With increased scrutiny of New Zealand farming practices and more intensive farming, scientists at Lincoln University and Agricom have been measuring the effect of using different forage pasture grasses, such as plantain and chicory,to reduce the environmental impact of dairy heifers, while maintaining their weight gain.
"Research at Lincoln University has shown positive environmental benefits from grazing dairy heifers on pastures containing plantain instead of conventional ryegrass-white clover," says Dr Paul Cheng from the Faculty of Agriculture and Life Sciences.
"However, we still don't know the mechanisms that are causing the lowered excretion of urinary nitrogen that we found.
"This information will be useful to the farming sector and rural professionals in order to support their decision-making about using plantain to raise dairy heifers while reducing nitrate leaching," he said.
"To help discover these mechanisms our most recent study, a short indoor trial, funded jointly by AGMARDT, Agricom and Callaghan Innovation, was conducted.
"This involved two sets of dairy heifers eating two different diets of cut plantain and ryegrass-white clover, and aimed to provide more data to back up the earlier outdoor grazing trial. It also aimed to establish the mechanisms involved in the differences in urinary nitrogen excretion with plantain by measuring feed digestion," says Dr Cheng.
In this trial, Dr Cheng and his colleagues were able to quantify how much the heifers each ate and defecated, and also analysed the nutritional content of the diets (water content, mineral content and protein content). One group of six dairy heifers received cut ryegrass-white clover and the other group of six received cut plantain.
The indoor trial was necessary to enable the collection of waste products for analysis, as this was impossible in the field. It is not hard to imagine the difficulties when collecting manure samples from cows grazing freely in a paddock, he said.
"The results of this current trial showed that feeding specifically the cultivar Tonic plantain to dairy heifers significantly reduced both urinary nitrogen concentration and excretion of nitrogen from the heifers," says Dr Cheng.
"In a practical sense, this offers a simple but powerful tool to reduce the nitrate loading in urine patches and subsequently reduce nitrate leaching under grazing system.
"This was partly achieved by heifers ingesting more water from the feed, without altering the feed digestion or intake.
"This information provides us with the opportunity to mitigate on-farm nitrogen pollution and contribute to improving the New Zealand environment, although more work still needs to be done to be sure of the others mechanisms involved in the reduction in these two parameters."
A follow-up study involved modelling two years of heifer growth and nitrogen excretion data collected in these trials indicated feeding plantain could maintain heifer productivity while reducing nitrogen leaching in comparison to feeding ryegrass-white clover at a farm scale.
Undertaking studies involved a large team of scientists alongside Dr Cheng with different specialities.
These included Prof. Grant Edwards, Dr Racheal Bryant, Prof. Tony Bywater, Dr Jim Moir and PhD student Miss Kirsty Martin, from Lincoln University, and Dr Glenn Judson from Agricom.