Thursday, 11 April 2019 10:37

Foliar-applied boron can reduce milk urea

Written by  Bryan McLeod, former dairy farmer who has consulted on soil, plant and animal nutrition for 36 years
Bryan McLeod. Bryan McLeod.

Boron is a fascinating element in that its presence affects many other elements and functions in a plant.

Importantly it helps a plant deal with and effectively process nitrogen into desirable amino acids and in doing so lowers protein and nitrate levels in pasture.

Milk urea (MU) levels are directly related to blood urea and the level of both is directly related to cow diet which in New Zealand is basically pasture. As pasture nitrogen increases so does MU, especially when pasture is stimulated with high levels of applied urea. 

NZ’s basic pasture species (ryegrass) can accumulate very high levels of N, e.g. 5.5%+ and high levels of NO3 at certain times of the year. 

It is important to understand that protein is total N multiplied by 6.25; protein contains 16% N. So flush ryegrass pasture at 5.5% N = 34.37% crude protein; this surplus protein (N) is degraded by rumen organisms to ammonia which goes to the blood giving us high blood N.

What if we could change the way pastures handle this N? That’s where boron comes in, helping a plant deal with N.

If we add high energy supplements (maize silage) or change our pasture species (to plantain) we lower the amount of degradable protein in the rumen and so lower blood N and MU and, most importantly, urea discharge.

I have discovered that the application of foliar boron at 100 - 200mg of B/ha quickly reduces pasture protein% from, e.g. 35% down to 25% with MU levels following, showing significant decreases. On clients’ properties I have seen MU levels decrease from about 35 down to 25 and in one case down to 18, which is significant and although not measured it would mean a significant decrease in discharged urine urea. 

Importantly, note that the boron needs to be applied as a foliar to fresh lush pasture; soil boron may be OK but one still needs to make the foliar application to get this effect and this would need to be done at least every six weeks or sooner. 

It also means that high urea users may be able to maintain their applications, still achieving significant reductions in nitrate leaching, helping to protect the environment in the process.

These cases are antidotes only and more research is needed to confirm responses.

We may tend to think of boron as only being essential for brassicas, not for our general pasture species. But boron has interesting functions in all plants: 

1. In pastures, boron helps to balance nitrogen levels and prevents excess accumulation of nitrate nitrogen, thus reducing excess protein in spring and autumn pasture. This is the key to reducing MU and urea discharge in cow urine.

2. It increases plant sugars (energy)

3. It is essential for photosynthesis and energy production in plants. This is critical in helping to increase energy production 

4. It reduces energy (ME) loss in the degradation of nitrogen by rumen organisms

5. It increases plant quality and density – critical for DM intake

6. It increases calcium absorption in plants and animals

7. Boron deficiency often gives plants a bushy appearance: we see this in lucerne crops 

8. It increases seed set and oil percentage in oil crops, e.g. canola and olives.

The secret lies in point no 1 (above): boron helps a plant deal with and process nitrogen and helps prevent the accumulation of NO3 which can be a serious problem in ryegrass. When we look at the known effects boron has on plants we can see why it can effectively reduce MU. If the foliar application of boron can effectively reduce pasture protein to an acceptable level, finally resulting in a lower percentage of nitrates being leached from our dairy pasture into our waterways, we see how it would have a significant effect on our environment without reducing our cow numbers.

My consultancy has taken me to many countries where it is common to see boron deficiency in many plant species including pasture. In NZ I have identified boron and copper deficiency in ryegrass pastures, which raises the question why are we not addressing these deficiencies? Boron, worldwide, is generally only believed to be essential in horticulture, e.g. brassicas, yet it is essential for all plant species. 

Foliar applied boron deserves a special place in our pastoral systems. Extensive research is needed to confirm that nitrate leaching could be achieved by foliar application of boron. Farmers would need to monitor pasture N levels and apply foliar boron when and as required. Farms with large irrigators could simply inject boron into their irrigation water on application.

• Bryan L. McLeod, a former dairy farmer, has consulted on soil, plant and animal nutrition for 36 years. 

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