supplementary feeds, it is tempting to recommend controlled mating so that most lambings take place during the rains. Such a strategy must, nonetheless, be matched against one that simply maximises the natural ability of these sheep to breed year-round and lamb every 8 months. It is also necessary to recognise that lambing during certain seasons can result in higher lamb mortality (Njau et al., 1988). In addition, as lamb growth is largely influenced by maternal and lactation ability, the economics of supplementing grazing ewes nursing twins with escape protein on milk yield and lamb survival needs to be evaluated.

unlikely that animal numbers will be deliberately reduced to ease pressure on grazing pastures. We therefore recommend, inter alia, the following: ( 1) Feeding regimes for breeding ewes need to be formulated through better utilisation of on-farm crop by-products or leguminous multipurpose trees. Better nutrition will increase ovulation and pregnancy rates, reduce pre- and postnatal losses, minimise postpartum anoestrus interval, increase lamb growth rates and enhance puberty. (2) Unproductive females should be culled. Farmers need to be encouraged into setting up rural small-scale fattening schemes to absorb culled females and excess males where market prices can reward for good quality animals. (3) Efforts must be made to control non-disease causes of lamb mortality and prevention of prevalent infectious diseases and endoparasitism. (4) Factors that make some farmers more successful need to be identified in order to facilitate their extension to other farmers.

7. Conclusions and recommendations These studies confirm the natural ability of Menz sheep for good fertility and short lambing intervals. Major constraints to production are high young mortality, morbidity and slow growth in survivors. Since most sheep in Africa are kept for meat production, efforts to optimise female reproduction, lamb output and number of animals for slaughter are vital. It is

Acknowledgement

We wish to express sincere thanks to Anne Nyamu for editorial help in preparing this manuscript.

176

E. Mukasa-Mugenva,

A. Luhlou-Kassi/Small

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