However, other experts say it’s not as simple a solution as it may at first seem.
Lucy Waldron of LWT Animal Nutrition first flagged the apparent oversight of nutrition as a tool to reduce nitrate loss during the Horizons Regional Council’s One Plan hearings in 2012, where she gave evidence to the Environment Court as an expert witness regarding the role of animal nutrition.
Now she’s taken data from the Lincoln University Dairy Farm (LUDF) to illustrate what’s possible, running pasture crude protein (CP) and milk yield data through a computer model that shows spring urinary nitrogen excretion might be three times what it could be if the herd received a more balanced diet.
“At times the crude protein of the grass was over 30%. That’s more than twice what’s required for optimum milk production and it never gets below 15% protein, which is about where you want the overall intake to be.”
The result of such high protein intake, as shown by Castillo et al in 2001 (Journal of Animal Science, 79:247-253), is calculated urinary nitrogen excretion of more than 1kg N/ha/day, or more than 300g/cow/day, yet for the same milk yield urinary nitrogen loss could be as little as 110g/cow/day.
“You need a low protein feed such as maize silage, that’s 10-11% protein, and it needs to make up over half the diet when the [grass] protein is that high.”
Waldron says there’s no clear relationship in the LUDF data about whether milk yield responds to changes in nitrogen content of the diet but notes there’s some evidence elsewhere that energy saved through not having to excrete excess nitrogen can be channelled into milk production instead.
It’s also well-proven that a higher starch content in the diet increases milk protein, so the milk produced from a better-balanced diet could be of higher value.
“It may seem odd to be feeding supplements when grass is lush, but this is the way to dilute protein. To keep the cost down farmers need to plan plantings so they have some low-protein forages available to achieve this in the long run, and allow for more silage to be made from spring grasses when CP is highest.”
To calculate whether such manipulation of the diet would be economic is Waldron’s next step in what she admits is “a project on the side” from her main research and consultancy work.
“I’m a nutritionist, not a farm economist or management specialist, and I’m not getting any funding to do this work, but I just want to show there are ways to dramatically reduce the nitrogen footprint of dairying through manipulation of diet before farmers start getting pinged with fines from regional councils. There seems to be a disconnect between the legislation being passed and information flow to the dairy farmers, the ones who are going to be impacted by it, about what they can do to comply with it.”
DairyNZ principal scientist in its feed and farm systems research group, David Chapman, says the numbers Waldron’s come up with for nitrogen excretion are consistent with the research but 30% crude protein grass would be the extreme and 22-25% is more typical, which, with a cow eating 18kg drymatter (DM)/day, would be a nitrogen intake of 500-550g N/day and excretion of about 200g N/cow/day.
If that nitrogen intake is reduced to 400g/day or below, which could be achieved by dilution in the diet, nitrogen excretion bottoms-out at 75-100g N/day. The problem is the cost of the feed required to do that.
“In the current economic environment it is not going to be cost-efficient but I agree that theoretically it is possible to reduce that nitrogen level.”
South Island Dairy Development Centre executive director Ron Pellow echoes that, saying such options have been looked at for the Lincoln University Dairy Farm but the economics were challenging, especially because the farm would still have been growing 15 tonnes DM/ha of spring grass.
“Do you increase the stocking rate because you’ve still got that 15t of grass on top of what you’re bringing in? You might end up with a lower concentration of nitrogen in each urine patch but if you’ve got more urine patches per hectare there might be just as much nitrogen excreted.”
Taking some of that spring grass as silage to fill a summer deficit would make sense in Waikato but with the more even, season-long grass growth at LUDF it’s much less likely to be economic, he adds.
Lincoln University’s professor of dairy production, Grant Edwards, acknowledges reducing dietary crude protein content can be a useful strategy to reduce nitrogen excretion but it’s not a simple fix because diluting dietary protein from 30% to 15% at an 18kg DM/cow/day intake would require about two-thirds of the diet to be a 10% crude protein supplement if the other third was spring grass. At such levels of supplementation, despite some substitution for pasture, total drymatter intake might increase, resulting in little change in total nitrogen intake per cow. He also echoes Pellow’s point about supplement leading to increased stocking rate and any effect of a low-nitrogen supplement being offset by more urine patches.
Edwards says it should also be noted optimum nitrogen content for pasture growth isn’t the same as the optimum for lactating cow nutrition, at more than 25% and about 18% respectively. Also, urine patches formed in spring in well-managed irrigated systems are a low leaching risk, so altering nitrogen excretion in spring might have little impact.
“It is more important to do it in autumn going into winter when the risk of nitrogen loss is high.”
