Each day our 50 million sheep deposit an estimated 864 million pellets of droppings weighing 26,000 tonnes, and cattle convert uncounted acres of lush pasture into 113 million pats weighing 205,000 tonnes. This excreta is estimated to be equivalent to the amount produced daily by some 275 million humans. Cow pats persist on the pasture for one to six months, and each day sees an additional 745 hectares inundated! In total, 40,000 to 190,000 hectares of “clean green New Zealand” are completely plastered with the stuff. Is this another environmental nightmare that we should be flagellating ourselves, government, farmers, and possibly their animals, about?
Probably not. The situation has existed for decades already, and isn’t getting any worse. Rather, it is a question of effective resource management.
Animal waste represents a lot of fine organic matter, rich in nitrogen and other plant nutrients. It is valuable food for both plant and soil. Yet in the form it is deposited, up to 80 per cent of the nitrogen may be lost to the atmosphere before it can be assimilated into the soil, and much of the grass buried under the effusion will suffer retarded growth.
Too much of a good thing can be detrimental, and an excess of riches often burns pasture plants. Dung blocks sun, too, so smothered plants set their solar panels in darkness, and photosynthesis grinds to a halt.
But almost as bad as a squandered resource, droppings constitute shelter and sustenance for various undesirables. Several species of fly call these murky confines home, and most if not all the internal parasites of stock are passed on through animal excrement.
Nematode parasites of gut and lung are a major animal health problem, and farmers spend about $50 million each year on anti-helminthic drenches. Parasites are developing increasing resistance to the drenches, so the problem is not improving.
Africa, home of wildebeest and nasty viruses, may hold an answer for us in the form of the humble dung beetle. Dung beetles burrow 10 to 50 centimetres deep into the soil beneath droppings, and in these galleries place two-gram balls of dung in which they lay eggs.
Larvae feed off the supplied dung, and eventually pupate to become young adults that pair and fly off to set up home in a fresh manure pat. A female may lay ten eggs under each cow pat, and inoculate nine pats during her nine-week lifetime. All stages of the 12 mm black insect feed only on dung, and plant roots are left untouched.
Since New Zealand lacks native mammals (with the exception of bats), we have no native dung beetles either. Jenny Dymock, an Auckland entomologist, believes that African dung beetles would be a worthwhile addition to our fauna. “There would be quite a few significant advantages,” she explained. “Dung beetles assist worms in improving the aeration of soil, and, by incorporating dung, increase nutrient cycling and soil fertility. By reducing the amount of dung on the ground, pasture growth should be increased, fly numbers reduced, and there will be less faecal contamination of waterways.”
Overseas studies have shown that the load of parasites on pasture can be reduced by 80-90 per cent in the presence of a vigorous population of dung beetles, and the numbers of intestinal worms affecting calves can be substantially knocked back.
We actually have some dung beetles resident here already. Mexican dung beetles were imported into Whangarei, Nelson and Canterbury in 1956, but now they only flourish north of Whangarei. Two Australian species were accidentally introduced more than a century ago, and, although widely distributed, have not had a great impact. They seem most adept at dealing with marsupial droppings.
Australia and the USA have already adopted the African species Jenny Dymock is recommending, Onthophagus binodis. Should the importation be approved, New Zealand would obtain fresh, surface-sterilised eggs from Tasmania, to reduce the possibility of also importing dung beetle pathogens or soil-borne microbes.
The proposal to bring in the beetles is being considered by the Ministry of Agriculture and other government bodies, and public comment is being solicited before a final decision is made.
Onthophagus alone would be unlikely to solve all our animal excrement problems, because it is not very active in winter. If it proves as beneficial as hoped, other species active in cooler areas and seasons could be evaluated for import subsequently.