Nutritive values and mineral contents of pasture can vary throughout the growing season due to a variety of factors such as weather, soil temperature, moisture, sward composition, and grassland management, going by studies conducted by Scotland’s Rural College (SRUC) and Agri-Food and Bioscience Institute (AFBI).

Work by Dr David Lawson (SRUC) in 2020 looked at the effect after Nitrogen application on a ryegrass sward which has been cut at two-week intervals from two to 12 weeks after spring N application. N was applied at two different application rates – 60 and 120kg N/ha. On analysis, the mineral content showed a gradual decline as the cutting date increased and was the same across all the major minerals (calcium, phosphorous and magnesium). There was little difference in mineral content between the application rates, but the levels were always slightly higher for the 120 kg N/ha.

AFBI in Northern Ireland, who are part of GrassCheck, recently published data from research carried out by Dr Francis Lively in 2023 on GrassCheck grass samples which were analysed for minerals and trace elements. The research found that during the grazing season the concentration levels of the majority of minerals and trace elements varied, indicating seasonal differences.

Unlike SRUC’s trial, there tended to be an increase in mineral content throughout the growing season. This difference is likely because GrassCheck farms are in a rotationally grazing platform and animals will naturally return minerals to the soil through dung and urine. There is also likely to be more clover in GrassCheck swards, whereas the SRUC trial was based on a pure perennial ryegrass sward. Clover starts growing when the soil temperature is 8⁰C, as opposed to 5⁰C for grass, so the different growth pattern could explain the elevated levels seen in GrassCheck farms later in the growing season as grass growth starts to decline.

The two studies show that the mineral content of grass is not static and can vary throughout the season. Analysis of grass samples will both, help to ensure grazing animals get their daily nutritional requirements to ensure optimal health and performance, and help with decision making on mineral supplementation.

Whilst weekly sampling of every field is impractical, a mid-season sample provides valuable insights, even retrospectively. For instance, if livestock underperform, but if the grass analysis does not highlight any issues, other factors may be responsible.

How to collect a grass sample

  1. Choose the right time: Ideally, collect a grass sample during peak growing season when the pasture is most representative of what animals are grazing. Early morning is often a good time.
  2. Select the area: Choose a representative area of pasture. Avoid areas that are overly grazed, near water sources, or recently fertilised, as these can skew the results.
  3. Sampling technique: Using clean scissors or shears, cut the grass at grazing height (about 5-7cm above the ground). Collect samples from multiple spots within the selected area to get a representative mix.
  4. Handling the sample: Place the collected grass in a clean, labelled plastic bag. Avoid crushing or compacting the grass as this can affect the analysis. Store the sample in a cool place and send it to the laboratory as soon as possible.

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