In previous articles we have discussed a number of aspects relating to pasture management, and provided an overview of a number of common grass species found in horse pastures in Australia.
In the Pastures for Horses series we touched briefly on non-structural carbohydrates (NSC) in the different species, but many horse owners may be wondering what they actually are, and what part they play in pasture and conserved forages (hay and chaff).
Horse owners may also want to know what other nutrients are available in pasture and conserved forages, and which are suitable for horses with high-energy demands such as breeding, growing and performance horses.
In this 3-Part Series, we will answer those questions, and in particular we will look at laboratory testing of forages (pasture, hay and chaff) to assess their quality and suitability for your horse’s diet.
Graziers and browsers
We know that horses are hindgut-fermenting herbivores adapted to eating a plant-based diet high in fibre. Graziers and browsers are able to survive on a wide range of pastures and forage feeds.
Fresh grasses, browse (plants other than grasses) and conserved forages such as hay and chaff, can cover the nutrient requirements of most horses, and should at the very minimum cover half the diet of horses that receive (cereal based) concentrate feeds.
In reality though, we frequently come across horses that don’t receive an adequate supply of forages (long stem hay or pasture), which can puts them at risk for developing digestive and metabolic disorders (which may present as colic, ulcers, and laminitis).
Although we may think that horses with high energy demands need supplementation with grain or other concentrated feeds, recent studies have shown that a forage-only diet can sustain racehorses (see page 42 of this issue).
This is why it is time that we take a closer look at the nutrient profile of forages.
The need to analyse
When you buy commercial concentrate feeds you will always find the nutrient analysis table printed on the bag; in contrast, and despite the fact that most horses diets comprise mainly of conserved forages, hay and chaff suppliers very rarely supply you with any information of what’s in their bales or chaff bags.
This said, the increased interest in low NSC forages for metabolically challenged horses, is pushing forage companies and suppliers to advertise and provide feed analysis of the forages they sell.
Why test conserved forages?
Regardless if you are dealing with metabolically challenged horses, breeding horses, growing horses, or performing horses, laboratory analysis is the only way to develop an efficient feeding program to reduce costs and/or prevent metabolic disorders.
What about testing pasture?
There is a bit of ambiguity about conducting pasture analysis on horse properties because they only represent a moment in time.
Nevertheless, pasture analysis can be helpful depending on the operation you run, the types of horses you manage, and your personal objectives.
The difficulty lies in that nutrient content in grass plants vary greatly from place to place on the same property and at different times of the year, and is affected by many factors such as soil quality, rainfall, temperature and grazing pressure.
The pasture sample we take in for analysis only provides us with a momentary depiction of what the quality was at the time the sample was taken.
Nevertheless, horse facilities can use the results to build a database of pasture quality from year to year and for different paddocks as a reference.
Testing may also help inform your feed formulation, and you can use the analysis when you want to review your soil improvement and biodiversity strategies over time.
Ultimately though, it remains tricky to really provide an estimation of what your horses are obtaining from the pasture. When at pasture, an individual horse’s intake per day varies, and horses will select a range of plants (not exclusively grasses). Providing the forage availability is adequate, every day they will mix and match different species according to their nutritional status at that time.
Assessing Feed Quality
Analytical testing can be used to predict how well a particular feed will meet the needs of the animal.
The status of the horse (non-working, performance, breeding, growing etc) will off course influence the quantity and quality of feed required to cover the nutrient requirements.
For optimum productivity and irrespective of the animals’ status, nutritionists rely on the following feed properties:
- Dry matter intake
- Crude protein content
- Carbohydrate composition
- Energy yield from the digested feed
- Mineral and trace element content
- Vitamin content
Dry matter is the dry weight of pasture or forage after the removal of moisture, and is usually expressed as a percentage (%) of the fresh weight.
The dry matter intake needs of a horse, depends on many variables, including live weight, workload, breeding status, stage of lactation, environmental conditions, feeding history, body condition and the quality of the feed.
The Nutrient Requirements of Horses (NRC, 2007) makes the following recommendations:
Horses in maintenance and light work – 2% of the body weight in dry matter daily.
For horses at maintenance and light work, Moderate exercise – 2.25% of the body weight in dry matter daily and,
for (very) heavy exercise, growing, breeding and lactating horses – 2.5% of the body weight in dry matter daily.
The protein content of the pasture or forage is directly related to the Nitrogen content which varies with growing conditions, plant species, and maturity of the plant. The crude protein (CP) content will decrease with increasing plant maturity. CP requirements are dependent on the live weight and class of horse being fed. For example, a 500 kg horse at maintenance with an average temperament requires about 630 grams CP per day and a lactating mare of the same weight in the second month of lactation requires more then 2 x the amount of maintenance – 1530 grams CP per day.
Plant carbohydrates can be conveniently classified as structural (cell wall or indigestible) carbohydrates and non-structural (cell contents or digestible) carbohydrates. Digestible carbohydrates are typically processed in the small intestine, whereas indigestible carbohydrates are fermented in the hind gut.
The diagram on the left indicates how plant carbohydrates are partitioned into fibre fractions. This breakdown may look very complicated, but it is vital to the forage analysis, because to get an overall picture of the digestible and indigestible carbohydrates in the forage, the different fractions require analysis.
Non Structural Carbohydrates: The key NSCs in forages are the soluble sugars such as sucrose, glucose, fructose, and starch. Plant soluble sugars fluctuate diurnally as a result of photosynthetic activity with highest levels generally found in the early to mid afternoon period. Typical levels for temperate (C3) grasses can range from 5 to 15 %.
Structural carbohydrates are dominated by cellulose and hemicelluloses. These are polymers that form the basis of fibre in all plant tissue.
The levels of structural carbohydrates increase as the plant matures, with a corresponding decrease in plant digestibility. Cellulose in particular, may become lignified to varying degrees. Lignin, which is not depicted in the figure as it’s not a carbohydrate, can become intimately associated with cell wall carbohydrates, reducing the nutritive value of the forage.
Acid Detergent Fibre (ADF) & Neutral Detergent Fibre (NDF) provide estimates of the less digestible structural carbohydrates in forages.
ADF consists mainly of cellulose and lignin (very undigestible) with small amounts of nitrogen and minerals.
The NDF fraction includes the hemicelluloses in addition to the ADF component of plant tissue. Very high fibre levels slow the rate of digestion and limit dry matter intake, but a certain amount of fibre is required to stimulate digestion by hindgut fermentation.
As you can see in the diagram, some carbohydrates such as pectins, beta-glucans and fructans overlap, ie. they can end up being processed in the hindgut.
Feed digestibility is simply defined as the proportion of forage dry matter able to be digested by the animal. It is largely influenced by the maturity of the plant species, as it declines as the plant matures because of increasing levels of the structural carbohydrates.
Within pastures, the plant species also influences digestibility. For example, lucerne retains a higher leaf:stem ratio with increasing maturity and so maintains a higher digestibility compared with bunch grasses.
When analysing forages, digestibility is measured in two quite distinct procedures:
- in vivo digestibility – determined directly by animal feeding trials by way of a mass balance from what is consumed, what is digested, and what is excreted.
- in vitro digestibility – determined by wet chemistry using caecal fluid or purified cellulase enzymes.
In vivo digestibility provides the most meaningful estimate of animal performance, but nowadays, the cost of setting up animal trials for measuring in vivo digestibility, or for providing caecal fluid is prohibitive, thus, most laboratories measure in vitro digestibility by incubating samples with enzyme preparations and use these data to predict in vivo digestibility.
The apparent digestible energy (DE) content of forages is calculated by subtracting the gross energy in faeces from the gross energy (intake energy) consumed by an animal.
The term ‘apparent’ is used because some of the materials excreted in the faeces do not originate from the feed but from cells in the gastrointestinal tract and digestive secretions.
There are two factors that impact the DE in feeds, which are the gross energy content of the feed and the digestibility of the energy-containing components.
As highlighted above, the most accurate way to estimate DE is by feeding trials but in horses, the numbers of studies that have done this are limited compared with other species such as livestock.
The DE value of feeds varies among species and the Nutrient Requirements of Horses (NRC 2007) has developed equations for estimating the DE content in horse feeds from the chemical composition of the feeds. There are different formulates for dry forages, roughage, pasture and range plants and energy feeds and protein supplements. However, these equations have limitations and cannot accurately predict the DE value of some feeds, so they are less relied upon when formulating a diet.
Minerals and trace-minerals
While minerals only constitute a minor part of the equine diet by weight, they play a critical role in the health of horses.
Minerals are involved in a number of functions in the body, including formation of structural components, enzymatic co-factors, acid-base balance and energy transfer.
Macro-minerals are the minerals that are required in larger amounts. These include Calcium (Ca), Phosphorus (P), Magnesium (Mg), Potassium (K), Sodium (Na), Chloride (Cl) and Sulphur (S).
Trace-minerals or micro-minerals are required in smaller amounts than the macro-minerals but are as essential as the big ones. The group includes Copper (Cu), Cobalt (Co), Zinc (Zn), Manganese (Mn), Selenium (Se), Iodine (I) and Iron (Fe). The most common minerals analysed in forages are Ca, P, Mg, K, Na, Fe, Zn, Cu, and Mn.
As herbivores, horses obtain most their minerals from plants. Plants in turn get their minerals from the soil and fungi association. Fungi play a very important role in transporting minerals and energy through the soil, storage of minerals and energy in living cells and transferring minerals to plants. This means the plant can only be fed when the soil biology and quality is healthy.
Plants will take up the inorganic minerals and convert them in the cells to organic forms that sustain the life of the plant and all the organisms that consume the plant. When a mineral is nutritionally organic, it means that it is chelated or bound to an organic compound such as proteins, polysaccharides, amino or organic acids. Horses ingest most of the minerals in these organic forms.
The mineral requirements of horses depends on the live weight and animal status. For example a 500 kg horse at maintenance requires daily 20 g of Ca, 14 g of P and 100 mg of Cu, whereas a lactating mare of the same weight requires 59 g of Ca, 38 g of P and 125 mg of Cu per day.
Vitamins are classified as either water-soluble (B and C) or fat soluble (A, D, E and K) organic compounds that can be naturally found in small amounts in plant and animal-derived foodstuffs. Organisms require small amounts of vitamins for proper function of the body and any deficiency can leads to metabolic and physical disorders. In an optimal environment horses can obtain most of their vitamins from fresh grass and other plants and in the case of vitamin K and B complex vitamins, additional amounts can be supplied by microbial synthesis in the intestine tract. But when horses are stabled, have limited excess to pasture or are kept on poor quality pastures they generally need some extra vitamins (and minerals). The requirements can also increase when horses are breeding, pregnant, lactating, growing, ageing, exercising or in poor health. When vitamins cannot be synthesized by the body or they cannot be made in adequate amounts, they need be supplied by the diet and/or supplementary sources.
In general vitamins are expensive to analyse in forages/feeds, therefore they are not standard in the forage testing packages. However, on request you could analyse for A, D and E.
The bottom line…
Optimum feeding is a balance between the amount (quantity) of available feed and the quality of that feed.
Analysing forages is indispensable if you want to increase feed efficiency, reduce supplementary feeds/additives and feed costs and prevent metabolic disorders.
Next month we will continue with reviewing the quality of forages and provide an overview of common conserved forages and other (commercial) fibrous sources fed to horses. Then in part 3 we will reveal the results of testing in an Australian laboratory, some different types of hay and chaffs, and how those results would impact the formulation of your horse’s diet.
- Frape, D. 2010. Equine Nutrition & Feeding. Wiley-Blackwell; 4th edition; UK.
- Hall, M.B, Hoover, W.H., Jennings, J.P. and Webster,T.K.M. 1999. A method for partitioning neutral detergent-soluble carbohydrates. Journal of the Science of Food and Agriculture 79, pp. 2079–2086
- Nutrient Requirements of Horses (NRC). 2007. 6th revised edition. National Academy Press, Washington DC, USA.
- Van Soest, P.J. 1994. Nutritional ecology of the ruminant. Second edition. Cornell University Press. Ithaca, NY, USA.