Applied Animal Nutrition-I

Factors affecting digestibility of a feed

FACTORS AFFECTING DIGESTIBILITY OF FEED

A. ANIMAL FACTORS

B. CHEMICAL COMPOSITION OF THE FEED

C. PREPARATION OF FEED

A. ANIMAL FACTORS

Species

• Roughages high in crude fibre are better digested by ruminants than by non-ruminants due to the pre-gastric fermentative digestion that occurs in the ruminants.
• In several non-ruminants, post-gastric fermentative digestion occurs which helps in digestion of crude fibre.
• Pre-gastric fermentative digestion is highly efficient since the nutrients released are digested and absorbed in stomach and small intestine.
• The ruminant is more efficient in the digestion of high-fibre, low protein forage; whereas the simple stomached pig is more efficient in digestion of high protein, low-fibre feedstuffs.

Age

• Very young or very old animals are usually less efficient in their digestion of feeds.
• The young ruminants can neither eat nor digest roughage until their digestive tracts, specially their rumens are developed.
• After 6 months of age there appears to be no difference in the digestibilty of the ration in the calf and adult dry animal.
• In newly born piglets, development of digestive enzyme system takes place gradually.
• Digestibility of fat in chickens is higher in adults than in young chicks.
• In case of old animals their ability to chew feed is impaired by worn out teeth.
• Declining health and reduced secretion of enzymes may adversely affect the digestibilty at an advanced age.

Work

• Light work seems to improve digestibility of feeds, while heavy work depresses it.

Individuality

• Individual variation of as much as 25% has been observed in the digestive ability of the same feed among animals. However, most animals have shown variation of about 4 - 5%.

Level of feeding

• Generally when more feed is consumed by the animal the rate of passage of the digesta in the alimentary canal is faster and the digestibilty declines due to lesser retention time. This effect has been significantly observed in ruminants and to some extent in swine as well. 

B. CHEMICAL COMPOSTION OF FEED

• Generally grains are well utilised by all classes of livestock.
• The digestibility of forages is closely related to the chemical composition.
• The chemical composition of the forage is affected by number of factors  like soil composition, manuring and fertilization, water supply, stage of maturity of the plant, frequency of cutting, variety and strain of the plant, climate, etc; the predominant factor being the stage of maturity when cut.
• Differences among varieties within the same species may be due to the physical composition of the plant.ie: leaf to stem ratio, soil fertility, etc. Early cut fodder has higher digestibility than late-cut.
• The protein, minerals and vitamins decreases while crude fibre increases as the plant matures.

C. PREPARATION OF FEED

Particle size of the feed

• Grinding of grains and other feed helps to improve digestibility in young piglets with undeveloped teeth and in older animals with worn out teeth.
• In general grinding increase digestibility, because of increase surface area for enzymatic action and disruption of grain coat. If grain or any other feed is ground to a fine particle size, the feed is less palatable and digestible. If roughages are ground to fine grinding, digestibility of fibre is decreased while total consumption is increased due to increased rate of passage.
• Rumen fermentation pattern is also changed due to fine grinding of feed. 

Soaking

• Soaking of grains and feed in water before feeding generally increases digestibility.

Processing of grains/feed

• Processing by boiling, steam processing, micronization, pelleting, extrusion cooking, improves their digestibility. However some processing methods depress digestibility due to increased dry matter consumption and the eventful faster rate of passage. This is more conspicuous in pelleting of roughages, where digestibilty of DM and crude fibre decreases.

Nutrient content in the ration/ration composition

• Protein level: When several feeds are fed in a ration, one feed may influence the digestibility of the other. This "associative effect" of feeds on one another's digestibility is more evident in the case of ruminants, when the addition of a protein or NPN compound to a low protein ration increases the microbial  digestion of the crude fibre by stimulating  the growth of microorganisms in the rumen. Thus as the dietary protein level increases, the digestibility of all the nutrient increases. Similarly, as the dietary protein level is lowered ,the digestibility of all the nutrients decreases.
• Carbohydrates: The nature and level of dietary carbohydrates affect the digestibility of all nutrients present in the diet. In ruminants, excessive levels of soluble carbohydrates (eg.molasses 7% and above) results in lower microbial breakdown of crude fibre. It tends to depress not only the digestibility of cellulose, hemicellulose, etc., but other nutrients also. High crude fibre content of mixed diets decreases their digestibility. The higher the percentage of crude fibre in a ration, the lower is the digestibility of dry matter and all other nutrients.
• Lipids: Addition of oil or fat in a diet increases the digestibility coefficient of ether extract, as such fats have higher digestibility than other constituents of the ether extract. Higher levels of fat in the diets generally reduce the digestibility of other nutrients, particularly of dietary fibre.
• Minerals: In the diets of pigs and poultry, mineral content does not seem to influence the digestibility of other dietary constituents, while mineral deficiency produces  more severe deficiency symptoms in their body. Deficiency of minerals in herbivorous animals limits the growth of microorganisms and this will reduce the digestibility of crude fibre and of other nutrients as well. Adequate amount of salt and water tend to improve digestibility.