Solving for N

By Tony Benny

AgResearch scientists show that the plantain-nitrogen calculation is not necessarily a simple one.

In an experiment that used an unconventional method to measure the nitrogen content of cows’ urine, AgResearch scientists have shown that plantain has to be at significant levels in pasture mixes to be effective in reducing N excretion.

Their research used sensors attached under the tail of cows to measure how much nitrogen they excreted. Lead scientist Dr Lisa Box said the project differed from previous studies in that it measured the cow’s N output throughout the day rather than by oneoff spot testing.

“Spot samples of urine are less useful because you can only look at the concentration of nitrogen and at one point of the day, but actually what happens is that both volume and N concentration vary significantly through the day so they’re not really representative of a day’s worth of nitrogen coming out of that cow in urine,” Box said.

To conduct the experiment, AgResearch engineers had to come up with a way to collect the urine and direct it through the sensors that were to be attached to each of the 20 non-lactating cows that were fed different, randomly assigned, diets with 0%, 20%, 40%, 60% or 100% plantain (dry matter basis), with the remainder comprised of ryegrass-white clover pasture and grass-silage.

“It looks quite funny, it’s moulded off a Russian stacking doll because it fits over the back end of the cows quite nicely and that funnels the urine into our sensors. They’re attached to the back of the cow and clipped to a cover to hold them on so the urine filters through the sensor each time a cow urinates.

“We measured the volume and concentration of nitrogen and what time urination happened so you get the entire days’ worth of information of what that cow’s done in terms of urinating.”

Reducing the total amount of urine nitrogen from cows is not quite as simple as, “Feed them more plantain time happened and the amount of N will magically drop correspondingly”, because plantain contains far more water than other pasture and therefore the cows pee more. And there’s a difference between nitrogen concentration and nitrogen loads.

“If you put a teaspoon of sugar in a whole bucket of water, you won’t really taste it, but if you put a teaspoon of sugar in a shot glass it will be quite sweet and that’s kinda what we’re talking about –you’re still consuming the same amount of sugar and cows are still urinating the same amount of N but the water portion changes,” Box explained.

The experiment suggests that a reduction in urine nitrogen concentration can be achieved on low levels of plantain (20% of the diet), but greater than 60% plantain diets are required to reduce N load per event, which is more important in terms of N leaching potential.

When the experiment was conducted in Waikato, the region was in drought so extra N was spread on plantain pasture to ensure there was enough available to feed the cows, and Box believes this may have had an effect on the outcome.

“Usually you’d expect plantain to have a similar or lower crude protein concentration than ryegrass/white clover and we had the opposite. If we take that into account, maybe we wouldn’t have needed so much plantain.

“That was probably the most exciting thing out of this, that the animals were on plantain that had been fertilised so they were eating a whole heap more nitrogen than those just on ryegrass and white clover and silage, yet they weren’t excreting any more nitrogen so really plantain was able to reduce the urine nitrogen a bit relative to intake.”

Like much research, this experiment has thrown up new questions that need to be answered, Box said.

“I thought it was good news but the practical side of me thought, ‘Oh man, if we need this much to have an effect through the animal, how is that going to be achieved, what does it mean for these people who’re putting in 15-20% plantain on farm, is that going to be meaningful?’, and I thought, this could be quite a challenge.”

But she was quick to add that the effect of plantain on urine nitrogen is only part of the story because the plant also works in the soil, reducing N loss by slowing down how quickly nitrogen is transformed in nitrate.

Box also identified something farmers planting plantain should be aware of: “Once you get plantain to 40% of the diet, cattle don’t drink any water from the troughs because they don’t need to, so if you’re putting stuff like zinc in the drinking water, you need to consider that they’re not going to be consuming it and find another way.” n