The importance of forage sampling after a drought

31st August 2018

2018 has turned out to be a difficult year for farming. With a late spring blizzard followed directly by a drought that for some is still a big issue and for others no so much of a current issue.

As Gary wrote in our last blog, measuring and assessing the amount of fodder available to you this winter is absolutely vital. So far for most of us, the autumn is giving the chance of gathering more forage for the winter. However, as the chart below shows, all across the British Isles, there are still many places that are on the knife edge of drought and are having difficulties growing forage and grazing cattle at the moment. What I want to focus on in this blog is the impact of the drought on the mineral composition of the forages for next winter.

United Kingdom drought severity index (August)and Met Eireann soil moisture deficit (29th August)


Plants get their nutrients from the soil to their roots in 3 major ways,

  1. Interception by roots - Any nutrients that the plant root touches it is able to take up.
  2. Mass flow and diffusion - As the plant uses the nutrients around it, it causes a deficit of nutrients around the root. This will cause nutrients in areas of higher concentrations to flow towards the lower area. Water is vital in this process to help the flow of nutrients.
  3. Soil biology - All living organisms in the soil make nutrients available to plants. The best example of this is mycorrhizal fungi which forms a mutualistic relationship between some plant. Fungi increase the size of the plant roots to allow for more nutrient capture. The plant in return feeds the fungi in exchange for nutrients.

Water plays a vital part in the flow of nutrients in the soil to the plant. It is also vital for the flow of nutrients inside the plant itself. The process is called transpiration. A plant takes up water through its roots, it moves up the plant through the xylem and is lost through the stomata on the leaves. This creates a slight pressure differential to suck water from the soil. Approximately 96% of the water a plant takes up is lost to the atmosphere, only 4 % is used by the plant. The other 96% is used to transport nutrients around the plant.

This process can be shut done in very hot weather like we received this year, as the plant senses that it is losing too much water and closes its stomata on the leaf. The upshot of this is that if a plant is not moving water through itself, itís not bringing nutrient into itself either. It sits there dormant, waiting for the drought to pass.

In practice what does this mean to us?

What I think is that this year, we have to be vigilant for -forages that are lacking in nutrients and protein. Every farm will be different and there will be variation from region to region and farm to farm. This is the year to make sure you get a forage test on all your forages and particularly those made in the drought or after the drought. Some of the issues that I expect to see are;

  1. Protein will be lower than the plant is analysing for. The reason for this is that during the start of a drought the plant senses that there is a drought coming and takes up a lot of nitrogen. As the drought bites the plant canít take up water to run its internal processes, it shuts down. The nitrogen gets stored in the leaves, and nothing is done with it. In normal circumstances the nitrogen would be converted to amino acids and then protein. The crude protein test is actually a test for nitrogen, you find out the amount of nitrogen in the sample and multiply by 6.25 (nitrogen is approximately 16% of protein). The effect I think we might see this year is that we will pick up the nitrogen, but it won't be in true protein form, therefore overestimating the protein.
  2. Low phosphorus (P) analysis in forages. P is hard to get into plants at the best of times, it is related closely to the biological activity of the soil. In a drought, this biological activity ceases as biology need water too. This will inhibit the uptake of P from soil for the plant.
  3. As strange as it sounds in fine weather keep an eye out for tetany. The reason for this is that when a drought breaks potassium (K) is the first to become readily available. Magnesium and potassium compete which each other to be used by the plant. If potassium is more available the plant will take it up as it is there (this is called luxury consumption), but wonít take up magnesium as it is not there and is being blocked by the potassium. For grazing grass, get the buckets out, but for winter forages look at magnesium sources like MAG12 to supplement the diet.

Impact on Trace Elements

Trace elements are likely to be lower than normal. The reason for this is luxury consumption again, this time for nitrogen. Nitrogen is antagonistic to the plant taking up trace elements (zinc, manganese, copper etc.). Plants will keep taking up nitrogen as they need in much larger quantities to grow than trace elements. Although they need the trace elements too, they need them in times 1000 less than nitrogen. The impact of the drought, combined with luxury consumption of nitrogen means there is a big chance that forages will analyse lower for trace elements this year. Theseh will have to be supplemented in the winter diet for the health of the animals eating those forages.

Tompson and Joesph analysis for grass, maize and whole crop silages

Lastly, I would like to talk about maize and whole crop silage. A lot of farmers will be using these types of forages for the first time this year and might not have experience of them. As we can see from the table, these crops are traditionally lower in minerals and trace elements than grass forages. This effect will be greater than ever this year.


These are some of the reasons why testing your winter forages is vital this year. If you have any questions. Please get in touch with us here at Devenish, and you can talk to one of our nutritionists on the best way to take forages samples from pits of bales. Get your forages sampled and start to plan your winter-feeding programme now.

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