Animal Nutrition and Digestive Systems

Three guts, three diets

Farm animals do not all digest food the same way, and the difference decides what you can sensibly feed them. **Ruminants** (cattle, sheep) have a stomach in four compartments and can live on grass, because microbes inside them break down cellulose that the animal itself cannot. **Monogastrics** (pigs) have one simple stomach, like ours. They cannot get much out of grass, so they need cereals and other concentrated feeds. **Birds** (poultry) have no teeth at all, so grinding happens further down, in a muscular organ that does the chewing for them. Two separate ideas run through this topic. **The gut decides the diet. The job decides the ration.** Keep them apart.

Where the grass actually breaks down

A cow's stomach has four compartments. Tap the one where microbes ferment grass and break down cellulose. It is by far the largest of the four.

Why a cow can live on grass and a pig cannot

Grass is mostly cellulose. What lets a cow use it when a pig largely cannot?

• The cow's rumen holds microbes that ferment cellulose for her; neither animal makes an enzyme that can digest it
• The cow produces a cellulose-digesting enzyme and the pig does not
• The cow's teeth grind grass finely enough to digest
• The cow is simply much bigger

Three digestive systems side by side

What each gut is built to handle, and what that means at the feed trough.

Label the hen's digestive tract

Five parts of this hen's digestive tract are marked. Drag the right name onto each one. Two of the labels belong to a ruminant, not a bird, so you will not use them.

Match each part to what it does

The feed vocabulary

Five words the exam expects you to use precisely rather than loosely.

How much dry matter is she really eating?

A dairy cow eats **45 kg of grass silage** a day. The silage is **30% dry matter**. How many kilograms of dry matter is she actually taking in? Give your answer in kg.

Why dry matter matters

Select the TWO correct statements about dry matter.

• Comparing feeds by fresh weight is misleading, because a wet feed is mostly water
• Rations are worked out in dry matter, since that is the part supplying energy and protein
• It means the animal does not need drinking water
• A feed with a high dry matter percentage is always the better feed

Maintenance first, production on top

An animal's ration is built in two parts, and mixing them up is the most common mistake on this topic. **The maintenance ration** keeps the animal alive and at a steady weight. It does nothing else. An animal that gets only this survives but produces nothing. **The production ration** is what you add on top for whatever job the animal is doing: growing, milking, carrying a pregnancy, laying eggs, growing wool. So two cows of the same weight can need very different amounts of the same feed. Requirements also vary with **species, sex, breed, age and pregnancy**, and a cow in late pregnancy needs noticeably more than the same cow dry and empty.

Feeding the herd

Cattle and sheep are _____, digesting cellulose with the help of microbes in the _____. A pig has a single simple stomach and so needs _____ rather than grass alone. Bulky farm-grown feeds such as silage and hay are called _____. The part of any feed left once the water is removed is its _____, and it is on this basis that rations are calculated. Feed that keeps an animal alive at a steady weight is the _____ ration.

Who needs the bigger ration?

Two ewes of the same breed and the same weight are housed together. One is six weeks from lambing with twins; the other is not in lamb. What does the pregnant ewe need, and why?

• Considerably more, because her maintenance requirement is unchanged but she now has a production requirement on top for two growing lambs
• The same, since the ewes weigh the same
• Less, because a pregnant ewe is less active
• The same amount but with all the fibre removed

A ration worked out

**"A dairy cow weighs 600 kg and is giving 25 litres of milk a day. Explain how her ration is made up. (6 marks)"** "Her ration has two parts. The maintenance ration is set by her body weight and keeps her alive at a steady 600 kg; it supports no milk at all. On top of that comes the production ration for 25 litres of milk. This is where the concentrates come in: forage alone rarely supplies enough energy and protein for a cow milking well, so silage is fed to appetite and concentrates are added according to yield. The ration is worked out in dry matter rather than fresh weight, because silage is roughly 30% dry matter while concentrates are nearer 86%, so equal fresh weights of the two are nothing like equal amounts of feed. Fibre must stay in the ration even though it is not the energy source, because the rumen stops working properly without it, and minerals are supplied separately since a shortage causes disorders even when energy and protein are right." **Notice the shape**: maintenance, then production, then the dry-matter point, then the two things people forget. That order works for any species this topic asks about.

Follow the feed through

Put these stages in the order grass takes through a cow.

• The cow grazes, swallowing grass with very little chewing
• Microbes in the rumen begin fermenting the cellulose
• Food is regurgitated as cud, chewed thoroughly and swallowed again
• Water is absorbed as the material passes through the omasum
• Acid and enzymes act on it in the abomasum, the true stomach

Build a dairy cow's ration

You are planning winter feeding for a herd of dairy cows housed indoors. Work through the decisions.

• What forms the base of the ration?
• Yields rise after calving and the forage is no longer enough. What do you do?
• You are comparing two feeds on paper: 45 kg of silage against 10 kg of concentrates. How do you compare them fairly?

Your turn: feed the animal

Explain how digestive anatomy and production demand together decide what a farm animal is fed.

• Name the four compartments of a ruminant stomach and say what two of them do
• Explain why a pig cannot be kept on grass alone but a cow can
• Define dry matter and explain why rations are calculated on it rather than fresh weight
• Explain the difference between a maintenance ration and a production ration, using one example of an animal whose requirement changes