W.O. OdenyaEgerton University, Animal Science Department
P.O. Box 536, Njoro, Kenya
Abstract
Introduction
Materials and methods
Results and discussion
Implications and conclusion
References
Reproductive and disease incidence characteristics of Dorper (D), F1 Dorper x Masai (DM) and Masai (M) ewes were compared under semi-arid conditions in Kenya.
Percentage of lambs born alive were 87.1,90.1 and 98.2 for D, DM and M ewes respectively. Gestation lengths were 149.1, 148.2 and 146.7 days respectively; birth weights were 3.1, 3.5 and 2.7 kg; ages at first service were 12.2, 11.8 and 12 months and weights at first service were 30, 32.2 and 26.2 kg respectively. Significant differences between breed groups were observed in reproductive traits and disease frequencies. The DM ewes were younger and heavier at first service than the parental breeds (D and M). The differences between breed groups kept under the same environmental conditions indicate that disease susceptibility has a genetic basis.
Caractéristiques de la reproduction et de l'incidence des maladies chez les brebis de race Dorper, Dorper x Massaï et Massaï élevées en milieu semi-aride au Kenya
Résumé
Les caractéristiques de la reproduction et de l'incidence des maladies chez des brebis Dorper (D), F1 Dorper x Massaï (DM) et Massaï élevées en milieu semi-aride au Kenya ont été comparées. Les pourcentages d'agneaux nés vivants étaient respectivement de 87,1, 90,1 et 98,2 pour les brebis D, DM et M. Les durées respectives de gestation étaient de 149,1, 148,2 et 146, 7 jours et les poids à la naissance de 3,1, 3,5 et 2,7 kg. L'âge à la première saillie était respectivement de 12,2, 11,8 et 12 mois et les poids à la première saillie de 30, 32,2 et 26,2 kg. On observe des différences significatives entre les types génétiques pour ce qui concerne les caractéristiques de la reproduction et la fréquence des maladies. Les brebis DM étaient plus jeunes et plus lourdes à la première saillie que les races parentales (D et M). Les différences entre types génétiques maintenus dans des conditions environnementales identiques montrent que la prédisposition aux maladies repose sur une base génétique.
Hair sheep form a major component of the livestock sector of agriculture in Kenya. Of a total of approximately 7 million sheep in Kenya (De Boer, 1981) about 95% are hair sheep (Odenya, 1984) which produce an estimated 15 to 20% of the country's red meat (Allonby, 1975). The predominant indigenous hair sheep breeds are Masai and Blackhead Persian. However, Dorper has emerged as the most important sire breed for use in crossbreeding for mutton production in the semi-arid areas of Kenya. The indigenous hair breeds have not been adequately evaluated against Dorper under similar conditions to ascertain genetic differences between them. It is only after genetic evaluation of individual, maternal and paternal traits that appropriate breeding plans can be formulated to optimise production in a given environment.
Reproductive traits and disease incidence within and between breeds are important traits that are taken into account in selecting sheep for meat production and culling. De Boer (1981) reported that the reproductive rate of hair sheep in semi-arid areas is low. Allonby (1975) reported prevalence of disease in hair sheep breeds in the same environment. Low reproductive rates and a higher disease incidence would ensure a low selection differential resulting in reduced genetic gains.
The objective of the research reported here was to examine flock records and determine reproductive traits (percentage of lambs born alive; gestation length; birth weight; age and weight at first service), disease incidence and reasons for culling in Dorper (D), Dorper x Masai (DM) and Masai (D) breed groups.
Animals and traits
Reproductive parameters were determined for 330 D, 260 F1 DM and 100 M ewes between 1982 and 1984. Parameters included percentage of lambs born alive in a lambing season, gestation length (days), birth weight of lamb (kg) as a trait of the dam, age of ewe at first service (months) and weight of ewe at first service (kg). Data on the number of animals treated or diagnosed for dystocia, mastitis, foot-rot, pneumonia, haemonchosis and other undefined problems and diseases were collected from 3110 ewes (860 D, 1200 DM and 1050 M) between 1974 and 1984. Similarly, the reasons for culling were determined from 428 ewes (200 D, 150 DM and 78 M) from 1979 through to 1984. The reasons for culling were the same as those for disease incidence, plus low milk yield.
The animals were located at Ol'Magogo farm in Naivasha, Kenya at the UNDP/FAO (United Nations Development Programme/Food and Agriculture Organization of the United Nations) Sheep and Goat Development Project.
Animals management
All the animals were managed in the same flock. They were maintained on natural pastures that consisted primarily of Naivasha star-grass. Once a month the animals were supplied with mineral supplements. Water was supplied freely. The animals were mated in December (after the short rains) to lamb in April-May (after the long rains). Flocks of purebred D and M were maintained. Some of the D breed sires were used for crossbreeding M ewes to obtain F1 DM animals. All animals were eartagged at birth.
Analysis of data
The data were analysed with a one-way (breed group) analysis of variance. Pairwise comparisons of means for reproductive traits were done using Duncan's multiple range test (Steel and Torrie, 1960). The Friedman test (Steel and Torrie, 1960) was used to find significant differences between breed groups for disease incidence.
Reproductive parameters
Results of reproductive parameters for D, DM and M breed groups are shown in Table 1. M ewes had the highest percentage of lambs born alive, followed by DM and then D. The increased reproductive performance of the crossbred ewes relative to that of D ewes is probably due to the increased oestrons and gonadal activities coupled with improved uterine and peri- and post-natal conditions for foetal and lamb survival. One important determinant of the number of lambs born is ovulation rate. Ovulation rate, however, shows little or no heterosis (Nitter, 1978). The little heterosis observed in this study in the crossbreds could be due to this reason. Another possible reason for the low heterosis shown in the crossbreds could be that there was little genetic variance in the ovulation rate of their purebred parents. Also, lack of heterosis may have been possible if both parental breeds (D and M) had been well adapted to the prevailing environmental conditions.
Table 1. Reproductive parameters for Dorper, F1 Dorper x Masai, Masai breed groups.
|
Parameters |
Dorper (N=330) |
F1 Dorpers X Masai (N=260) |
Masai (N=100) |
|
% Lambs born alive |
87.1a |
90.1b |
98.2c |
|
Gestation length (days) |
149.2a |
148.1a |
146.7b |
|
Birth weight (kg) |
3.1a |
3.5b |
2.7c |
|
Age at first service (months) |
12.2a |
11.8a |
12a |
|
Weight at first service (kg) |
30b |
32.2b |
26.2a |
Means with different superscripts within the same row are statistically different, P<0.05.
Studies by Dickerson and Laster (1975) and Jakubec (1977) showed that heterosis may contribute to earlier sexual development in sheep. Their observations were confirmed in this study (Table 1), although the differences were not significant (P>0.05).
M ewes had the shortest gestation length of the three breed groups (Table 1). Studies by Ricordeau (1981) have shown that larger animals have longer gestation length than smaller ones. In this study, M ewes were significantly (P<0.05) lighter than DM and D ewes at first service (Table 1). This may have accounted for the shorter gestation length in the M ewes.
Disease incidence
Table 2 shows the number and frequency of different diseases by breed group. The DM breed group had the highest incidence of dystocia while M had the lowest. The DM breed group also had the highest birth weight while M had the lowest (Table 1). Breed groups with higher incidence of dystocia also tended to have longer gestation lengths (Tables 1 and 2). This concurs with results by Philipsson (1976) who reported that gestation length is positively correlated with dystocia. The results therefore suggest a possible positive phenotypic correlation between dystocia, birth weight and gestation length.
The D breed group had the highest incidence of foot-rot, pneumonia and haemonchosis (Table 2). The M breed group had the lowest incidence of all specified diseases but also recorded the highest incidence of unspecified diseases (Table 2).
Differences in the frequency of diseases between breed groups kept under the same environmental conditions indicate that disease susceptibility has a genetic basis.
Reasons for culling
Table 3 shows the frequency distribution (%) of reasons for culling among ewes of the three breed groups. D ewes were culled most frequently for udder problems and low milk yield, while the M breed group had the lowest culling rate in all categories of known reasons for culling.
Table 2. Number (No.) and frequency (%) per breed group of different diseases in ewes.
|
Diseases |
Dorper |
F1 Dorper x Masai |
Masai |
|||
|
No. |
% |
No. |
% |
No. |
% |
|
|
Dystocia |
86 |
10.0b |
145 |
12.1b |
21 |
2.0a |
|
Udder (mastitis) |
172 |
20.0b |
286 |
23.8c |
66 |
6.3a |
|
Feet (foot-rot) |
185 |
21.5c |
153 |
12.8b |
56 |
5.3a |
|
Respiratory (pneumonia) |
133 |
15.5c |
155 |
12.9a |
126 |
12.0a |
|
Digestive (haemonchosis) |
176 |
20.5c |
180 |
15.0b |
107 |
10.2a |
|
Others |
108 |
12.6a |
281 |
23.4b |
674 |
64.2c |
|
Total |
860 |
100 |
1200 |
100 |
1050 |
100 |
Friedman test: Significant differences between breed groups, P<0.05.
Table 3. Frequency distribution (%) within breed groups for reasons for culling ewes.
|
Reasons for culling |
Dorper |
F1 Dorper x Masai |
Masai |
|
Dystocia problems |
12c |
10b |
4a |
|
Udder problems |
40c |
30b |
4a |
|
Feet problems |
9b |
8b |
5a |
|
Respiratory problems |
7b |
5ab |
4a |
|
Low milk yield |
23c |
20b |
10a |
|
Unknown problems |
9a |
27b |
73c |
|
Total |
100 |
100 |
100 |
Means with different superscripts within the same row are different (P<0.05).
The difference in reproductive traits, the known culling reasons and the known disease frequencies in Dorper, Dorper-Masai and Masai breed groups may have a genetic basis, probably due to differences in adaptation to their environment. In disease epidemic areas it may be cost-effective to rear Masai rather than Dorper sheep as Masai seem to be the most appropriate breed for farmers in semi-arid areas, followed by F1 Dorper-Masai crosses.
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Steel R D G and Torrie J H. 1960. Principles and Procedures of Statistics. Mcgraw-Hill Book Company, New York, USA. 481 pp.
