Case Studies: 3 Pasture Tests

Both pasture and hay can widely vary in nutrient levels, not just between types of pasture and hay, but also high variability for each type of grass or hay.

You can’t tell the nutrient profile by looking at photos of paddocks or hay. With the enormous range of different feeds and supplements on the market the choices of what to feed a horse is very challenging. Many horses are over supplemented leaving horse owners faced with expensive feed bills and horses still may not be getting what they need.

Everyone wants to ensure that what our horses are being fed is meeting their nutritional requirements, however to do this, it’s important to look at the whole diet rather than parts of it in isolation. Feed manufacturers generally offer well balanced feeds but if the bulk of the diet, pasture or hay is out of balance then these bagged feeds or supplements will not correct any imbalances.

For example, the ideal ratio for copper and zinc in the overall diet for horses is 1:3, that is, one part copper to 3 parts zinc. some say 1:4 but environmental contaminants like sulphur can prevent uptake of copper. Too much zinc in the diet is known to interfere with the uptake of copper and it is strongly suspected that too much copper will interfere with zinc uptake. The feed given to the horse may have copper at roughly one third the level of zinc but what about the bulk of the diet coming from pasture or hay? Grass has a lower requirement for copper and zinc and species of grasses differ in their uptake of minerals from the soil. To find out if the whole diet is providing the minerals and other nutrients to satisfy a horse’s requirements and that the ratios are optimally balanced then the first step is to look at the bulk of the diet, the forage.

Looking at pasture or hay won’t work

Unfortunately it is impossible to tell the nutrient levels of pasture by simply looking. Horse owners know when their pasture increases in energy value, especially with the arrival of spring with warmer temperatures and some rain but it’s impossible to tell levels of minerals like calcium, phosphorus, magnesium, copper and zinc just by looking. If any feed program, regardless of the level of sophistication, uses photographs or averages for determining nutrient content of pasture, its validity should be questioned. The very best looking pasture can be very mineral deficient, that is, deficient for horses but not for the species of grass.

Pasture testing is the simplest and most accurate way to find out the nutrient profile. It doesn’t have to be a costly exercise either. If the main forage is hay, a representative sample can be sent to a laboratory.

A pasture test can give results for digestible energy (horse), crude protein, calcium, phosphorus, magnesium, iron, copper, zinc and manganese – sulphur, molybdenum and selenium can also be requested. All these nutrients at the right levels are necessary for optimum health. For hay, testing the sugar and starch levels will help with determining if the hay is suitable for sugar sensitive or insulin resistant horses.

Case Studies

Highlighting the differences in pasture which may not be visible to the eye, the following are case study examples from NSW and the ACT. They also show how varying levels of nutrients within each pasture may change the rate of mineral supplements needed for horses for whom a significant intake of the diet is made up of pasture. This applies to hay as well.

All values in these cases represent dry matter which means the water content was removed for testing. The following assessments are for a 500 kilogram (kg) horse eating 10 kg pasture.

Pasture 1 – Predominantly Kikuyu pasture on the mid NSW coast

Pasture on this paddock consists of approximately 70% Kikuyu pasture mixed with 30% native species. Test results indicated a high level of protein and very low sodium. Unfortunately being an oxalate grass species any calcium coming from Kikuyu is unavailable to the horse so calcium supplementation is a must. The phosphorus and magnesium were high enough for a horse in little to no work but extra supplementation would be needed for horses in a higher needs group such as growing, breeding or those on a high workload. Copper was deficient for horses at 8 mg/kg but zinc was a surprisingly high 97 mg/kg. This gives a ratio of 1:12 for copper to zinc.

Pasture 2 – Predominantly Kikuyu pasture on the mid NSW coast

Located less than 500 metres away from Pasture 1, the Pasture 2 looks the same with 70% Kikuyu and the rest a mixture of native species. Again, test results showed a high level of protein so it’s highly unlikely protein would need to be supplemented for horses on these two properties if the diet was mainly pasture. The phosphorus and magnesium levels were a bit higher than Pasture 1, however both do require significant calcium supplementation. Copper (10 mg/kg) was similar but zinc was a lot lower at 53 mg/kg, a ratio of 1:6. Horses on both properties require copper and zinc supplementation to meet requirements and correct the copper to zinc ratio.

Why there is such a significant difference in the level of zinc is interesting as the two pastures are similar to look at. Other factors such as soil acidity (pH), moisture, oxygen and nitrogen levels come into play for influencing a plant’s uptake of minerals and Pasture 1 was used as a chook farm in the long distant past, perhaps that’s why the higher than usual levels of zinc.

Pasture 3 – Highly diverse native species pasture in the ACT

Many horse owners favour highly diverse native species pasture for horses with many people believing that the greater the number of plant species in the paddock, the less chance of nutritional deficiencies for their horses. Test results for this paddock indicated that protein levels were fine but the biggest surprise were the major minerals; calcium was okay although would need supplementation for a higher needs horse but phosphorus was extremely deficient and magnesium was not much better. Regardless of whether a horse was in work or not, he would be significantly phosphorus deficient grazing on this pasture.

Copper (7 mg/kg) and zinc (33 mg/kg) were very low and not optimally balanced at 1:5. All three pastures samples were low in sodium so any horses on these properties need additional salt added to their feed, at least a tablespoon for maintenance needs, more if sweating on a hot day.

For the long term heath of pasture, periodic soil testing is recommended. On the basis of the soil tests, decisions can be made on which soil treatments should be applied. But if you want to know what your horse is actually eating, testing what he eats is the most accurate way. The information the test provides can then be used to work out the best choice of supplementary feeds, if any are actually needed. In many cases, the highest quality feed in a horse’s diet is the grass portion. The more supplementary feed fed the less need for the horse to eat the pasture and the horse can end up nutrient deficient. And of course, this will depend on the horse and it’s workload and any special needs. Horses that are growing, or breeding/lactating or on a demanding workload will have higher protein and mineral needs than a horse in little to no work. Any mineral deficiencies can be corrected and the mineral ratios balanced.

There are many variables in managing horses, many of which are out of our control but one aspect we can control is their nutrition for maintaining optimum health – a very good reason to ensure that the feed they eat is of high quality and contains all of the necessary vitamins and minerals.

Do it yourself pasture testing

Oberve your horses to see what plants they like to eat and which ones they prefer to ignore. Walk over the paddock and collect 12 to 20 (or more) random samples by using stainless steel scissors to cut at the same height that the horses graze, trying to collect the different plants in the similar proportions to what is in the paddock, representing what they eat. For example, if you have 75% of plant A and 25% of other plants in your pasture use one plastic bag for Plant A and a second plastic bag for the other plants. Keep collecting till you have 200 – 300 grams, you will need a set of kitchen scales to determine this.

Mix the plants together in a small clean bucket with the different plants in similar proportions to what is in your paddock. The aim now is to dry the sample as soon as possible as the grass continue to metabolise sugars after being cut. A microwave or food dehydrator is an excellent way to dry the sample. Place a sheet of newspaper and a glass of water on the turntable in the microwave. Use the medium/low or low setting and check the grass every 3 minutes to see how dry it is, turning it over regularly to help dry the sample evenly. Every time you check, refill the glass with cold water but be very careful not to burn yourself when touching the glass as the microwave is very good at heating up glass and glass is very good at retaining heat.

When the grass feels dry to touch it has had enough drying. We only want to partially dry the sample. The colour of the grasss will still be green though may look a bit faded. Depending on the size of the microwave, you may choose to do the sample in 2 to 3 batches.

If you don’t have a big enough microwave you can spread the grass out on newspaper in the sun. Once the sample feels dry, fill a small snap lock bag and squeeze all the excess air out. Now it is ready to send to any laboratory for testing.

If in doubt, contact your chosen lab for instructions on how best to prepare a sample. To help with understanding the lab test results or where to send a sample, read Understanding a hay or pasture test.

Originally published in the December – January 2010 (Vol 31 No 4) issue of Hoofbeats magazine, updated and added to.