The Micronutrient Content of Forage & How This May Impact Supplementation

Written by Briony Witherow MSc, BSc, RNutr.

This month’s blog focuses on the micronutrient content of forage (grass, hay, haylage) and how this may impact supplementation.

Forage nutrient levels fluctuate throughout the year depending upon various factors such as environmental conditions, management, and type of forage (grass, hay, haylage). However, as a rule it is unlikely that forage alone will guarantee a balanced diet. While many horses at rest or in light work may not show any signs of ill health without additional micronutrient supplementation in the short term, for those in harder work or with clinical issues signs of deficiency may be apparent sooner. 

The table below shows typical micronutrient (vitamin and mineral) levels for grass pasture and hay with how these compare to the daily requirement of a 500kg horse at rest. In red you can see minerals that are often deficient when a forage only diet is fed and in green those that are often abundant.

Typical micronutrient levels found within forage (per Kg Dry Matter)

^aMinerals deficient at maintenance when fed 2% bodyweight dry matter (10kg) of grass-based forage as sole dietary intake based on average nutrient levels.

Deficiency rarely seen at maintenance when forage is not restricted.

Levels are less abundant in forage or more variable in nature.

Equi-Analytical data grass Hay 2004-2022; Grass Pasture 2004-2022 n= number of samples. (Equi-analytical laboratories, USA 2023; NRC, 2007, NRC, 2001)

Below we tackle some common forage feeding scenarios, discuss their impact on micronutrient balance, and suggest solutions.

Feed Scenario: Forage only diet

Impact on Micronutrient Balance: Forage only diets are unlikely meet micronutrient requirement and are notably deficient in copper, zinc, and sodium. See below graph illustrations of nutrient provision versus requirement for horses fed average winter pasture and average meadow hay restricted to 1.5% of bodyweight (weight loss ration) in isolation. Note that quality protein (lysine) can also be deficient on these rations.

Figure 1: Nutrient provision of a pasture (winter average) only diet for a 500kg horse at rest. With kind permission from PC-Horse (https://pchorse.se/en/)

Figure 2: Nutrient provision of average meadow hay at 1.5% BW Dry Matter only diet for a 500kg horse at rest. With kind permission from PC-Horse (https://pchorse.se/en/)

Solution: Provide the recommended amount of a compound feed or broad-spectrum vitamin and mineral supplement alongside salt.

Feed Scenario: Weight loss or restrictive diets

Impact on Micronutrient Balance: Where the forage ration is restricted (particularly if hay is soaked) or where dry matter is not considered this can lead to a reduced dry matter intake (low fibre intakes) that will compromise micronutrient intake.

Solution: Provide the recommended amount of a compound feed or broad-spectrum vitamin and mineral supplement alongside salt.

Feed Scenario: Soaking hay for purposes of reducing respirable particles (10-30minutes)

Impact on Micronutrient Balance: Significant losses of sodium (36-43%), potassium (27-40%), phosphorus (14%), copper (27-30%) and magnesium (14-15%) have been reported (Moore-Colyer, 1996; Blackman and Moore-Colyer, 1998) even in 10-30 minutes of soaking. 

Solution: Where forage analysis is available, actual losses can be considered when rationing (hay with an initial high mineral content may still meet requirements). Alternatively, where analysis is not available, elevated levels of compound feed or broad-spectrum vitamin and mineral supplement may be recommended to counteract losses alongside salt. For example, feeding 38g of Selenavite E instead of 25g for an adult horse at rest.

Feed Scenario: Soaking hay for purposes of reducing sugars/calories (water soluble carbohydrates) (e.g. > 3 hours)

Impact on Micronutrient Balance: Micronutrients decreased: Sodium (20-60%); potassium (58-61%); Phosphorus (35-43%); Magnesium (19%) (Moore-Colyer, 1996; Blackman and Moore-Colyer, 1998; Argo et al., 2015).

Solution: If forage analysis is available, actual losses can be considered when rationing. Alternatively, where analysis is not available, elevated levels of compound feed or broad-spectrum vitamin and mineral supplement may be recommended alongside salt.

Feed Scenario: Relying on naturally occurring micronutrients within fibre feeds.

Impact on Micronutrient Balance: Many fibre-based concentrate diets are available which are not fortified with additional vitamins and minerals, consequently, these feeds may provide a more variable micronutrient intake. These diets can therefore lack micronutrients and may not be ideal as the sole source of micronutrients for those where deficiencies may impact clinical issues or performance.

Solution: Provide an additional source of micronutrients or the recommended amount of a compound feed or broad-spectrum vitamin and mineral supplement alongside salt.

Take home points:

• Micronutrient provision and overall balance is key to supporting health and performance in the horse.
• Forage only diets, particularly where intake is restricted, are unlikely to meet micronutrient and in some cases quality protein (amino acids)
• As a baseline to balance forage rations, a broad-spectrum vitamin and mineral supplement will be required alongside salt.
• Rations that are low in fresh forage (grass) will likely require additional antioxidant (e.g. vitamin E supplementation).