P. JEANNIN, A.S. GRIEVE, K. AGYEMANG, D.J. CLIFFORD, C.D. MUNRO and R.H. DWINGER
Introduction
Materials and methods
Results
Discussion
References
Herd reproductive performance determines herd growth, herd replacement and offtake rates. The age at first calving, the length of the reproductive life of the breeding female and the calving interval are the key parameters involved in herd reproductive efficiency.
The cattle population in the Gambia is approximately 300,000 head. (Straw and Hoste, 1987). The high density of about thirty cattle per square kilometre land illustrates the cattle keeping tradition.
The study was undertaken to determine reproductive efficiency in the village herds of N'Dama cattle in the Gambia. The relationships among reproductive efficiency, environmental and disease factors were investigated. To assess effect of nutrition on the onset of ovarian activity in the post-partum period, a study was conducted on twenty-four first-calving N'Dama cows kept under improved nutritional conditions.
Reproductive performance under village management
Villages
The data presented in this study were collected on the south bank of the Gambia River in the Kiang West District, in the villages of Keneba and Nioro Jattaba which are 80 and 100 km, respectively, inland from the sea.
These two villages are in an area of savanna and riparian woodland, long-term fallows and fallow fields with low bush. Seasonally, high densities of Glossina morsitans submorsitans may occur in this area (Snow and Rawlings, personal communication).
Climate
In the Kiang West District, the rainy season lasts from 3 to 5 months, between June and October. The rainfall varies considerably from year to year in quantity and distribution. The highest precipitation is usually in August. The remaining seven months of the year are hot and very dry, except for two transitional periods, cold and dry from November to January and hot and damp but rainless from May and June. The severity of the long dry season is accentuated by diurnal ranges of temperature through 25°C (Giglioli and Thornton, 1965).
Nerd management
The Gambian cattle are tethered individually overnight. From January to June, village herds are not herded. From early morning to late in the evening the animals trek long distances in search of feed and water. They maintain themselves on standing dry grass if available and woody browse. During the cropping period from July to December, the cattle are herded to graze extensively on communal grazing land. During this time, although there is an abundance of natural forage and crop residues, grazing time never exceeds 6 hours a day. Major aspects of herd management were similar to those described by Dunsmore et al., (1976).
Data collection
The village herds were identified and animals ear-tagged in October 1985 and in February 1986 in Keneba and Nioro Jattaba, respectively. At ear-tagging, the dam number, the sex, the estimated year of birth, the rank of calving and the number of carvings were collected for each animal. Frequent herd visits by livestock assistants stationed in each village provided information on the subsequent entries and exits. Monthly weighing of all animals using crushes and portable electronic scales (Barlo) was performed.
Blood and faecal samples were collected every month. Each month, the PCV was determined using the microhaematocrit method. Subsequently the haematocrit tube was cut and the huffy coat examined for the presence of trypanosomes using the dark ground method (Murray et al., 1977). Thin bloodsmears were made from all animals with PCV below 20% to investigate the presence of other protozoan parasites. Faecal samples were examined for coccidia, oocysts and helminth eggs using the McMaster technique, as well as for liverfluke eggs using a zinc sulphate method (Murray et al., 1983). Serum samples of all the animals were collected every six months. Investigations on antibodies against trypanosomes and Brucella were performed.
Onset of ovarian activity in first calving N'Dama cows kept under improved nutritional conditions
Animals and management
The animals were kept at the headquarters of the ITC. Although grazing, the animals had supplementary feed from 12 months before calving and onwards. Each cow was given 4 kg a day of a mixture of rice bran and sesame or groundnut cake (ratio 8 to 1). All cows calved between January and March 1987.
Data collection
Twenty-four cows from this herd were bled daily from early post-calving until pregnancy was established, or until 130 days post-partum. For plasma progesterone estimations blood samples were collected by jugular venipuncture into 5 ml EDTA vacutainer tubes. The samples were carried on ice to the laboratory, centrifuged within two hours after collection and stored at -20°C until assayed.
Progesterone assay
The progesterone assays were carried out using a modification of the Enzygnost Serum Progesterone Test Kit (Hoechst) and the assays read using a microelisa plate reader at 492 mm (Titertek Multiskan). The test was modified by using plasma from a dexa-methazone suppressed N'Dama ox to reconstitute the kit progesterone standards. Unknowns were predicted from a linear regression equation based on the log transformed optical density readings. Quality control samples gave an interassay coefficient of variation of 21%. Active ovarian function was supposed to have resumed when levels of plasma progesterone were equal to or greater than 2 ng/ml.
Reproductive performance of N'Dama village herd
Nerd composition
Table 1. Composition of Keneba and Nioro-Jattaba village herds on 1/01/1987.
|
Age Group |
Number of animals |
Percentage/Total herd (%) |
||
|
Keneba |
N. Jattaba |
Keneba |
N. Jattaba |
|
|
Suckling Calves (SC) |
96 |
217 |
24.4 |
22.2 |
|
female SC |
44 |
113 |
11.2 |
11.5 |
|
male SC |
52 |
104 |
13.2 |
10.7 |
|
Weaned Calves (WC) |
113 |
100 |
28.7 |
20.3 |
|
female WC |
49 |
123 |
12.4 |
12.6 |
|
male WC |
60 |
67 |
15.2 |
6.8 |
|
castrated male WC |
4 |
9 |
1.1 |
0.9 |
|
Adults (A) (>3 yrs old) |
185 |
563 |
46.9 |
57.5 |
|
A female |
148 |
467 |
37.6 |
47.7 |
|
A male |
16 |
53 |
4,0 |
5.4 |
|
A castrated male |
21 |
43 |
5.3 |
4.4 |
Keneba herd size = 394 animals
N. Jattaba herd size = 979 animals
Table 1 summarizes the herd composition on 1/01/1987 in Keneba and Nioro Jattaba, respectively. The higher percentage of adult breeding females in Nioro Jattaba than in Keneba was associated with a lower percentage of suckling and weaned male calves. The ratios, adult entire males per adult breeding females, were high, 10.8 and 11.3% in Keneba and Nioro Jattaba, respectively.
Age at first calving
Although a few animals are able to calve between 3 and 5 years of age, the mean age at first calving is between 5 and 6 years.
Calving interval
The second calf is produced at 7 years, the third between 8 and 9 years and the fourth between 10 and 11 years. However, the annual calving rate in 1986, calculated as the ratio of number of carvings per average number of adult breeding females kept during the year, was 52 and 47% in Keneba and Nioro Jattaba, respectively. These calving rate figures would correspond to a calving interval of 23 to 25 months.
Productive life time
The number of breeding females is constant from 3 to 7 years. In 1986, the exit rate for that age group was 5.3%. From 7 to 10, the number of breeding females was reduced by 50%. In 1986 the exit rate for that age group was 14%. From 10 years onwards, the number decreases rapidly. In 1986 the exit rate for that age group was 37%. Details on exit type for these age groups of breeding females are summarized in Table 2.
Table 2. Exit types and rates of adult breeding females per age category in 1986 in Keneba and Nioro-Jattaba.
|
Age Category (years) |
Mortality Rate (%) |
Disposal Rate (%) |
Total Exit Rate (%) |
|
3-7 |
0.9 |
4.4 |
5.3 |
|
7-10 |
4.2 |
9.7 |
13.9 |
|
>10 |
5.6 |
31.5 |
37.1 |
Calving distribution
Although large numbers of mature bulls are in the herd throughout the year, there was a marked seasonal block calving, with 86% of all carvings occurring between August and December 1986. The inferred natural block conception period, November to March, is the early part of the dry season.
Monthly liveweight changes
Figure 1 shows the pattern of mean monthly liveweight for both dry and lactating adult females in Keneba from December 1985 to September 1987. During that period, 60% of the breeding females were lactating while 40% were dry.
The overall mean liveweight for all adult females (n=2813) was 213 (s.d. 29.3) kg. The mean body weight for dry cows (223, std. 29.0) was significantly higher (P<0.01) than the mean body weight for lactating cows (207, std. 27.8). Severe liveweight losses, 15 to 25% of the mean, in both lactating and dry cows coincided with the later part of the dry season in both 1986 and 1987. The rains started later in 1986, mid-July, than in 1987, early June. Accordingly, the liveweight losses during the late dry season were more severe in 1986 than in 1987. The mean monthly liveweight of lactating females at month of conception and conception rate were positively correlated, (r=0.73, P<0.1, n=13). Moreover the mean liveweight for all cows in December and January (221, std. 27.2) was significantly higher (P<0.01) than the mean liveweight for all cows in the remaining months (211, std. 29.4).
Figure 1. Mean monthly liveweights for dry and lactating cows in Keneba (in-1986/87).
Monthly PCV changes
Figure 2 shows the pattern of mean monthly PCV for both dry and lactating adult females in Keneba from December 1985 to September 1987. The overall mean PCV for all adult females (n=2813) was 26.4, std. 4.5. The overall mean PCV for dry cows (27.6, std. 4.8) was significantly (P<0.01) higher than the overall mean PCV for lactating cows (25.8, std. 4.1). Moreover the mean PCV for all cows in December and January (28.8, std. 4.3) was significantly higher (P<0.01) than the mean PCV for all cows in the remaining months (25.9, std. 4.3). Furthermore, mean monthly PCV of dry cows was positively and significantly correlated with mean monthly liveweights (r=0.63, P<0.001, n=22). The same applied to the mean monthly PCV of lactating cows, which were correlated with mean monthly liveweights (r=0.74, P<0.001, n=22).
Figure 2. Mean monthly PCV for dry and lactating cows in Keneba (in 1986/87).
Diseases
Blood microscopic examinations revealed a low trypanosome prevalence in village herds, 2 and 0.5% in Keneba and Nioro Jattaba, respectively, throughout the study period. This result was confirmed by the serological results. In Keneba, high titres of antitrypanosome antibodies at ear-tagging decreased dramatically confirming that tsetse challenge had fallen.
Other blood parasites, such as Babesia and Anaplasma, were never diagnosed. Strongyles and coccidia were mainly found during the rainy season and in the young animals. Liver fluke and other trematodes were never encountered.
Brucellosis was identified as a major problem in Nioro Jattaba. The abortion rate was 5%. Of 436 serum samples collected in May 1987, 14% were found to be positive after Rose Bengal and seroagglutination tests. Although a few light calves, less than 10 kg, were born in Nioro Jattaba, birth weight was not significantly lower in Nioro Jattaba than in Keneba. Male and female birth weight in Keneba and Nioro Jattaba were 16.6 (s.d. 3.0), 16.2 (s.d. 2.8), 17.0 (s.d. 3.1) and 15.7 (s.d. 2.8), respectively.
Onset of ovarian activity in the post-partum period of 24 supplementary fed N'Dama cows
All 24 animals succeeded in rearing their calves up to weaning. By 60 days post-partum, 12 of the 24 animals showed an increase in plasma progesterone levels (P4>2ng/ml), indicating resumption of ovarian cyclicity. For 4 out of the 24 animals, the record of service was not collected. In the other 20 animals, the mean calving to first service interval was 69 days, ranging between 19 and 194 days. At the end of October 1987, 8 months after calving, four cows were not pregnant. Two cows were pregnant without any service date information. Of the 18 remaining animals, conception occurred 26 to 194 days post partum with an average calving-to conception interval of 80 days.
Existing knowledge of cattle reproductive performance in The Gambia is based largely on the results from the Gambia/Glasgow University collaborative trypanosomiasis research programme, now continuing under the ITC (Straw and Hoste, 1987). The results gave the estimated age at first calving as between 40 and 60 months and estimated calving rates were 50 to 60% and 35 to 43% in areas of low and high tsetse challenge, respectively, (Clifford, 1986). Dunsmore et al. (1976) believed that calving rate varied substantially from year to year, from high to low trypanosomiasis challenge areas, but estimated the mean to be only 41%. Although slightly different, both estimates from literature and from this study indicate a poor reproductive efficiency of N'Dama cattle kept under village management in The Gambia. The results of this study suggest the importance of the effects of various environmental and disease factors on reproductive performance.
Undernutrition and reproductive performance interactions in cattle are well documented (Haresign, 1983; Holness, 1983; Gauthier et al., 1984; Spitzer, 1986). One of the major difficulties in interpreting nutrition-reproduction interactions is that of measuring nutritional status. Nutritional status can be defined in terms of requirements (physiological status) and supply of energy, protein and other dietary components. Unfortunately, it was not possible under these village conditions, characterized by extensive grazing and browsing, to assess feed -intake. Liveweights, used in the current study, were the best estimate of the nutritional status of the animals. Such measurements describe long-term nutritional status only, while many of the components of the reproductive process in the female (e.g. ovulation, fertilisation, implantation) take place over a relatively short time-period (Haresign, 1983). However, even if the small number of pairs of values (n=13) was responsible for a poor confidence interval, the mean liveweight of lactating females at month of conception was positively correlated with conception rate.
In this study, the liveweight of the adult breeding females varied considerably throughout the year. Liveweight losses coincided with the mid and late dry season, while liveweight gains coincided with the onset of the rains and consequent growth of grass. Animals reached their best body condition in the early part of the dry season after they had benefitted from the green forage during the rains and from crop residues after harvest. It was mainly during this period of the year that cows successfully conceived, probably because of the influence of feed intake on body condition. Similar findings in dairy cows in Europe showed that actual feed levels, body condition and interaction between these two parameters around the time of mating were positively correlated with reproductive performance (Youdan and King, 1977).
In spite of much information on the observational components of the nutrition-reproduction relationship, relatively little is known of the underlying physiological mechanisms involved (Haresign, 1983). However, it has been demonstrated that the key to early return to oestrus after calving were adequate pre-partum levels of nutrition, a consequent good body-condition score at calving and adequate levels of nutrition after calving to prevent important liveweight losses (Holness, 1983; Gauthier et al., 1984). The relationships between nutrition and both ovulation rate and length of the calving to the conception interval are well established (Haresign, 1983). A recent study carried out on N'Dama cattle in village herds in The Gambia demonstrated that post-partum anoestrus was a major factor responsible for extended calving intervals. Of 176 cows, only 6.8% started cycling within 110 days post-partum (Gyawu, in press).
The preliminary results from the 24 supplementary fed N'Dama cows at ITC already demonstrate that calving to resumption of the ovarian-cyclicity interval could be shortened by improved feeding.
In addition to its effect on ovulation, resumption of postpartum cyclic activity and fertility, underfeeding can delay puberty in both males and females and therefore delay age at first calving (Gauthier et al., 1984). In 14-month-old creole heifers, the percentage of cyclic females was a function of the mean daily weight gain (Gauthier and Thimonier, 1982 cited by Gauthier et al., 1984).
The mean PCV throughout the study period was lower than PCV values for N'Dama cattle reported by other workers (Walshe and Gilles, 1962; Roberts and Gray, 1973; Toure et al., 1978; Poivey et al., 1983). This low mean PCV value indicates that N'Dama cows in the two villages studied suffer from chronic anaemia. However, the prevalences of helminthiasis, trypanosomiasis and other blood parasites were low. Therefore these infections could not account significantly for this anaemic status. Moreover mean monthly PCV was highly and positively correlated with mean monthly liveweights in both dry and lactating adult breeding females.
Consequently PCV and liveweight changes may only reflect the underfed status of the N'Dama cow in the two study villages. This emphasizes the need to improve the nutrition of the cows to enhance the reproductive efficiency of the N'Dama kept under village conditions. Moreover, it shows that, when evaluating the importance of trypanosome infection through its effect on PCV, it will be essential that other anaemia producing factors such as poor nutrition are identified and their effects estimated.
Unfortunately, because the trypanosomiasis challenge was low it was impossible to assess the effect of this disease on herd reproductive performance. Brucellosis was identified as a major problem in Nioro Jattaba. Although the study of the effects of brucellosis on reproductive performance is difficult (Plommet, 1971), its importance should not be neglected in the village herds of The Gambia.
Little is known about the effects of climatic factors on the reproductive process of both the N'Dama cow and the N'Dama bull. In this study the successful conception period coincided with major seasonal climatic changes. November to March are the months with the lowest mean monthly humidity rates. This period also coincides with the lowest mean monthly minimum temperatures and with the lowest mean monthly temperatures recorded throughout the year (Figure 2).
Suppression of conception rates during hot months of the year has been well documented for Holstein cows. They respond with reduced sexual activity, reduced length and intensity of heats. Moreover heat stress may lead to embryonic death (Thatcher, 1983; Monty, 1983; Thatcher and Collier, 1986). The N'Dama may differ from the Holstein cow, but the need for further research in that field becomes more important at a time when the trypanotolerant trait of the N'Dama cattle is to be developed. How the spermatogenesis, libido and sexual behaviour of the N'Dama bulls are affected by climatic factors are also areas needing further research.
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