Forage intake is a measure of the amount of food that a herbivore can eat in a given time. 

If approximate animal numbers are known, intake figures in Table 1 can be used to estimate the likely relative magnitude of the impact of grazing on a patch of vegetation by a range of different herbivore species.

The list of breeds in Table 1 is not exhaustive. When using the figures to calculate stocking density you may find the breed that you have is not included. If you know the live weight of your animals you can calculate dry matter intake yourself using the calculation details in the box below.

Add 40% to ewe intake for a following lamb at the start of summer, rising to 66% at the end of summer.

Add 45% to cow intake for a following calf up to six months old.

Heifers can be anything from 60% to 100% the weight of a mature cow, depending on their maturity.

Supplementary feeding

The effect of supplementary feeding of hay and /or concentrates has not been included in Table 1, though it can have a significant impact on how heavily stock graze the woodland habitats.

The most obvious impact will be in winter when supplementary feeding is most likely to take place. However it is very difficult to give guidance on what this impact might be. It depends on the breed of animal, the season, the amount of feed that is supplied and the amount of forage available within the woodland. If you can estimate how much of the animals’ intake is likely to come from supplementary feed then, as a rough approximation, you can assume that the rest will come from the woodland vegetation. The digestibility of supplementary feed is likely to be higher than that of the woodland vegetation in winter. This will increase the amount that the animals can eat. Winter estimates made in this way may therefore be under-estimates.

Horses in particular are likely to be fed year round and though their forage intake is high the proportion that comes from the woodland vegetation may be significantly less than suggested in Table 1.

Supplementary feeding will also impact on foraging patterns and the intensity with which habitats are grazed. Grazing pressure, as well as poaching impacts, are likely to be relatively heavy around supplementary feeding sites and relatively light in habitats far from the supplementary feeding sites.

If supplementary feeding is necessary, you will need to factor this into your calculation of stocking densities and show the location of feeding sites on your Grazing Management Map.  

Calculating Dry Matter Intake: some technical notes

The amount that a herbivore can eat when there is unlimited food available is directly related to the digestibility of the diet and the animal’s size.  When the digestibility is low, the gut takes longer to digest the food so less can be processed. Large animals have larger guts so can process more food.

Intake is linearly related to metabolic live weight. This relationship can be defined as live weight raised to the power of 0.75.

N.B. If your maths is a bit rusty, you can calculate ‘powers of’ using a Microsoft Excel spreadsheet. For example, if the live weight of a Luing cow is 600 kg, enter this figure in an Excel cell, click on the function button and in the ‘Paste Function’ dialogue box, click on ‘Maths & Trig’ then ‘POWER’ and the spreadsheet will do the rest for you, giving a figure for metabolic live weight of 121.309 kg.

The following equation for arriving at an intake figure has been derived for sheep:

Intake (kg. dry matter/day) = metabolic live weight (kg.) x (0.167 x digestibility) - 0.044,

where summer diet digestibility is assumed to be 0.7, winter digestibility 0.5 and average digestibility for year-round grazing 0.6. 

As a crude approximation, this equation can be used for other animal species to compare intakes. Thus, using the example above, summer intake (kg of dry matter/day) for the Luing cow would be 121.309 x (0.167 x 0.7) – 0.044 = 8.84. 

It is worth noting that, with the possible exception of Highland cattle, cows have become heavier in recent years. If you have more traditional herds, you will want to reduce the figure for live weight and dry matter intake.

The values for horses and ponies are derived using the equation above and the summer digestibility value then multiplying the result by 1.84 to account for differences in their digestive system. They are relatively unaffected by quality so the same value is used in winter.