Coppiced hardwood trees for reuse of farm dairy effluent

Abstract

Alternatives to the traditional treatment of effluent, irrigation back onto pasture, may prove valuable for farmers. Here we present the results from the first two years of a trial set up to test the potential of cut-and-carry coppiced hardwoods (poplars and willows) in taking up nitrogen from fresh effluent and providing fodder on a dairy farm. Three blocks each of Argyle poplars and Tangoio willows were planted as 1.2 m stakes on a dairy farm in southern Wairarapa in September 2001. One block of each species was irrigated with fresh farm dairy effluent at a high rate, about 5 mm per week, the second was irrigated at a low rate of about half that amount, and the third control block of each species was left unirrigated. The first coppicing, conducted in March 2002, yielded 6, 13, and 24 t DM/ha from the Willow-Control, -Low, and -High treatments. The corresponding yields from the poplar blocks were 6, 14, and 11 t DM/ha. The depressed yield of the Poplar- High was due to a rust infection. Growth was much slower in 2002/03 due to a cold October and dry summer. The yields were about a third of those measured in the previous year. The amount of nitrogen in the harvested biomass of the Willow- High treatment was 440 and 100 kg N/ha in the two years. Coppice blocks are likely to be most useful where the amount of land suitable for irrigation is limited, where there may be heightened concerns about the effects of nitrate leaching, or where wet weather storage of effluent is limited. The coppice blocks accumulate a large amount of animal fodder in late summer when many farms experience feed gaps and the fodder from coppice blocks may also have animal health benefits. Keywords: willow, poplar, forage crops