Low Phosphorus Enviropigs

Hog ranches have gone industrial in most of the developed countries of the world and no longer bear much resemblance to storybook images of Old-MacDonald-style family farms. In a modern swine factory, huge rectangular barns house hundreds or thousands of pigs who seldom see the light of day. The animals are fed and bred indoors, and their urine and feces are washed through floor slats to be piped into waste lagoons. As these impoundments fill, surplus offal is sprayed on surrounding fields to further decompose and dissipate. Intensive hog farming increases animal production per unit of land and lowers the farmers' costs, but this often comes at great expense to the well-being of nearby property owners and to the environment.

Hog-farm odors can be overpowering, but they are only a part of the broader stink for society. Hog excrement is loaded with nutrients that can become pollutants when they find their way into streams and aquifers. This sometimes occurs in dramatic fashion. In June 1995, a large hog-waste lagoon ruptured in North Carolina, sending 25 million gallons of putrefying mass into the New River. Three weeks later, another spill of 1 million gallons spread into tidal creeks adjoining North Carolina's Cape Fear estuary. Apart from assaulting the senses, such catastrophes cause toxic algal blooms that result in massive fish kills.

Other leaks from hog farms are small or chronic, but cumulatively no less harmful to the environment over the longer term. When leached into streams or lakes, hog-farm nutrients promote a eutrophication process typically characterized by excessive growth of phytoplankton (aquatic algae) that depletes oxygen, disrupts food webs, and seriously degrades the ecological health and biotic richness of natural waterways.

One of the worst such pollutants is phosphorus (P). In the past, pigs obtained this essential dietary element from phosphate (PO4) compounds in plants and other foods they ate. On commercial farms today, however, a primary hog food—corn seed—is a poor source of phosphorus for the animals. In the kernels of corn (unlike in germinated plants), about 70% of the phosphate is locked up as phytate, a substance that remains mostly unusable by nonruminant animals with simple stomachs, such as hogs and chickens. Thus, most of the phosphorus in phytate passes through the animals' guts to pollute the environment.

To alleviate this dietary deficiency, hog ranchers routinely supplement their animals' food with phosphate. However, this additive adds to total feed costs and likely will become more expensive in the future as humans exhaust the finite supplies of accessible phosphorus that can be mined from the earth's crust.

Thus, other strategies to deliver usable phosphorus to hogs have been attempted. One approach is to include phosphate-rich animal by-products (such as meat meal, bone meal, or processed food wastes) in pig feed, but this raises concerns about the spread of animal diseases. Another approach is to feed the swine low-phytate strains of corn (genetically modified or otherwise), thereby making more of the phosphorus bioavailable to the animals. A third practice has been to supplement hog feed with phytase, a commercially available enzyme isolated from bacteria that breaks down the phytate in corn seed, thereby releasing phosphorus (and other minerals that phytate sequesters) in a form that pigs' bodies can use. But this approach also costs money and suffers because this enzyme sometimes denatures (breaks down) prematurely (e.g., if overheated during production or storage).

Is there another economical way to deliver usable phosphorus to swine, preferably in an environmentally acceptable manner? Some biotechnologists think the answer lies in transgenic "enviropigs," genetically engineered to produce phytase on their own. Genes for phytase could be isolated from bac teria or other microbes and attached to promoter regions specifically directing gene expression in mammalian salivary glands. The molecular construct would then be inserted by GM techniques into swine. If all went well, saliva in the transgenic hogs would soon be dripping with the precise enzyme needed to convert the natural phytate in ingested corn kernels to phosphorus compounds that swine could use. This would mean no more supplemental feeding of costly phosphorus or phytase to the pigs, no need to abandon otherwise nutritious corn kernels as hog feed, and less ecologically damaging phosphorus being excreted from the animals.

In 2001, scientists succeeded in engineering experimental pigs in almost the exact manner described above. These GM porcines produced copious saliva containing phytase from a bacterial transgene, and the enzyme appeared to work as intended in the pigs' mouths and guts, releasing usable phosphorus from phytate. As a result, the GM pigs required almost no dietary supplements of inorganic phosphate for normal growth, and, as an added environmental benefit, the animals excreted up to 75% less fecal phosphorus than non-GM controls.

Due to economic costs, social factors, and ecological considerations, these transgenic pigs would more likely be used in high-density hog ranches rather than on small family farms. Similar GM approaches can be envisioned for improving phosphorus management on massive chicken farms, another prime agricultural source of environmental pollution. On the other hand, some people argue that it might be better, in the long run, to rethink the entire concept of high-density ranching and high-tech genetic solutions (although the alternatives are not necessarily clear). They abhor the basic concept of mass-rearing "artificial" farm animals under artificial and perhaps inhumane conditions, especially when environments also are harmed by the process.

So, before transgenic enviropigs can have significant impact on ranching practices, several important hurdles remain to be cleared, including gaining acceptance by regulatory agencies and the public. This new approach to the phosphorus problem holds some promise, but its potential is far from realized.


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