E. Mukasa-MugerwaInternational Livestock Centre for Africa (ILCA)
P.O. Box 5689, Addis Ababa, Ethiopia
Abstract
Introduction
Technical constraints to production
Disease constraints of small ruminants
Reproductive problems of small ruminants
Future prospects
References
Small ruminants contribute significantly to meat (and sometimes milk) production in Africa and hence in meeting the current shortfall created by the fast rising human population. Flock productivity may, however, be affected by a wide range of disease problems and reproductive wastage. The impact of disease may be through mortalities and abortions or through morbidity and such subclinical effects like weight loss or reduced gains, reduced carcass weight and quality and the cost of time and money involved in controlling or overcoming the effects of disease. Thus, a top priority in operating successful small ruminant enterprises must be adequate attention to health problems and the causes of reproductive wastage. Losses due to mortality often have severe and easily quantifiable economic impact, but losses in production due to morbidity are commonly underestimated because they are difficult to estimate. This paper provides a background against which specific effects of the role of health problems and reproductive wastage on production can be discussed.
The problem
Five major trends are taking place today which need to be recognised to put this presentation into perspective. First, there is a rapidly growing human population in the world's developing regions, illustrated for sub-Saharan Africa (SSA) in Table 1. In comparison to Latin America and Asia, where the rate of human population increase respectively peaked at 2.8% and 2.4% annually in the 1955-60 and 1975-80 period and has since declined to 2.3 and 1.7% today, the SSA population increase peaked to 2.8% in 1990-95 and is only just beginning to show some decline. A rapidly growing population is frequently associated with insufficient human nutrition and low intake of food of animal origin. Meat consumption in developed countries is, for example, six times as high as in developing countries and the disparity in milk consumption is over sevenfold (Krostitz 1991). Significantly, these gaps appear to be increasing.
Second, the rapid SSA human population increase has tended to reduce regional economic growth. For instance, per capita GDP decreased 2.2% in the 1980s resulting in lower personal incomes. Third, the rates of urbanisation as people move from rural to city locations in developing countries are growing at a faster rate that the overall rate of population increase (Table 2). But fortunately, forth, rapid urbanisation often tends to influence animal agriculture partly because of changing patterns in food demand brought about by the increased purchasing power of the city people. Such a positive trend ought to create better markets for farm products and increase commercialisation of agriculture. In fact, mean response in the demand for milk, meat and eggs is estimated at 8,9 and 9.5%, in contrast to 2% for cereals, whenever urban peoples' income has increased by 10% (Jahnke 1982). However, fifth, rural farmers are already also asking for higher income for their products.
Table 1. Human population projections for sub-Saharan Africa, 1990-2025.
|
Year |
Population (millions) |
Annual growth rate (% 5-year period) |
|
1990 |
498 |
- |
|
1995 |
580 |
3.1 |
|
2000 |
676 |
3.1 |
|
2005 |
784 |
3.0 |
|
2010 |
902 |
2.8 |
|
2015 |
1028 |
2.6 |
|
2020 |
1159 |
2.4 |
|
2025 |
1294 |
2.2 |
Source: Winrock (1992).
The projected target
The present low supply of animal products in the developing regions is, unfortunately, against the background that 52% of world cattle, 77°/0 of buffaloes 24% of sheep and 63% of goats are found in the tropics. Meat production efficiency, estimated as kg meat per livestock unit, is just 118 kg in SSA in contrast to 123 in Africa as a whole, 126 in Asia and 135 in Latin America. All are, however, still lower than the 155 average for all developed countries.
To meet the rising demand for animal products from indigenous sources, the World Bank estimates that animal production (meat and milk) must increase by 4% per year until 2025 to feed the population, improve nutrition and eliminate imports. At this rate milk production would reach 19 million tonnes and cow and goat milk 43 million tonnes. Ruminant livestock, including small ruminants, are expected to account for 60% of the expected increase in meat production and almost all of the milk. The rest of the meat will come from poultry and pigs. Currently, about 66 million sheep and 55 million goats are slaughtered annually for meat in Africa from respective populations of 206 and 171 millions. About 57% of the sheep and 65% of the goats in SSA are found in the arid/semi-arid agro-ecozone, 22 and 26%, respectively, in the humid/subhumid ecozone and 21 and 9%, respectively, in the highlands (ILCA 1987). The most rapid livestock population growth rate of 2.2% is taking place in the subhumid/semi-arid ecozone, followed by 1.0% in the highlands and just 0.2% in the arid areas.
Ruminant livestock are primarily raised under traditional systems characterised by low inputs. These systems cannot produce the increased amounts of crop and livestock products for the projected increases in human population. It is estimated that unless steps are taken to address major technical constraints to production and if current use of inputs remain unchanged, the land area required to support the population by the year 2010 would have to be expanded by more than 100 million ha, or 5% (FAO 1986).
Table 2. The rate of urbanisation as per cent of the total population in sub-Saharan Africa, 1960-2025.
|
Year |
% urban |
|
1960 |
11.8 |
|
1965 |
13.7 |
|
1970 |
15.9 |
|
1975 |
18.8 |
|
1980 |
22.0 |
|
1985 |
25.4 |
|
1990 |
29.0 |
|
2000 |
36.3 |
|
2010 |
43.5 |
|
2025 |
54.2 |
Source: Winrock (1992).
Major technical constraints to livestock production include inadequate feed supply, poor animal health, low-yielding animal genotypes and minimal levels of management. In particular, the need for increased human food and cash crop production in the tropics has reduced the size of grazing lands and land for intensive fodder production. This has made nutrition a major constraint to livestock production. Undernutrition further increases the risk to animals of disease.
Temperate ruminant animals are adapted to feed sources of relatively high fibre content but sufficiently unlignified to provide most dietary energy in the form of cellulosic carbohydrates. In these modern management regimes animals have access to well balanced arrays of nutrients, and the level of production is largely determined by intake levels. But this is not necessarily so among tropical animals. Tropical pastures and/or crop residues are often deficient in essential nutrients particularly trace elements and nitrogen needed by rumen microbes for efficient growth (Leng 1990). It is also possible that a low protein:energy ratio that results from this translates into a high heat increment in the rumen and high metabolic heat production in the body, which at times interacts with climate to produce heat stress which in turn reduces feed intake. Overall reproduction and productivity are thus further compromised. Inability to feed animals adequately throughout the year is the most important technical constraint for most traditional animal farmers. And, feed availability and cost will be the major determining factor as to whether the 4% annual increase target in production can be met.
Although indigenous breeds of goats and sheep are fairly well adapted to the tropical environments, the majority of animals are raised traditionally under extensive free-roaming management systems with no specialised input into housing care, nutrition or disease treatment or prevention. A small percentage of animals are, however, raised intensively, sometimes even under a cut-and-carry system of management. Here, animals are afforded better shelter, nutrition and health care and often achieve higher productivity. The type and impact of diseases on production varies with the level of management.
Common diseases
Table 3 lists the more important and common diseases of small ruminants by etiological agent. Diseases significantly reduce animal productivity irrespective of ecological zone.
Contagious caprine pleuropneumonia (CCPP) and peste des petite ruminants (PPR) are widely distributed diseases which unless properly controlled, can limit animal production over wide areas. Parasitic and viral infections are mainly vector transmitted and their prevalence is therefore largely influenced by the environment. No effective vaccine or chemotherapeutic control method is currently available for most of these ailments. It is estimated that SSA's annual losses due to mortality alone may be equivalent to 2 billion dollars with about that same amount due to losses in morbidity.
Table 3. Some common diseases of sheep and goats.
|
Infectious |
Broncho-pneumonia, contagious caprine pleuropneumonia (CCPP), caseous Iymphadenitis, contagious ecthyma, enteritis, peste des petite ruminants |
|
Parasitic |
Coccidiosis, helminthiasis, mange, trypanosomiasis, tick-borne disease |
|
Nutritional |
Energy and protein deficiencies |
|
Reproductive |
Abortions, metritis, perinatal mortality, stillbirths, mastitis |
|
Traumatic |
Hernias, fractures |
Parasitic infestations
Some studies have noted that the extensive grazing system of management for most sheep and goats, results in animal faeces being deposited widely which significantly reduces the chances of infection with third stage larvae (ILCA 1979). This would support the observation by Smith et al (1986) that despite being irregularly drenched, parasitic gastro-enteritis (PGE) in goats was negligible and clinically unimportant in housed goats maintained on slatted floors. However, other investigations have found a high incidence of PGE among roaming animals (Adeoye 1985; Ndamukong 1985).
Haemonchus contortus is the most important species of helminth parasites responsible for PGE in most ecozones except the cool highlands. It causes high mortality particularly among young animals in the wet season. Other species include Ostertagia, Trichostrongylus, Cooperia, Nematodirus and Oesophagostomum.
Coccidial oocytes are common in the faeces of grazing animals but clinical cases are more overshadowed by helminthiasis. Mortality rates of up to 6% have been observed in confined goats (Otesile et al 1983).
Literature reports are available of small ruminant infection from Babesia motasi, Anaplasma ovis, A. marginale, Trypanosoma vivax and T. congolense (Sadiq 1985; Smith et al 1986; Opasina 1987). There is, however, no definite association between the serological demonstrations of these infections and clinical disease.
On the other hand the mange mite (Sarcoptes, Chorioptes and Psoroptes spp) is common, sometimes with incidence of 24 to over 80% even for roaming animals (Akinboade 1982; Adeoye 1985). With regular washing this may be reduced to under 5% (Smith et al 1986).
Bacterial and mycoplasmal diseases
Pneumonia is easily the most important bacterial health problem to small ruminant production, being responsible for 6 to 31 % of morbidity rates (Ndamukong 1985; Njau et al 1988). In addition to verminous pneumonia caused by lung worms, microorganisms commonly associated with small ruminant pneumonia are: Mycoplasma mycoides, Pasteurella multocida, Streptococcus spp, Staphylococcus aureus and Corynebacterium pyogens.
Gastro-enteritis is a fairly common ailment of goats and sheep. The extent to which this is attributable to bacteria is, however, difficult to ascertain. This is because enteritis is also commonly associated with helminthiasis and dietetic diarrhoea The other condition that can be mentioned is Caseous Iymphadenitis which may have a morbidity rate of 13 to 18% especially in confined goats (Addo 1982; Smith and van Houtert 1988). It tends to have low mortality.
Viral diseases
The two most common viral diseases of small ruminants are contagious ecthyma (Orf) and peste des petite ruminants (PPR) with morbidity rates of 50 to 100%. Mortality rates are often high for PPR but low for Orf.
Metabolic disorders
Metabolic disturbances like pregnancy toxaemia, mineral deficiencies and rickets have occasionally been observed. They are more likely to occur among confined institutional flocks with limited regard to proper house design, emphasising the role of management system and level on the spectrum and impact of diseases on production.
Effects of disease on inputs
The effects of disease on inputs revolve around the cost of treatment or prophylaxis. This includes drugs, chemicals and vaccines plus the cost of application and/or veterinary care. The cost will accumulate substantially in case treatment must be repeated or performed regularly (e.g. for tick or tsetse control), or when animals must be gathered over long distances for treatment.
Common problems
The major reproductive problems of confined or free-roaming small ruminants include: abortions, still births, agalactia, mastitis, metritis, dystocia and perinatal mortality (Falade and Sellers 1976; Smith and van Houtert 1988).
Many of the above problems are associated with systemic diseases that lower the overall performance of the animal, while others specifically cause foetal mortality, abortion or male infertility. Foetal mortality and abortion can be caused by vibriosis (due to Camphylobacter fetus intestanalis), salmonellosis (due to Salmonella abortus-ovis and S. dublin), Listeria monocytogenes, Chlamydia, sheep ticks (Ixodes ricinus) carrying tick-borne fever, Border disease, mycotic abortion, Q' fever (Coxiella burnetti) and Toxoplasma gondii. It is noteworthy that Brucella abortus is not a major cause of abortion in sheep.
Infertility in male animals may arise from infectious epididymitis (caused by B. ovis and Actinobacillus seminis).
Effects of disease on output
The effects of disease on output may be direct or indirect. Direct losses include mortality and morbidity. Overall losses due to livestock mortality in sub-Saharan Africa have been estimated at US$ 2 billion (de Haan and Bekure 1991). Losses due to morbidity as reflected by reduced growth, lactation, work output and reproduction (judged by lambing interval, lambing percentage, and delayed puberty etc) are probably of the same magnitude. This is best reflected by offtake rates for meat, the main purpose for which small ruminants are raised, which remain low (32%) in Africa. Furthermore, mean carcass weight is only 12-13 kg per head with the result that regional annual mutton/lamb and goat meat production in Africa is only 880 and 653 l/year.
Another important effect of disease on production is that many times diseases alter the value of the animal by changing its conformation or rendering the products unfit for human consumption. The products are therefore condemned for human use. Furthermore, substantial revenue is lost annually because of the failure of many potential producers to meet the sanitary requirements of lucrative export markets.
Among the more indirect effects of disease is the inability of farmers or producers to utilise favourable grazing lands or resources, adopt new systems of animal management, introduce more productive genotypes or fully utilise specific animal products such as draft power because the presence of disease increases the fear for subsequent morbidity or mortality.
Diseases may also indirectly affect production through restrictions imposed on animal products as a result of treatment or vaccination, and the implementation of control regulations following the outbreak of certain diseases. The latter may range from the enforcement of animal movement to the actual slaughter of sick animals or even those in contact. Where animals are restricted in their movement, it is not uncommon for overgrazing to follow as another complication.
Combined effects of disease and reproductive wastage on production
The impact of disease and reproductive wastage on small ruminant (sheep and goat) production is best estimated by the annual reproductive rate (RR) which is defined as the number of lambs or kids weaned per ewe (or doe) of reproductive age per year. This estimate is influenced by litter size (S), lamb mortality (M) and lambing interval in years (I) as follows: RR = S(1M)/I. The trait is thus significantly influenced by litter size, young mortality, interval between parturitions and morbidity.
Litter size
Litter size in tropical sheep ranges from 1.0 to 1.5 indicating that twinning rate in sheep ranges from 0 and 50%. But, mean litter size may be less than 1 indicating that the rate of pre- and perinatal losses in the form of stillbirths is greater than the twinning rate in the flock. There are some recognised prolific breeds in Africa, e.g. the West African Dwarf and the Moroccan D'Man breed which commonly deliver two or more young.
Litter size is largely determined by ovulation rate but is also modified by fertilisation rate and embryonic and foetal losses. For example, although the ovulation rate of D'Man ewes averages 2.5, only about 42% of the eggs develop to full-term foetuses. Ovulation rates vary among breeds, increase with ewe age up to 6-7 years and among seasonal breeders are greatest in the first half of the breeding season. The trait is highly repeatable within individuals.
Young mortality and the role of the dam
Young mortality rates in sheep and goats from birth to weaning range from 5 to more than 50% and represent a serious reduction in biological efficiency because resources invested by and in dams to initiate and maintain pregnancy are wasted. Major causes of mortality include starvation-mismothering-exposure (the SME syndrome), pneumonia, enteritis, accidents, navel infection and septicaemia (Njau et al 1988; Jordan and LeFevre 1989; Chaarani et al 1991).
Because starvation and enteritis are two of the most important causes of neonatal mortality, it is necessary to ascertain if lambs/kids receive adequate colostrum and subsequent nutrition. Maternal behaviour is therefore very critical to the newborn immune and nutritional status. The ability of dams to recognise and bond with their young occurs during the initial sensitive period, a narrow window of time limited to the first few hours. Dam ability to recognise progeny increases with parity and is often impaired by overcrowding (Alexander 1984; Poindron et al 1984).
Significance of maternal behaviour in young survival varies with management strategy and environmental conditions. But, whatever the subtle signals may be, failure of the maternal-neonatal bond in the first few hours may result in poor maternal behaviour and failure to nurse adequately. The trait is probably also genetically influenced but this has not been adequately investigated in tropical breeds.
Among other dam traits commonly associated with reduced young viability and survivability is weight and body condition score during gestation. Adequate nutrition not only improves conception rates at mating, it increases young birth weight and decreases perinatal mortality (Alexander 1984; Holst et al 1986; Barlow et al 1987; Jordan and LeFevre 1989; Yapi et al 1990; Mukasa et al 1994). Low nutrition in early pregnancy also results in 90-day foetuses with smaller linear body measurements (Parr et al 1986).
Placental size is a reasonably good index of placental function. Decreased placental size limits foetal growth (Bell 1984). Placental weights vary widely due to endocrine factors, nutrition, heat stress, and such maternal factors as age and genotype. Although the number of cotyledons is also quite variable, foetal weight is more correlated to the functional cotyledon weight and the chorio-allantoic weight (Hinch et al 1985; Parr et al 1986).
Finally, newborn rectal temperature is an important factor for survival (Peelers et al 1991). Many times, perinatal death results from the inability to maintain adequate body temperature outside the uterus. Barlow et al (1987) concluded that lambs which died had a significantly lower average body temperature one half hour after birth than those which survived. Wind conditions often tend to exacerbate the heat loss.
Young adaptation to cold stress
During cold exposure many metabolic hormones like thyroid hormone, growth hormone, insulin, glucagon, gut hormones and adrenal hormones may be involved in altering intermediary metabolism. These may be influenced by somatostatin. In fact, growth potential of young lambs can be maintained during cold stress by active immunisation against somatostatin. Somatostatin activity in lambs is suggested to be concentrated in the lower gut where it may be involved in nutrient absorption. This has also been reported for humans, dogs and rats. These observations have important implications for neonatal mortality rates among sheep raised in the high altitude tropics and may provide useful tools for selecting for perinatal survival.
Flock productivity indices
Despite some limitations, productivity indices can be used to measure the level of flock productivity. Illustrated below for a study that involved Menz ewe lambs (Mukasa et al 1994), initial lamb weaning weight and subsequent parturition interval were used to construct three indices (Fall et al 1982; Wilson 1983; Wilson et al 1985) as follows:
|
Index I |
= (total live weight of litter at 90 days x 365)/subsequent parturition interval in days |
|
Index II |
= Index I/post-partum dam weight (ppwt) in kg |
|
Index III |
= Index I/ppwt0.73 of dam (kg) |
Index I is derived in kg per ewe and represents lambing interval as a percentage of a year and considers lamb performance until weaning. Because the dam is the source of major input up to weaning, productivity was also assessed in g per kg dam weight (Index II) or in kg per kg metabolic weight (Index III). Indices for ewes whose offspring died before weaning were zero, the zeros being incorporated in the means and standard errors for the variables under consideration. The mean value of Index I was 11.0±0.87 kg, Index II was 520±41 g and Index III was 1.18±0.09 kg. Because Indices II and III are calculated in terms of dam body weight or metabolic weight, it becomes possible to gauge the efficiency of available feed utilisation
Disease through morbidity and mortality and poor reproduction largely manifested as reproductive wastage, significantly reduces flock productivity and may make small ruminant rearing uneconomic. The following sequence of measures is suggested to minimise the impact: major causes of disease and low reproduction must be identified followed by the application of appropriate and well thought out prophylactic measures to control diseases and minimise reproductive wastage.
The type of package recommended will, therefore, among others, vary with: production system, farmer acceptability, drugs and vaccine availability and cost and disease profile etc. particularly if the other management inputs are adequate.
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