Feed Assessment

Feeds are assessed so that a judgment can be made as to their nutritive value and the part that they can play in meeting an animal’s requirements for different nutrients. Ideally, the information gathered from feed assessment can be used to select a range of feeds that will together make a balanced diet for the animal, meeting all its requirements for energy, protein, vitamins, minerals and of course water. Laboratory methods have for many years been used to help define animal feeds, assess their nutritive value and provide data for the prediction of animal performance. The sections on assessment of the different dietary components - fibre, starch, protein and energy etc., and on chemical composition of feeds provides information that helps the user to gain a greater understanding of the nutritive content of different feeds.

The Value of Forages Changes with the Season Assessing the Nutritive Value of Feeds Chemical Composition of Feeds Estimating Fibre Content of Feeds Estimation of Starch and Water Soluble Carbohydrates (sugars) Assessing Protein Quality of Feed Assessment of Energy Value of Feeds Guidelines for Formulating Livestock Diets Assessing the Nutritive Value of a Novel Feed Research Projects References and Further Reading

Feed assessment is much more difficult when animals are grazing or browsing as it is very difficult to know exactly what they are eating, which makes assessing what is needed to balance the diet, or supplement for those components of the diet that are insufficient, much less precise. Even when feeds and forages are cut and carried to livestock, they still exercise a degree of selection (considerable selection if there is sufficient forage to enable them to reject less palatable or nutritious fractions of the feed), and so again it is very difficult to assess exactly what has been consumed. Common practice when feeding hay, for example, is to supply an excess - the animals select from what is available and leave those parts of the hay that they do not want.

In the case of tropical livestock, particularly those managed by the resource poor and/or landless livestock keepers, there is very little choice that can be made in the selection of feeds and forages anyway, as it is often a case of feeding what is available rather than what may be best for the animal.

There is often also a great shortage of information on the composition and nutritive value of tropical feeds, and acquiring such information is expensive, time consuming and rarely if ever feasible in an extension situation. However, a number of tropical feeds are described in the FAO database AFRIS (Animal Feed Resources Information System).

AFRIS Animal Feed Resources Information System

The purpose of this section is therefore to help in the interpretation of the information that is provided in typical feed analyses.

An important consideration in the assessment, particularly of forages, is their growing characteristics.

Forages which may have little perceived value to farmers at times of the year when other forages are abundant may have a high intrinsic value later in the dry season when forage availability is low if they are able to persist.

If tree forages are browsed, or lopped for use in cut and carry systems, then an important consideration in assessing their value is how quickly they will grow back to provide more forage.

Farmers’ Assessment

A feed provides a range of different nutrients, and possible toxic or anti-nutritive factors as well. It may be palatable and readily eaten by livestock, or unpalatable and avoided by livestock. It may be suitable for some classes of livestock but not others. It may be ideal as a supplement to an otherwise marginally deficient diet, or a good basis to the diet provided other key nutrients are provided by other supplements.

All these factors contribute to the overall value of the feed, and this ultimately requires the feed to be fed to an animal to determine what its effect on the animal is. This is the basis to farmers’ assessment of a feed, and any information on farmers’ perceived value of a feed and its ranking compared with others is extremely important. However, gathering such information has rarely been done systematically and so there are relatively few reports that provide information on the farmers’ assessment of different feeds. An important first step, therefore, is to find out what experience farmers have of a particular feed. Some work that has been done on collating farmers’ assessments of forages can be found in the publication by Komwihangilo et al. (1995), and also in many of the other publications listed under References.

Animal Assessment

Ultimately, any assessment of a feed’s worth must be based on its ability to support life, growth and reproduction in the animal, and all other feed assessments seek to predict an animal’s response to a particular feed or diet. Feeds are frequently assessed at research stations and on farms by feeding them in different combinations to animals, and measuring the animals’ response.

Although such work is valuable in providing data of how feeds actually "work" in practice, there are a number of limitations to this approach. It is extremely expensive and time consuming, and so this limits the amount of data available on any given feed. Data that are collected also refer only to those particular animals, in that particular situation, with that particular sample of feed. Different animals (even of the same species, breed, age and sex), different situations (climate, weather, exposure to different diseases, changes in the composition of the rest of the diet), and different samples of the same feed (since there can be enormous variation in the composition of feeds, particularly tropical forages) are likely to result in different responses by the animal.

Such tests are therefore really only any good at demonstrating how different feeds compare with each other, rather than providing a definitive estimate of how an animal will respond to a particular feed.

 

The limitations associated with animal assessments of feed quality mean that a number of laboratory estimates of the chemical composition of feeds have been developed, to provide a clearer description of what nutrients are actually provided by a feed. The main series of chemical analyses that are performed are called "proximate analysis", and this seeks to estimate the different components of a feed. (Note: This analysis can accurately measure content of nutrients in a sample of feed, but cannot measure the amount that will be utilized by an animal).

The proximate analysis of feeds is based on the following description.

The plant cell walls constitute the fibre fraction of the feed. Fibre is made predominantly of carbohydrate, and so is potentially a source of energy for the animal, but it is much more slowly digested than starch or sugar. Most fibre, in fact, is indigestible to pigs and poultry although they are able to digest some fibre from vegetables and young leaves. However, their limited ability to digest fibre means that the amount of fibre that can be included in their diet should be restricted to less than 10% of the total diet. Ruminant animals (sheep and goats) on the other hand, are able to digest fibre. However, even sheep and goats struggle to digest very fibrous feeds (such as straw or very woody stems from mature plants and trees).

Pigs and poultry are not able to digest large amounts of fibre, and so much of their energy must come from carbohydrates such as starch or sugars, or from fat. These are also the main sources of energy for humans, and so this does put pigs and poultry in competition with humans for foods, and so the starch or sugar content of the diet for tropical pigs and poultry is usually much lower than that of temperate pigs and poultry kept in industrialized systems in the West.

While the estimation of crude protein content will give a reasonable estimate of the AMOUNT of protein in a feed, of perhaps greater importance is the QUALITY of that protein. The very different digestive systems of pigs and poultry compared with sheep and goats mean that the assessment of protein quality differs for these two classes of animal.

The amount of energy that a feed will supply to an animal can be estimated by measuring the amount of energy that is produced when the feed is completely burnt. This gives the Gross Energy (GE) content of the feed, measured in MJ (or kilocalories). However, it is only of limited use since all feeds (apart from those with a very high fat or oil content) have about the same gross energy content (18 MJ/kg DM). Some other means of evaluating energy values is clearly needed, since straw obviously has a lower energy content than maize grain, but the two have very similar gross energy contents.

The table below gives general guidelines as to how much fibre and fat can be fed to different classes of livestock, and a guide as to how much protein different livestock need. It may not be possible in many instances to provide this much protein, in which case the animal will be more at risk from disease and may not be able to withstand other challenges (such as high temperatures or other stresses) that it may be subjected to. These are rough guidelines only, and a lot will depend on the digestibility of the feeds used, the composition of the whole diet, and the particular situation in which the animal is kept.

When a novel feed is available for feeding to livestock, the first thing to do is try and get any information (from farmers, the literature and other advisory institutions) about its value as a feed. In particular, any experience of its acceptability to livestock (are they prepared to eat it?) and whether any problems with toxicity have been encountered. If the animal will eat the feed, and if there are no toxic side-effects, then it is worth trying to determine what nutrients the feed will provide.

Is there enough of the feed to offer it regularly? Is it abundant enough that it could form the basis of the diet? If so, are there any problems with toxicity if large amounts of the feed are fed? If the feed is not that plentiful, or there is a danger of toxicity if it is eaten in large amounts, it may still be useful as a supplement to the diet to provide particular nutrients that might otherwise be limiting in the diet. The diagram below is intended to give some indication of the type of nutrients that are likely to be supplied by different classes of feed. It is not a complete categorization of all feeds, but may help in deciding when a feed might be useful in supplementing an animal’s diet.

Morrison, B.J., Gold, M.A. and Lantagne, D.O. (1996). Incorporating indigenous knowledge of fodder trees into small-scale silvopastoral systems in Jamaica. Agroforestry Systems, 34:101-117.

Proceedings of the National Level Workshop on Improved Strategies for Identifying and Addressing Fodder Deficits in the Mid-Hills of Nepal, Kathmandu, Nepal, 5th-6th September, 2000. Department of Forest Research and Survey (DFRS), Kathmandu, Nepal: 2000. pp73-79.