J.M. C. JIBBO, J. DURKIN, D.E. LIGHT, M. MURRAY, K. SONES and J.C.M. TRAIL
Tsetse-transmitted African trypanosomiasis is a highly significant factor in Africa's deteriorating food production situation. Tsetse flies infest an area of Africa larger than the USA and affect thirty-seven countries (FAO/WHO/OIE, 1982). It is considered that 7 million km of this area would otherwise be suitable for livestock and mixed agriculture. The cost of tsetse control, the lack of a field vaccine, the limited prospects of any new trypanocidal drugs appearing in the near future and the relatively small numbers of trypanotolerant livestock, make reliance on the currently available trypanocidal drugs an unavoidable necessity.
The amount of trypanocidal drug used in Africa is small in relation to the number of animals at risk. Currently, the number of doses employed is around 25 million per year (reviewed by Murray and Gray, 1984) despite the fact that an estimated 50 million cattle, 30 million sheep and 40 million goats are exposed. Even if animals were treated only twice per year, 240 million doses would be required, ten times the number currently used.
Chemoprophylaxis against bovine trypanosomiasis has been in widespread use for more than thirty years. However, there have been few attempts to assess its effectiveness in terms of animal performance achieved. Studies undertaken to date have usually considered viability and growth only and have tended to be on a small scale and of relatively short duration. This lack of convincing information on production possibilities under chemoprophylaxis and the belief that repeated use of the same trypanocidal drugs must lead to drug resistance, are probably the main reasons for insufficient funds being available for the purchase of trypanocidal drugs.
Ford and Blaser (1971) drew attention to the existence of matching cattle health and performance records at the Mkwaja Ranch, Tanzania. By virtue of their completeness and volume, they represented a unique opportunity to evaluate the long-term effectiveness of chemoprophylaxis, in a production situation where cattle cannot survive unless protected by trypanocidal drugs (Blaser et al., 1979).
This paper reports the viability, reproduction and associated growth rates achieved by grade Boran cattle maintained by chemoprophylaxis at Mkwaja Ranch over the ten years, from 1973 to 1982 (Trail et al., 1985). The performance traits involved were calving interval, pre-weaning mortality, pre-weaning growth and the resultant cow productivity (weight of weaner calf per cow per year). Calf pre-weaning growth must be evaluated simultaneously with cow reproduction, as Trail et al. (1971) noted for Ankole, Boran and East African Zebu cows in Uganda, that heavier cows had the highest conception rate; cows heavier at parturition weaned heavier calves; but cows weaning heavier calves lost more weight (or gained less) during lactation.
Mkwaja Ranch is located on the coast of Tanzania about 100 km south of Tanga. The ranch occupies approximately 47,250 hectares of coastal forest-savanna mosaic vegetation. The area is heavily infested with three species of tsetse, Glossina morsitans morsitans, G. pallidipes and G. brevipalpis (Gates et al., 1983). The level of tsetse challenge is maintained throughout the year (Figure 1; Gates et al., 1983) and is such that cattle cannot survive unless protected by trypanocidal drugs (Figure 2; Blaser et al., 1979).
Over the last ten years, the ranch has supported some 12,000 head of grade Boran, including an average of 4800 cows. The ranch was established with local East African Zebu which have been progressively upgraded to 70 to 80% Boran by the importation of Boran bulls and Boran semen from Kenya. There is also a large wildlife population on the ranch.
A rigorous animal health programme is employed. Since 1964, a chemoprophylactic regime based on the use of Samorin (May and Baker Ltd.) has been progressively adopted. The only other control measure tested was that of sterile male release of G.m. morsitans in a trial carried out during 1978 (Williamson et al., 1983). All animals are vaccinated against rinderpest and annually against anthrax, blackquarter and pasteurellosis. In addition, breeding stock are vaccinated against foot-and-mouth disease and Campylobacter fetus and heifers against Brucella abortus. Tick-borne diseases are controlled by a strict programme of dipping/spraying, twice weekly using an organo-phosphorus acaricide in the wet season and once a week using toxaphene in the dry season. Anaplasmosis is treated with oxy-tetracycline. The principal internal parasite is Haemonchus placei and animals less than twenty-four months of age are treated with an anthelmintic approximately every three months.
From 1964 to June 1980, breeding cows, heifers, bulls, young bulls and steers were maintained under Samorin prophylaxis at 0.5 mg/kg on a herd basis. Trypanosome infection was monitored by thick blood smears. One month after the last herd prophylaxis, 30 to 40 animals per herd (herds averaged 225 to 300) were tested every one or two weeks depending on a subjective assessment of the level of challenge. The animals selected usually included any animals that appeared to be in poor condition. When approximately 20% of the sample was positive the entire herd was treated with Samorin. In addition, individual animals that looked sick when entering or leaving night paddocks were tested. If positive for trypanosomes and the next Samorin treatment of the herd was not yet due, that animal would be treated with Berenil (Hoechst) at 3.5 mg/kg. All pre-weaning calves were treated at monthly intervals with Berenil. In June 1980, the criteria for herd treatment and the drug regime employed were changed. Beginning two months after the last prophylaxis, as soon as routine examination revealed the first positive case, all animals in a herd were treated with Berenil and then one week later with Samorin at 1 mg/kg. However, in late 1981 and 1982, only Samorin was used as Berenil was not available.
Figure 1. Level of tsetse challenge (from Gates et al., 1983).
Figure 2. Untreated weaners do not survive (from Blaser et al., 1979).
The effects of year (A), season (B), age (C), area (D), number of Samorin treatments (E), timing of first Samorin treatment (F), Berenil treatment (G), sex of calf, location/area and interactions of A x B. A x D, A x G. B x D, C x E, C x G. D x E, D x F. D x G. E x G and F x G on calving interval, calf mortality, weaning weight and cow productivity were studied.
Performance levels achieved at Mkwaja Ranch
Cow reproduction, cow and calf viability and calf growth values achieved are shown in Table 1 and are combined to build up the index of total weight of weaner calf produced per cow per year.
Reproductive performance and viability are key traits in beef cattle enterprises. Possible genetic improvement in virtually all traits of economic importance is closely tied to them. The annual cow replacement rate of 18.7% indicated an average of 5.3 years in the breeding herd. Thus with calving intervals of 15.9 months, five calves would be produced by each cow in its lifetime. An appropriate definition of mortality rate in the context of genetic progress is the percentage of females that die before calving. By adding a post-weaning mortality rate of 4% based on Mkwaja Ranch Annual Reports, to the pre-weaning mortality, it was approximately 12% at Mkwaja. The rearing proportion is the proportion of births that produces a heifer that survives and is fertile. The rearing proportion here was 0.44. This means that once in 2.27 carvings, a cow produced a heifer calf that could in turn calve in the herd. The average length of reproductive life was five carvings: therefore, approximately 45% of the females born were required as replacements to maintain herd size.
Genetic improvement per unit of time is more important than per animal generation. The average age at first calving was 47.0 months and the average calving interval 15.9 months. Thus, to replace herself, a cow had to calve 2.27 times; so the average generation interval was 83.1 months or 6.9 years. This is within the range well defined for tropical indigenous breeds. Overall, the proportion of females required as replacements and the length of the generation interval would indicate considerable scope for implementation of selection programmes on growth traits. More stringent culling based on reproductive performance could also well make a contribution.
Table 1. Overall performance achieved by grade Boran cattle at Mkwaja Ranch from 1973 to 1982.
|
Trait |
No. of records |
Mean |
s.e. | |
|
Reproductive performance | ||||
|
|
Age at first calving (months) |
4,460 |
47.0 |
0.12 |
|
|
Calving interval (months) |
11,999 |
15.9 |
0.06 |
|
|
Calving percentages (%) |
11,999 |
75.3 |
n.a. |
|
Viability | ||||
|
|
Pre-weaning mortality (%) |
18,266 |
8.0 |
0.20 |
|
|
Annual cow mortality (%) |
45,852 |
5.8 |
0.91 |
|
Breeding cow replacement rate | ||||
|
|
Annual culling rate (%) |
45,852 |
12.8 |
1.01 |
|
|
Annual cow replacement rate (%) |
45,852 |
18.7 |
1.42 |
|
|
Calf weaning weight at 8 months (kg) |
16,275 |
133.5 |
0.23 |
|
|
Cow productivity index b (kg) |
11,999 |
101.9 |
0.47 |
|
|
Herd productivityc (kg) |
11,999 |
96.0 |
n.a. |
a Calving percentage was obtained by dividing 365 by calving interval (days).
b Total calf weaning weight per cow per year.
c Cow productivity index x annual cow viability.
n.a. = Not applicable.
Performance levels in an overall African context
There is a serious lack of fully documented information on livestock productivity levels attainable using trypanocidal drugs in trypanosomiasis risk situations. Since Table 1 contains considerably more information than has been reported in Africa since chemoprophylaxis was first used, the key performance traits are contrasted in Table 2 with those from another major ranching situation in Africa.
Performance data have been obtained from 11 Boran ranches in Kenya comprising around 16,000 data per trait. Individual studies covering three of these ranches have been reported (Trail and Gregory, 1981; Gregory et al., 1984; Trail et al., 1984). The Kenya Boran ranches were in tsetse-free areas; thus, a major environmental difference between them and Mkwaja Ranch was the trypanosomiasis risk at the latter situation.
Mkwaja Boran were inferior in all performance traits to those in Kenya. They were approximately 3% poorer in viability, 16% poorer in reproductive performance and 26% poorer in body weights. Calculated cow and herd productivity indices reflected these, differences in productivity per cow between the two situations being relatively greater than differences per unit metabolic weight of cow, due to the lower mature cow weights attained at Mkwaja. The herd productivity, which is probably the most meaningful comparison to be made, expressed as weight of an 8-month-old weaner calf produced per 100 kg metabolic weight of cow per year, was 20% less for the Mkwaja Borans.
Table 2. Comparison of Mkwaja Boran cattle with Kenya Boran.
|
|
Mkwaja Boran High trypanosomiasis risk Prophylaxis |
Kenya Borana No trypanosomiasis risk. No prophylaxis | |
|
Reproductive performance | |||
|
|
Age at first calving (months) |
47.0 |
39.7 |
|
|
Calving percentage |
75.3 |
87.0 |
|
Viability | |||
|
|
Pre-weaning viability (%) |
92.0 |
94.6 |
|
|
Annual cow viability (%) |
94.2 |
98.0 |
|
Body weights | |||
|
|
Weaning weight at 8 months (kg) |
133.5 |
174.0 |
|
|
Estimated mature cow weight (kg) |
293 |
414 |
|
Calculated herd productivity | |||
|
|
Productivityb per cow per year (kg) |
87.1 |
140.3 |
|
|
Productivityb per cow 100 kg 0.73 of cow per year (kg) |
137.8 |
172.5 |
a From 11 ranches, approximately 16,000 data per trait, constructed from J.C.M. Trail, unpublished.b Productivity = weight of 8-month-old weaner calf.
Chemoprophylactic regime and its relevance to drug resistance
This work was carried out in a situation where it had been shown clearly that without chemoprophylactic treatment cattle would die (Blaser et al., 1979). The decision for treatment was based on blood smear examination and assessment of herd condition. As a consequence, the number of treatments required reflected the level of trypanosomiasis risk to which animals were subjected. Thus, an association between lowered performance and increased numbers of treatment would be expected (Table 3).
Table 3. Effects of number of Samorin treatments required from parturition to 240 days after parturition, on performance traits.
|
|
Number of Samorin treatments required |
||||
|
1 |
2 |
3 |
4 |
Average s.e. |
|
|
Calving interval (days) |
511 |
499 |
504 |
505 |
9.0 |
|
Calf viability (%) |
96.1 |
92.5 |
89.5 |
86.4 |
0.89 |
|
Weaning weight (kg) |
135.8 |
133.6 |
130.1 |
129.5 |
0.87 |
|
Cow productivity index (kg) |
96.1 |
95.4 |
93.4 |
90.3 |
2.35 |
A major consideration in the long-term use of trypanocidal drugs is concern over the possible appearance of drug resistance. If resistance to Samorin existed at Mkwaja, it would be expected that use of an occasional Berenil treatment would be more effective than an additional Samorin treatment as cross-resistance between Samorin and Berenil is thought to be rare. However, in terms of maintaining productivity overall, the effects of an occasional Berenil treatment were very similar to those of a Samorin treatment. Furthermore, if resistance had developed, it is more likely to have done so in a high trypanosomiasis-risk situation where Samorin treatments were required more frequently rather than in a low-risk situation where fewer treatments were necessary. However, as shown in Table 4, Berenil had relatively less impact in a high-risk than in a low-risk situation.
Table 4. Interaction constants between Berenil treatment (s) given and number of Samorin treatments required from parturition to 240 days after parturition, on performance traits.
|
|
Number of Samorin treatments required |
||||
|
1 |
2 |
3 |
4 |
Average s.e. |
|
|
Calving interval (days)a |
-12 |
1 |
7 |
4 |
5.9 |
|
Calf viability (%)b |
1.2 |
-0.5 |
0.3 |
-1.0 |
0.59 |
|
Weaning weight (kg)b |
3.3 |
2.0 |
-0.6 |
-4.7 |
0.56 |
|
Cow productivity indexb |
8.0 |
0.5 |
-2.5 |
-6.0 |
1.54 |
a Lowest value most desirable.
b Highest value most desirable.
Old cows required exactly the same number of treatments as young cows (Table 5). It was shown that cows did not start dropping in overall productivity until 9 years of age and older (Table 5), a result consistent with well-established findings in breeding females in non-trypanosomiasis, beef producing situations. Thus maintenance under continuous prophylactic treatment until 9 years of age did not lead to increased requirements for treatment or to an early decline in productivity. Also, the fact that there was no reduction in the number of treatments required by older animals, indicated that acquired resistance had not occurred under the chemoprophylactic strategy practiced. At the same time, there was no indication that the use of Samorin in pregnant animals had any negative effects. In fact, a treatment between 30 days before and 10 days after parturition appeared beneficial.
Table 5. Cow-age effects on treatments required and performance.
|
Age (years) |
No. of treatments required |
Calving interval (months) |
Calf mortality (%) |
WW (kg) |
Cow productivity (kg) |
|
3-4 |
5.0 |
18.0 |
8.9 |
132.6 |
86.0 |
|
5-6 |
5.4 |
16.5 |
8.7 |
136.3 |
97.8 |
|
7-8 |
5.5 |
15.8 |
9.6 |
133.6 |
97.8 |
|
9+ |
5.3 |
16.1 |
8.5 |
126.5 |
93.6 |
|
Av. s.e. |
0.03 |
0.25 |
0.8 |
0.78 |
2.02 |
Tsetse-transmitted African trypanosomiasis is the most significant single factor in Africa's deteriorating food production situation. There is a serious lack of fully documented information on livestock productivity levels attainable using trypanocidal drugs in trypanosomiasis-risk situations. The availability of ten years of matching animal productivity and health data at Mkwaja Ranch offered the unique opportunity of evaluating more than twenty times as much information on livestock productivity under chemoprophylaxis as had been made available in the whole of Africa over the previous twenty-five years.
At Mkwaja it has been clearly demonstrated in planned experiments that cattle cannot survive without trypanocidal drugs. If left untreated, all die of trypanosomiasis or are killed by predators. The cattle maintained under Samorin prophylaxis appear to be highly successful, being 80% as productive as Boran cattle on trypanosomiasis-free ranches in Kenya. In a situation where bush clearance and tsetse control had been carried out, it was in fact possible to achieve the average Kenyan ranching productivity level. There were no indications of resistance to Samorin developing over the ten-year period. The drug appeared to have no deleterious effects on reproductive performance, as gauged by its completely non-significant influence on calving intervals. The multiple inoculations did not raise any serious problems for animals in terms of local reactions and did not lead to increased requirements for treatment or to an earlier decline in productivity.
The results of this study have thus shown that cattle production under Samorin prophylaxis is possible in areas heavily infested with tsetse. The fact that this result is based on one of the largest data sets ever analysed, offers immediate hope for increased exploitation of tsetse-infested areas by encouraging the more widespread, rational use of chemoprophylaxis as an integral part of management.
Blaser, E., J.M.C. Jibbo and W.I.M. McIntyre. 1979. A field trial of the protective effect of Samorin and Berenil in Zebu cattle under ranching conditions in Tanzania. Report of the 15th meeting of the International Scientific Council for Trypanosomiasis Research and Control. Banjul, The Gambia, 1977. OAU/STRC No. 110, 383-386.
FAO/WHO/OIE. 1982. Animal Health Yearbook 1981. V. Kouba, ed. FAO, Rome.
Ford, D. and E. Blaser. 1971. Some aspects of cattle raising under prophylactic treatment against trypanosomiasis on the Mkwaja Ranch, Tanzania. Acta Trop., Basel 28: 69-79.
Gates, D.B., P.E. Cobb, D.L. Williamson, B. Bakuli, E. Blaser and D.A. Dame. 1983. Integration of insect sterility and insecticide for control of Glossina morsitans morsitans Westwood (Diptera: Glossinidae) in Tanzania. III. Test site characteristics and the natural distribution of tsetse flies. Bull. Entomol. Res. 73: 373-381.
Gregory, K.E., J.C.M. Trail, J. Sandford and J. Durkin. 1984. Crossbreeding cattle in beef production programmes in Kenya. I. Comparison of purebred Boran and Boran crossed with the Charolais, Ayrshire and Santa Gertrudis breeds. Trop. Anim. Hlth. Prod. 16: 181-186.
Murray, M. and A.R. Gray. 1984. The current situation on animal trypanosomiasis in Africa. Prevent. Vet. Ned. 2: 23-30.
Trail, J.C.M., G.D. Sacker and I.L. Fisher. 1971. Crossbreeding beef cattle in Western Uganda. 1. Performance of Ankole, Boran and Zebu cows. Anim. Prod. 13: 127-141.
Trail, J.C.M. and K.E. Gregory. 1981. Characterization of the Boran and Sahiwal breeds of cattle for economic characters. J. Anim. Sci. 52: 1286-1293.
Trail, J.C.M., K.E. Gregory, J. Durkin and J. Sandford. 1984. Crossbreeding cattle in beef production programmes in Kenya. II. Comparison of purebred Boran and Boran crossed with the Red Poll and Santa Gertrudis breeds. Trop. Anim. Hlth. Prod. 16: 191-200.
Trail, J.C.M., K. Sones, J.M.C. Jibbo, J. Durkin, D.E. Light and M. Murray. 1985. Productivity of Boran Cattle Maintained by Chemoprophylaxis under Trypanosomiasis Risk. Research Report No. 9. Addis Ababa: ILCA.
Williamson, D.L., D.A. Dame, D.B. Gates, P.E. Cobb, B. Bakuli and P.V. Warner. 1983. Integration of insect sterility and insecticides for control of Glossina morsitans morsitans Westwood (Diptera: Glossinidae) in Tanzania. V. The impact of sequential releases of sterilized tsetse flies. Bull. Entomol. Res. 73: 391-404.
