Most developing countries are experiencing difficulties providing sufficient food (staples) for their resident population. In general in most third world countries food production is increasing at a slower rate than the population growth rate. Developing countries on average are increasing food imports by 10% per year and this is likely to continue into the foreseeable future. Whilst ruminants need not compete with humans for food there are few economic, large scale systems for pig and poultry production established in developing countries which do not rely on grain based concentrate or on crops such as potatoes and other root crops which also take up land that could potentially be used for human food production. The recent developments of sugar cane as a source of feed (juice) for pigs, however, has a major advantage which resides in the high biomass production per hectare and a great potential to form the basis of an integrated farming system (see Preston, 1990).
There has been far too little emphasis on developing systems of production of pigs and poultry on agro-industrial by-products. The exception being the molasses and swill-feeding systems for pigs that have developed in Cuba (Figueroa, 1989).
By-products high in sugar such as molasses, palm oil sludge and fruit and vegetable pulps lend themselves to feeding to monogastrics. The constraints to production are a low protein content and in some cases an associated high level of fibre which is an intestinally indigestible carbohydrate.
Pigs are able to utilise some fibrous feed, possibly up to 30% of their digestible feed intake because of caecal fermentation. It appears desirable that in the future, pig production should utilise at least a proportion of fibrous carbohydrate polymers. Free-range pig production, where breeding stock obtain a fair proportion of their feed from pastures is a good beginning forced on producers in Europe by the high cost of grain feeding in intensive fattening systems.
If the microbial ecosystem in the caecum of the pig is well developed, basal diets high in sugar and low in fibre combine well with vegetable proteins such as cottonseed meal which are fibrous.
The use of oil-seed protein residues for pigs and poultry are generally limited by the fibrous components and in some cases the presence of secondary plant compounds which may be toxic or simply lower production (e.g. gossypol in cottonseed meal). To overcome the constraint created by too much fibre it may be possible to develop pre-treatment to protect the protein and then find mechanisms for decreasing the fibre (e.g. growing fungi, such as the white rot species on the materials to decrease fibre and increase protein) or find methods to hydrolyse the cellulose to simple sugars before feeding.
Chemical treatment of the feed to hydrolyze fibre to monocarbohydrates may also be potentially possible (e.g. with pressurised steam and sulphur dioxide).
Delignification with say Coprinus fungi and treatment by pressure/steam of fibrous crop residues to produce simple sugars for inclusion in pig rations is a further possibility requiring research.
At least one group in the U.K. is attempting to introduce cellulase genes from Clostridial sources into embryos of pigs with the concept of producing pigs with a capacity to digest more cellulose. The enzyme, if expressed will be controlled by a promoter sequence encoding only in the pancreas (this is given in a little more detail below as an example of (a) the possibilities, (b) the problems and (c) the requirements for basic research prior to any developments which might be applied (Hazelwood & Gilbert, 1989)).
By virtue of their hindgut fermentation, pigs have about the same capacity as humans for digesting plant structural polysaccharides (Engelhardt et al. 1985).
In theory, substantially more energy would be available to pigs (and other simple animals) if cellulose and hemicellulose could be digested in the small intestine and the sugars absorbed. In addition the complex carbohydrates of grains (e.g. β-glucans of barley), fibrous protein sources (e.g. cotton seed meal) could be more efficiently used. In the latter case it would allow a much higher proportion of protein from such sources to be incorporated into the diets replacing more expensive less fibrous protein meals such as fish or meat meal.
The ability to digest cellulose and hemicellulose by intestinal enzymes is not present in any animal. It is probable that in early evolution these abilities were not advantageous.
However, if encoding a gene for microbial cellulase or hemicellulase could be incorporated into an animal's genome and expressed in the pancreas it may allow these enzymes to be produced and secreted along with the other pancreatic secretions. This is being tested by Hazelwood & Gilbert (1989) by introducing into the early embryo of mice (and they suggest they will do this with pigs) the cel E gene of Clostridium thermocellum which encodes a thermostable endogluconase with xylan-hydrolysing activity.
Expressions of this gene in transgenic mice will be regulated by the elastase I promoter/enhancer region (from rats) which is located “upstream” of the elastase gene and is responsible for restricting the synthesis of the digestive enzyme elastase to target sites of expression of other genes, to the exocrine enhancer cells in the pancreas of transgenic mice.
To modify porcine digestive enzyme secretion to include the enzymes, cellulase and hemicellulase, will require tissue specific expression of the cel E gene and secretion of mature active endogluconase enzyme into the intestinal lumen. For this reason gene constructs incorporating the elastase promoter/enhancer and cel E coding sequence have been designed.
The concept of transgenic pigs is at the ideas stage, the research will undoubtedly require a return to basic investigations, undoubtedly over and under expression of the genes when transferred will be a major problem.
Fibre breakdown in the pig by cellulase is likely to be slow and therefore gut capacity is likely to be a major constraint even when transgenic pigs are produced.
Undoubtedly porcine growth hormone injected in controlled amounts into pigs on grain based concentrates increase their growth rate, efficiency of growth and reduce fat deposition. The place of PSt in pig production, as it pertains to developing countries, is unknown. The small production units, the likely high cost of injections and the “scavenger system” generally operated at the small farmer level in developing countries suggests that PSt is unlikely to be used. The exception may be in grain-based feeding systems for pigs aimed at the markets provided by the higher income groups and tourists.
The technology for production of transgenic pigs has certainly been developed. Problems of over and under expression of the genes are still apparent. Overexpression results in considerable problems of leg weakness and reproductive inefficiencies whereas under expression gives no additional benefits. The nutrition of pigs expressing for growth hormone again has been under-studied with the usually emphasis being placed on the thrust to produce transgenic animals. The need to develop special feeding and management systems for pigs seems to have been ignored.
Where these techniques have been applied in developed laboratories the absence of disease is mandatory. It is conceivable that in addition to a “special type of nutrition” there may be special requirements for disease and parasite control. Disease and parasitism are more likely to impact heavily on pigs and other animals with a greater potential for growth.
The problems to be faced in pigs transgenic for porcine growth hormone are well summed up in a paper by Wilmut et al. (1988):
“The pig exhibiting human growth hormone did not grow any faster than normal pigs but they had less back fat which might be of potential value to pig farmers. Unfortunately, these transgenic pigs also suffered from a number of abnormalities, including arthritis, lack of co-ordination of their rear legs, susceptibility to stress, anoestrus in gilts and lack of libido in boars. So it is back to the bench to attempt to redesign the gene to avoid these problems.”