Genetic aspects of control of anaemia development in trypanotolerant N'Dama cattle.

148 one-year-old N'Dama cattle, progeny of 29 sires, were exposed for 92 days to a medium natural tsetse-trypanosome challenge in Gabon, Central Africa. Matching health and performance data were recorded on 11 occasions. Average packed red cell volume percent (PCV) and lowest PCV reached during the period were evaluated as measures of ability to control the development of anaemia. Attempts were made to systematically control other possible causes of anaemia. In animals detected as parasitaemic, those with above average average PCV values or above average lowest PCV reached had 34% and 35% respectively higher daily weight gains than those with below average. Even when not detected as parasitaemic, those with above average average PCV values or above average lowest PCV reached had 14% and 12% respectively higher gain indicating that a proportion of these animals actually were parasitaemic. When all environmental and parasitaemia information was taken into account, the heritability of growth, average PCV and lowest PCV reached was 0.39 +/- 0.31, 0.64 +/- 0.33 and 0.50 +/- 0.32 respectively. The genetic correlation between average PCV and growth was 0.70 +/- 0.42 and between lowest PCV reached and growth was 0.28 +/- 0.55. While the standard errors are large, the higher heritabilities of PCV measures compared to animal growth and the positive genetic correlations between PCV and growth do indicate an opportunity for selection on PCV when animals can be detected as parasitaemic. All heritabilities and genetic correlations increased in size when parasitaemia information was utilized in the analysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of trypanosome infection, control of parasitaemia and control of anaemia development on productivity of N'Dama cattle.

One hundred and forty six calving interval records were built up from 64 N'Dama cows maintained for 3.5 years under a high natural tsetse challenge in Zaire. Matching health and performance data were recorded monthly to allow simultaneous evaluation of the effects of different criteria of trypanotolerance represented by time detected parasitaemic, parasitaemia score and packed red cell volume percent (PCV) on reproductive performance, calf weaning weight and cow productivity. Control of development of anaemia, measured by PCV value during trypanosome infection, had the major effect on all three performance traits. The repeatability of this criterion (0.33) was almost equal to that of calf weaning weight, indicating PCV measurement might be suitable for identification of more trypanotolerant animals. Simultaneous evaluation of the relative effects of control of development of anaemia in both the pre-weaner calf and its dam, on calf performance, suggested that its measurement in an animal might be feasible at an early post-weaner stage. Guidelines for work to develop practical field tests for trypanotolerance involving post-weaners maintained for varying lengths of time in high natural challenge situations are suggested.

Use of antigen-detection enzyme immunoassays in assessment of trypanotolerance in N'Dama cattle.

Antigen-detection enzyme immunoassays (ELISA) were used for the diagnosis of Trypanosoma vivax, T. congolense and T. brucei in N'Dama cattle in Gabon, Central Africa. The assays are based on monoclonal antibodies which recognise trypanosome antigens specific for each of the three species and animals were termed 'antigenaemic' when found positive by this technique but not found parasitaemic by the buffy coat technique. 148 one-year-old animals were exposed to a medium natural tsetse challenge and an average of 6 assays per animal were carried out over a 92-day period. Blood samples were routinely examined 11 times over this period and 28% of animals were detected as parasitaemic by the buffy coat technique. 90% of these were antigen-ELISA positive. More importantly, 40% of the animals with negative parasitological findings were also found to be antigenaemic. Parasitaemic animals with above-average packed-red-cell volume percent (PCV) values had 32% higher daily weight gains than those with below average, while antigenaemic animals showed no significant linkage between PCV values and weight gain. Thus only the 28% of animals with detectable parasitaemias could have been used for selection decisions based on PCV values. Antigenaemic animals grew at the same rate as negative animals and had 22% superior growth rates to parasitaemic animals. When antigenaemic animals were classified as having more ability to control parasite growth than parasitaemic animals, a significant sire effect suggested some possibility of a degree of genetic control being involved. Thus the ELISA could offer a practical possibility for selection of trypanotolerant animals based on infection criteria.

Epidemiology of bovine trypanosomiasis in the Ghibe valley, southwest Ethiopia. 1. Tsetse challenge and its relationship to trypanosome prevalence in cattle.

In this, the first of a series of papers on the epidemiology of bovine trypanosomiasis in the Ghibe valley, southwest Ethiopia, the tsetse populations and their relationships to the prevalence of trypanosome infections in cattle are described. The tsetse challenge to cattle at two sites sites in the area was estimated as the product of tsetse relative density and the trypanosome infection rate in flies. The proportion of feeds taken by tsetse from cattle was also considered. Three tsetse species were detected in the area, Glossina pallidipes, G. fuscipes and G. morsitans submorsitans. A significant correlation (r = 0.60, P < 0.001) was observed between the mean monthly estimates of tsetse challenge due to G. pallidipes and the prevalence of trypanosome infections in cattle the following month at one site, whilst at the other, no significant relationship was observed (P = 0.08). The tsetse density at both sites showed seasonal changes which were related to the monthly rainfall. Finally, variations in tsetse density appeared to be the main factor responsible for variation in tsetse challenge and thus trypanosome prevalence in cattle.

Evaluation of a field test for trypanotolerance in young N'Dama cattle.

In three separate tests in 1987, 1988 and 1989, a total of 436 one-year-old N'Dama cattle were maintained for 12, 18 and 24 weeks under a medium natural tsetse-trypanosome challenge in Gabon, Central Africa. Matching health and performance data were recorded on 4, 10 and 13 occasions respectively, to allow simultaneous evaluation of the effect of different criteria of trypanotolerance on animal performance. Under trypanosome prevalences of 25, 31 and 9%, respectively, ability to control the development of anaemia had a very major effect on daily weight gain, four times that of the ability to control parasitaemia, while previous exposure to trypanosome infection from birth to one year had no effect. Anaemia control, measured by average packed red cell volume percent (PCV) over the test period or by lowest PCV reached, was more closely associated with animal performance than when measured by average PCV when detected as parasitaemic. Above average PCV values in the first two measures resulted in a 44% to 48% superior daily weight gain over below average PCV values. PCV post-test recovery was shown to be rapid following a single trypanocidal drug treatment. In practice, it appeared that a suitable field test would be where natural infection could be effected as early in the test as possible and anaemia control measurements carried out over 6 weeks of detected parasitaemia. A field test would become even more feasible if satisfactory correlation could be obtained between the results of natural infection and those of an experimental alternative.

Patterns of Trypanosoma vivax and T. congolense infection differ in young N'Dama cattle and their dams.

Trypanosome infection was detected by the dark ground/phase contrast buffy coat microscopic technique in N'Dama cattle in a high natural tsetse challenge situation in Zaire. The data were used to compare the pattern of infection in very young animals and in their dams, and to evaluate how the pattern evolved in calves from birth to maturity, and thereafter in the different age groups represented by their dams. Five hundred and fourteen calves were evaluated at 3 week intervals for an average of 26 months each, over varying periods between birth and 42 months of age. Two hundred and sixty nine dams had matching records from parturition to calf weaning at 10 months. One month after weaning, animals were equally infected with Trypanosoma vivax and Trypanosoma congolense. From then until 42 months, the proportion of time an animal was infected with T. vivax relative to T. congolense gradually decreased. In the dams this trend continued from 4 years to at least 8 years of age by which time T. vivax infection was only one-third that of T. congolense infection. This finding is regarded as strong evidence of the ability of N'Dama cattle, in this region of Africa, to acquire significant control of the development of parasitaemia following T. vivax infection but apparently not following T. congolense infection. Pre-weaner calves, grazing with their dams, appeared to have considerable protection from, or be more resistant to, both T. vivax and T. congolense infections compared with their dams and to their own immediate post-weaning situations.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative phenotyping of N'Dama cattle for aspects of trypanotolerance under field tsetse challenge.

Matching animal health and performance data were recorded over the 2 year period from weaning at 10 months of age on 255 N'Dama cattle in a high natural tsetse challenge situation in Zaire. Four parameters that are regarded as possible indicators of trypanotolerance, species of trypanosomes detected, length of time parasitaemic, intensity of parasitaemia (parasitaemia score), and anaemic condition as estimated by packed cell volume (PCV) values, were measured and the relative effects of changes in these parameters on trypanocidal drug requirements and growth were assessed. The effects of species of trypanosome on drug requirements and growth were directly measurable. In the case of the other three indicators, the effects on drug requirements and growth that would be brought about by a change of one standard deviation in each were calculated. This allowed comparison of similar sized changes in these three indicators that are of necessity recorded in dissimilar units. Trypanosoma vivax and Trypanosoma congolense infections had equal effects on the number of trypanocidal drug treatments required, an average of 0.61 treatments being administered to each infected post-weaner. A reduction of one standard deviation (SD) in length of time infected reduced the number of treatments required by 0.23 or 36% and an increase of 1 SD in PCV reduced the number required by 0.27 or 43%. Changes in parasitaemia score were not important. In the case of growth, a T. congolense infection reduced growth by 12.4 g day-1 or 8% more than a T. vivax infection. A reduction of 1 SD in length of time infected increased growth by 9.8 g day-1 or 6.5%, a reduction of 1 SD in parasitaemia score increased growth by 9.0 g day-1 or 6.0%, and an increase of 1 SD in average PCV increased growth by 8.4 g day-1 or 5.6%. The necessity to simultaneously measure the four criteria is clearly indicated by their approximately equal effects on the final performance trait of daily liveweight gain. Thus, absence of information on any of these criteria would significantly affect the accuracy of the estimate of an animal's overall trypanotolerance phenotype in this central African situation and reduce the progress possible in production projects involving N'Dama cattle.

Relationships between trypanosome infection measured by antigen detection enzyme immunoassays, anaemia and growth in trypanotolerant N'Dama cattle.

Relationships were evaluated between trypanosome infection as measured by antigen detection enzyme immunoassays (antigen ELISA), anaemia as determined by average packed red cell volume (PCV), and animal performance as assessed by daily weight gain in 99 N'Dama cattle in Gabon exposed to natural tsetse challenge at 11.5 months of age and recorded 14 times over a 13 week period. Approximately half the animals were found to be infected for an average of five of the 14 times that they were examined: 38% with Trypanosoma congolense, 13% with Trypanosoma vivax and 49% with a mixed infection. Trypanosoma congolense infections had significant deleterious effects on animal growth, while T. vivax infections did not. Animals found on several occasions to be infected with T. congolense had significantly lower PCV values than those demonstrated to be infected on fewer occasions. No relationship was found between mean optical density (OD) values in antigen ELISA and PCV values. Animals capable of maintaining PCV values, even when antigen ELISA positive on a high number of occasions, grew at the same rate as uninfected animals. Animals that could not maintain PCV values when infected had poorer growth. Antigen ELISA has the potential to increase the efficiency of selection of trypanotolerant N'Dama cattle under tsetse challenge in the field, in three main ways. (1) Accurate identification of trypanosome species, especially in mixed species infections, clarifies relations between infection, anaemia and animal performance. (2) Detection of animals antigenaemic without patent parasitaemia could allow individuals with superior ability to control trypanosome infection to be identified. (3) More accurate measurement of the proportion of time an animal is infected allows more accurate evaluation of its anaemia control capability.

Measurement of trypanotolerance criteria and their effect on reproductive performance of N'Dama cattle.

One thousand and twenty-eight cow-year records were available from 260 N'Dama cows each having at least 2 years of monthly matching health and performance data over a 5-year period under a medium natural tsetse challenge in Gabon. Four hundred and fifty-eight calf/dam pairs were also available where the calf had been reared to weaning, both had monthly matching records and each cow had weaned at least two calves. Evaluations were carried out on effects of, and linkages between, environmental and stress factors, number and species of trypanosome infections, curative drug treatments given, anaemia measured by packed red cell volume (PCV), and performance measured by calf weaning weight, cow calving rate and cow weight change over the lactation period. Major findings were that over the period from calf birth to weaning, while calves and their dams grazing together had similar numbers of trypanosome infections detected, the Trypanosoma vivax: T. congolense ratios were very different: 1:0.7 in calves; 1:2.8 in cows. This indicated that some ability to control the development of parasitaemia following T. vivax infection might be being acquired, from weaning onwards. In cows, relationships between lowest PCV recorded and curative drug treatments given suggested that between 20 and 32% of trypanosome-infected cows were not being identified by the buffy coat parasitological diagnostic technique. The high level of curative treatment given (to 13.7% of cows over the calendar year, and to 40% of calves from birth to weaning) will have tended to reduce the variance and linkages between aspects of infection and PCV values, especially in calves. In calves, the influence of trypanosome infections, in both calf and dam, on their respective PCV values and hence on calf weaning weight was apparent. There was a 0.91 +/- 0.40 kg increase in calf weaning weight for each 1% increase in calf average PCV, and a 0.95 +/- 0.39 kg increase for each 1% increase in cow average PCV. In cows, there was a similar pathway of influence of T. congolense infection through the PCV values to calving rate--not significant with T. vivax infection. There was a 3.3 +/- 0.65% increase in calving rate for each 1% increase in average PCV. Repeatabilities of performance traits were in the normal range. Repeatabilities of numbers of trypanosome infections detected by the buffy coat technique were too low to have any practical significance. Repeatability of average PCV at 0.40 +/- 0.03 could allow PCV when infected to be used as one criterion of trypanotolerance.

Trypanotolerance in cattle and prospects for the control of trypanosomiasis by selective breeding.

Tsetse-transmitted trypanosomiasis is one of the major constraints on the expansion of the livestock and agricultural industries in Africa. The disease affects animals and man, with direct and indirect losses estimated in billions of dollars annually. Because of the phenomenon of antigenic variation, no vaccine is available. Current prophylactic efforts must rely on tsetse control by the use of insecticides and on trypanocidal drugs. However, recent advances in our knowledge of tsetse and trypanosome biology are offering hope for alternative methods of trapping tsetse, new drugs and even vaccination. Possibly of even greater significance is the increasing sense that Africa herself might be able to contribute to the resolution of this problem. Over a period of several thousand years, she has generated cattle, such as the taurine N'Dama and West African Shorthorn breeds of West and Central Africa, that are now known to possess a significant degree of innate resistance to trypanosomiasis and several other important infectious diseases. These cattle are extremely well adapted to the environment and are now recognised as having considerable production potential. The ability to resist the development of anaemia in the face of infection, as assessed by packed red cell volume percent (PCV), has been shown to be correlated with the capacity to be productive, thereby identifying regulation of PCV as a key trait of trypanotolerance. Thus, an estimate of the ability of an infected animal to maintain PCV, following either experimental or field infection, could be used as a method for identifying trypanotolerant individuals. This could provide a means of estimating trypanotolerance heritability, thereby permitting rational breeding programmes to be instituted. Africa may thus provide the answer.

Characterization of breeds of Bos indicus and Bos taurus cattle for maternal and individual traits.

Angus, Boran and Red Poll sires were mated to Ankole, Boran and small East African Zebu (Zebu) females to produce the cows characterized in this study, which produced calves by Friesian, Brown Swiss, Simmental, Boran and Red Poll sires. Individual traits included cow parturition weight, cow weight at weaning, cow mean weight and cow weight change from parturition to weaning; maternal traits included calf crop born percentage, preweaning viability percentage, overall viability percentage, birth weight, weaning weight, weight at 12, 18 and 24 mo and calf weight weaned per cow exposed to breeding (productivity index). Cows by Angus and Red Poll sires significantly exceeded cows by Boran sires (all cows in this comparison had Ankole and Zebu dams) in weaning weight of progeny and in cow weight at parturition and at weaning. Cows by Red Poll sires significantly exceeded cows by Boran sires in birth weight of progeny. Progeny of cows by Boran sires significantly exceeded progeny of cows by Angus and Red Poll sires in 24-mo weight. Cows by Angus and Red Poll sires did not differ (P greater than .05) in any of the traits analyzed. Cows with Ankole and Boran dams significantly exceeded cows with Zebu dams (all cows in this comparison had Angus and Red Poll sires) in progeny weight at birth, 18 mo and 24 mo and in cow weight at parturition and at weaning. Cows with Boran dams significantly exceeded cows with Zebu dams in weaning and 12-mo weight of progeny and significantly exceeded cows with Ankole dams in weaning weight of progeny. The significantly heavier weaning weight and significantly lighter 24-mo weight of the 3/4 Bos taurus progeny of cows with Angus and Red Poll sires relative to the 1/2 Bos taurus progeny of cows with Boran sires suggest that cattle that are 1/2 Bos taurus breed composition have greater general adaptation than cattle with 3/4 Bos taurus breed composition in the postweaning nutritive and climatic environment under which this experiment was conducted. Significantly heavier weights of Angus and Red Poll crossbred cows relative to Boran crossbred cows (all breeds crossed on Ankole and Zebu cows) indicate that the two Bos taurus breeds exceed the Boran (Bos indicus) breed in additive direct genetic effects for size when they have general adaptation to the environment.

Heterosis and breed effects on maternal and individual traits of Bos indicus breeds of cattle.

Data were analyzed to estimate the effects of heterosis and breed on a series of maternal and individual traits. Crossbred cows were Boran X Ankole and Boran X Zebu; straight-bred cows were Ankole, Boran and Small East African Zebu (Zebu). Cows of all breed groups were mated to Friesian, Brown Swiss and Simmental sires to produce crossbred progeny. While not generally significant, the average effects of heterosis of both crosses for the traits analyzed were: calf crop born, 7.0%; preweaning viability, 7.2%; overall viability, 7.3%; birth weight, 6.0%; weaning weight, 5.4%; 12-mo weight, 4.2%; 18-mo weight, 3.7%; 24-mo weight, 3.6%; calf weight weaned per cow exposed to breeding (cow productivity index), 24.5%; cow parturition weight, 3.5%; cow weaning weight, 4.2% and cow mean weight, 4.0%. Boran cows weaned 31.8 kg (48.0%) more (P less than .05) calf weight per cow exposed to breeding than Ankole cows. Boran cows were generally superior to Zebu cows in progeny weights at all ages (P less than .01). Boran cows weaned 34.5 kg (54.3%) more (P less than .05) calf weight per cow exposed to breeding than Zebu cows. Boran cows weighed an average of 70.8 kg more (P less than .01) than Zebu cows. Although progeny of Ankole dams were heavier (P less than .05) than the progeny of Zebu dams at all ages, the two breeds did not differ (P greater than .05) in calf weight weaned per cow exposed to breeding. Mean weight of Ankole cows was 75.8 kg heavier (P less than .01) than mean weight of Zebu cows.

Comparison of Bos taurus-Bos indicus breed crosses with straightbred Bos indicus breeds of cattle for maternal and individual traits.

Data were analyzed to compare crossbred females produced by crossing exotic Angus and Red Poll males to indigenous Ankole, Boran and Small East African Zebu (Zebu) females with straightbred females of the Ankole, Boran and Zebu breeds at the Ruhengere Field Station in the Ankole District of Southwestern Uganda. Progeny of the straightbred and crossbred dams were by Friesian, Brown Swiss and Simmental sires. Crossbred exotic X indigenous dams were favored over straightbred indigenous dams by 13.7% (P less than .01) in calf birth weight and by 14.8% (P less than .01) in calf weaning weight. Cross-bred cows exceeded (P less than .01) straightbred cows by 61.9% (48.5 kg) in calf weight weaned per cow exposed to breeding. Crossbred cows weighed 37, 46 and 42 kg more (P less than .01) than straightbred cows at parturition, weaning and cow mean weight, respectively. Angus X Boran and Red Poll X Boran crossbred dams were compared specifically with straightbred Boran dams. Exotic (Angus, Red Poll) X Boran crossbred dams exceeded straightbred Boran dams by 27.0% (P less than .05) in calf crop born, by 8.3% (P less than .05) in progeny birth weight and by 14.7% (P less than .01) in progeny weaning weight. Weight of calf weaned per cow exposed to breeding favored (P less than .01) the Angus X Boran and Red Poll X Boran crossbred dams over the straightbred Boran dams by 50.5% (50 kg).(ABSTRACT TRUNCATED AT 250 WORDS)

Heterosis, additive maternal and additive direct effects of the Red Poll and Boran breeds of cattle.

Data on 202 calves born, 191 calves weaned and 182 calves at 30 mo of age produced in 1969 from the Boran and Red Poll breeds and their reciprocal crosses were analyzed to estimate heterosis, additive maternal effects and additive direct effects on survival and growth traits from birth to 30 mo of age. The calves were produced in the Ankole District of south-western Uganda as a part of a beef cattle research program designed to provide information to support a commercial ranching development scheme in an area that had been cleared of the tsetse fly. Breed of sire effects were significant for birth and weaning weight, and breed of dam effects were significant for birth, weaning and 12-mo weight. Breed of sire and breed of dam effects that were significant showed the Boran breed to have higher values for all traits except breed of dam effects on birth weight; calves with Red Poll dams were 3.6 kg heavier (P less than .01) at birth than calves with Boran dams. Steers were significantly heavier than heifers at all ages except birth. Young cows (3 and 4 yr old) and old cows (9 yr old and older) produced calves that weighed significantly less at weaning, 12 mo and 18 mo than calves with dams that were 5 through 8 yr old. Heterosis effects were significant for weights at weaning, 12, 18, 24 and 30 mo of age; generally, heterosis effects on weight increased with age. Heterosis effects approached significance for survival from birth to 30 mo. Additive maternal effects were significant for birth weight; crossbred calves with Boran dams weighted 6.0 kg less at birth than crossbred calves with Red Poll dams. There was a reversal of the reciprocal effect between prenatal and postnatal gains; crossbred calves with Boran dams weighed 7.1 kg more at weaning than crossbred calves with Red Poll dams. Examination of additive direct effects showed the Boran breed to be significantly heavier than the Red Poll breed for birth weight and weaning weight. Additive direct effects showed the Boran breed to have generally higher (P greater than .05) values than the Red Poll breed for most other traits analyzed. Differences between straightbreds of the Boran and Red Poll breeds, which reflect differences in additive maternal and additive direct effects combined, significantly favored the Boran breed for weaning weight and 18-mo weight and favored (P less than .05) the Boran breed for all other traits except birth weight. It was concluded that the Red Poll breed had higher values for additive maternal effects for birth weight than the Boran breed in the climatic and nutritive environment in which this experiment was conducted; for most other traits evaluated, the Boran breed generally had higher values for additive maternal and higher values for additive direct effects than the Red Poll breed.

Production characters of the Sahiwal and Ayrshire breeds and their crosses in Kenya.

Data were analysed for lactation character of Ayrshire, Ayrshire x Sahiwal, Sahiwal x Ayrshire and Sahiwal females and on survival and growth characters of progeny of Ayrshire, Sahiwal, Simmental and Mixed breed sires mated to these females in the temperate highlands ecological zone of Kenya. The Ayrshire females had significantly higher total lactation yield than the other breeding groups; however, the Ayrshire females received preferential treatment. Despite this the Sahiwal-Ayrshire crosses were superior (3.8%) to the Ayrshire females in productivity index per cow per year because of a shorter calving internal and a higher level of calf survival. Sahiwal females had significantly lower total lactation yield than Ayrshire and Sahiwal x Ayrshire cross females. At 2 years of age female progeny of Sahiwal and Simmental sires were significantly heavier than the female progeny of Ayrshire sires but not of Mixed breed sires, viz. Sahiwal x Ayrshire and calves with sires not known. The Ayrshire breed was shown to be superior to the Sahiwal breed in transmitted effects for milk production and the Sahiwal breed was superior to the Ayrshire breed in transmitted effects for growth rate. It is concluded that the Sahiwal breed has potential to contribute to both milk and beef production programmes when combined in crosses with Bos taurus breeds of cattle in ecological zones similar to the lower to mid-potential areas of the temperate highlands of Kenya.

Trypanotolerance and the value of conserving livestock genetic resources.

Studies have been made in two main areas of genetic research on African trypanotolerant (N'Dama) cattle. The first is of the significance for performance and the heritability of trypanotolerance traits and the second is the search for markers of the breed type and the traits of interest. Results demonstrate significant effects of the ability of an animal to control parasitaemia and anaemia on its performance. Initial estimates suggested that parasitaemia measures had a very low heritability, but ability to maintain packed cell volume levels when detected as parasitaemic and to generate an immune response could form the basis of a practical selection approach. The search for markers has so far concentrated on the major histocompatibility complex and on a polymorphic system of common leukocyte antigens. Phenotypes that appeared more characteristic of the N'Dama in comparison with those of East African zebu cattle were examined for associations with trypanotolerance traits. One major histocompatibility complex encoded phenotype and two common leukocyte antigens gave indications of important associations. Planned research will further characterise the breed and define traits of importance to assist in increasing its productivity in Africa and thus in its conservation. The addition of a molecular genetics component to an integrated genetics research programme, principally through involvement in development of a linkage map of the bovine genome, will strengthen these efforts and make it possible to conserve specific N'Dama genes related to productivity.

Crossbreeding cattle in beef production programmes in Kenya. II. Comparison of purebred Boran and Boran crossed with the Red Poll and Santa Gertrudis breeds.

Two series of breed group comparisons for maternal traits were conducted at one location in Kenya. Data were collected on breed group series I for three years (Boran, 1/2 Red Poll-1/2 Boran (1/2 RP-1/2 B), 1/3 Red Poll-2/3 Boran (1/3 RP-2/3 B) and 2/3 Red Poll-1/3 Boran (2/3 RP-1/3 B) and on breed group series II for five years (Boran, 1/2 Santa Gertrudis-1/2 Boran (1/2 SG-1/2 B) and 1/2 Red Poll-1/2 Boran (1/2 RP-1/2 B]. The advantage in calf weight weaned per year per cow calving of 15.4% for the mean of three Red Poll-Boran crossbred groups over the purebred Boran in breed group series I can probably be accounted for by maternal heterosis. Thus the Red Poll breed probably does not exceed the Boran breed in additive maternal genetic effects in the environment where these data were collected. But the results suggest that the Red Poll breed exceeds the Boran breed in additive direct genetic effects for size while the Boran breed exceeds the Red Poll breed in additive maternal genetic effects as expressed by calf weight weaned per year per cow calving. Breed group series II comparisons show the Boran and the 1/2 SG-1/2 B breed groups do not differ (P greater than 0.05) in calf weight weaned per year per cow calving. The 1/2 SG-1/2 B breed group has maximum expected maternal heterosis; thus, it is indicated that the Boran breed exceeds the Santa Gertrudis breed in additive maternal genetic effects but the Santa Gertrudis breed exceeds the Boran breed in additive direct genetic effects for size.

Crossbreeding cattle in beef production programmes in Kenya. I. Comparison of purebred Boran and Boran crossed with the Charolais, Ayrshire and Santa Gertrudis breeds.

Data collected on a privately owned ranch located in the Machakos District of Kenya at approximately 2 degrees latitude south of the equator at an elevation varying from 1,675 to 2,000 m were analysed on five breed groups of cows: (1) purebred Boran, (2) 1/2 Charolais-1/2 Boran (1/2 C-1/2 B), (3) 3/4 Boran-1/4 Charolais (3/4 B-1/4 C), (4) 1/2 Ayrshire-1/2 Boran (1/2 A-1/2 B) and (5) 1/2 Santa Gertrudis-1/2 Boran (1/2 SG-1/2 B). The maternal traits evaluated included age at first calving, calving interval, calf weight at weaning and cow productivity index (calf weight weaned annually per cow calving). Mean cow productivity index for all cows was 192 kg; for purebred Boran, 174 kg; for 1/2 C-1/2 B, 200 kg; for 3/4 B-1/4 C, 191 kg; for 1/2 A-1/2 B, 210 kg; and for 1/2 SG-1/2 B, 185 kg. Cow breed groups 1/2 C-1/2 B, 3/4 B-1/4 C, 1/2 A-1/2 B and 1/2 SG-1/2 B exceeded (P less than 0.01) purebred Boran by 14.9, 9.8, 20.7 and 6.3%, respectively, in cow productivity index.

Genetic resistance to African Trypanosomiasis.

Genetic resistance to African trypanosomiasis occurs in certain breeds of livestock, many species of wildlife, some strains of mice, and possibly humans. The term trypanotolerance is used to describe this trait, which in domestic livestock is best exhibited by the indigenous West African taurine breeds of cattle, N'Dama and West African Shorthorn, that have been in Africa for 5,000 to 7,000 years. Confirmation not only that these breeds are genetically resistant to trypanosomiasis but also that they are at least as productive as other indigenous breeds is currently leading to their increased use in livestock-development projects in tsetse-infested areas. Trypanotolerance appears to be related to the control of parasitemia, a capacity associated with an event that regulates parasite growth and determines how rapidly the immune response is triggered. Identification of the factors that regulate this event and definition of their genetic basis may have important implications for the development of novel strategies for control of African trypanosomiasis.