January 3, 2009

Sheep Farm Husbandry - Feeding sheep: Nutrition principles

Sheep, farming, principles of nutrition, nutrients, protein, carbohydrates, fats, minerals, vitamins, water, energy units, dry matter, maintenance & production, digestibility, metabolisable energy

By Dr Clive Dalton

This is not the most exciting subject as there’s always too much dry theory and too many tables. But feeding, which is mainly pasture in New Zealand, is the main input in a sheep enterprise so it’s important to know how to grow and utilise it without waste, so it all goes through productive stock to end up as profit.

In practice, feeding sheep is simply about when you should open the gate into the next paddock, and how long you should leave them there. It’s easy to assume there’s not much science behind this simple decision, but read on and you may be surprised.

What goes in and what comes out?
The basic concept is that the feed eaten by the sheep must meet the animal’s total needs. The difficult part is to know what these needs are, because they vary greatly throughout the year, from being dry to pregnant, putting on body weight and growing wool. Then pasture varies in quality and quantity almost every day of the year and keeping all this in balance is why nutrition and feeding are described as being more art than science.

Here are some basic principles which you can skip to get on to the practical stuff. If you always have enough feed for your stock and they are performing well, miss the chapter out altogether!

Dry matter (DM) – what is it?
  • This is feed with all the water removed, and it’s very useful when you want to compare feeds and especially when buying them.
  • For example if you are offered surplus apples at 30cents/kg in the orchard, and the DM is only 10%, is this a good buy or not? It’s maybe better to buy meal at 80cents/kg which has 86% DM. You could be buying very expensive water in the apples - even if you get free delivery.
  • DM is an easy concept to understand as you can visualize a dry kilogram of a feed, and you can measure it yourself in a feed by drying it.
  • Energy Units (see later) which are now being used to supercede DM are measured in Megajoules (units of heat) and are much harder to visualize, and you certainly can’t measure them yourself.
  • So to start off thinking about feeding, think DM first.
The concept of Maintenance and Production
  • An animal uses part of its feed for “maintenance” which is to keep its basic functions working. It’s like a stationary car, parked with the handbrake on and the engine just ticking over.
  • So a sheep needs nutrients like protein, carbohydrates, minerals and trace elements to maintain basic body functions like body temperature, digestion, blood flow, organ function and minimal movement.
  • Then there are the feed nutrients needed for “production” which is over and above maintenance. This is needed when the animal starts to grow, become pregnant, lactate or walk long distances to find feed or water. It’s a car now moving with your foot on the accelerator.
  • How much an animal needs for maintenance is based mainly on its live weight.
Important nutrients in a diet
  • These are needed for growth and muscle so are important for young stock such as young lambs that need 13-14% protein of their total DM intake.
  • Lactating animals need about the same (as above) with finishing animals slightly less at 10-11%. Mature stock on a maintenance diet only need 8.5% protein.
  • Once animals get on to good green pasture – a protein deficiency is not very likely. Indeed in the spring flush there is too much protein and it is wasted, passing rapidly through the animal and not being digested.
  • Clovers have three times the amount of protein found in grasses.
  • Carbohydrates in feed provide energy, and meeting the animal’s energy needs will be your main concern.
  • Good short green pasture is rich in energy as well as protein, but the problem is to provide this all year round.
  • Silage made from pasture or a crop when it was short and green and hence highly digestible is an ideal source of energy and protein, especially when pasture is in short supply in winter and droughts.
  • Cereal grains are the highest source of energy but they are costly in New Zealand.
  • Fats are similar to carbohydrates but supply more energy and you’ll see they are included in diets for young rapidly-growing animals kept intensively.
  • Fat is nature’s way of storing large amounts of energy in a concentrated form in the body. People trying to slim and get rid of fat know all about this.
Vitamins, minerals and trace elements
  • There’s a wide range of these each with a specific function, and it’s easy to be concerned about possible deficiencies in your stock. Copper, selenium and cobalt are the main ones.
  • The key is to provide a balanced diet containing high-quality green pasture and you need not worry too much about deficiencies.
  • Seek veterinary advice if your stock are not performing well, to check if it’s a vitamin or mineral (trace element) deficiency.
  • Water is not a nutrient but don’t forget about it. It’s essential to life and good health.
  • It must be good quality (potable) water so it’s important to have it tested if you suspect the water source may have problems – and there are plenty of them.
  • Examples are bacteria (coliforms and faecal coliforms) and algae that can cause disease, and toxic minerals like manganese and iron, as well as chemicals like nitrates. These are not good for humans and they are not good for animals either.
  • There is also the problem of dead birds and vermin in water supplies which risk both animal and human health.
Digestibility – what nutrients stay inside the sheep?
When a sheep eats feed, only some of the nutrients are digested and end up in the bloodstream to be used for maintenance and production. What is left passes out in the faeces so the “digested” nutrients are the important bits.

It’s good to know the digestibility of the DM, the protein, and the energy in the feed as these are what cost money. Here’s the full definition of digestibility written as an equation.

Digestibility % = [(Feed Eaten – Feed excreted) / Feed eaten] x [100 / 1]

It’s not surprising that feeds vary greatly in their digestibility, as it’s controlled a lot by the maturity of the plants which make up the feed. As plants mature, fibre content increases (which is harder to digest) and protein decreases (which is easy to digest).

Key points from the table
  • Milk is a highly nutritious feed. Nature got it right!
  • Young grass is a good feed but it declines in value with maturity.
  • Note the importance of always feeding good hay and good silage.
  • Straw has very little feeding value but it’s useful if you want to make the animal feel full.
  • Root crops have good feeding value but are very high in water providing bulk which restricts how much the animal can eat.
Feed conversion efficiency (FCE)
  • This is also called Feed Conversion Ratio (FCR) and is used to measure how much live weight gain (LWG) you get for the feed eaten.
  • It’s the basis of profit and is expressed as kg DM/kg LWG.
  • FCE is easy to measure in pigs and poultry that are fed exclusively on grain, but measuring pasture intake in individual grazing animals is too difficult on the farm.
  • All we can do is assess feed before and after grazing on a hectare basis, and not quote FCE values for individual animals.
Energy Values
  • Here you go a step further than measuring just the DM%. You find out from a laboratory feed analysis how much energy there is in the DM – expressed as an “Energy Unit” and this will cost you money to find out.
  • Energy is what animals are most often short of for most of the year, and it’s what you’ll have to buy when things get difficult.
  • When broken down, the term is a bit of a mouthful. It’s the “Megajoules of Metabolisable Energy per Kg of Dry Matter” in the feed.
  • It’s abbreviated to “MJ/ME/Kg DM” and then to “ME units” which thankfully avoids the need to pronounce “metabolisable” (met-a-bol-iz-able)!
  • Farmers aware of costs now can get ME values done on their hay or silage before they start feeding out, and certainly on any feed before they buy it.
What is ME?
  • If you take a feed and explode and burn it in a special chamber in the laboratory, you can measure the total heat (energy) produced from it. This is called Gross Energy.
  • But the animal doesn’t use all that, as energy is lost in faeces and urine and methane belched from the rumen.
  • What’s left after those three parts have been removed is the energy that can be “metabolised” for maintenance and production – called Net Energy.
  • A part of ME is used to provide heat to keep the system going, so when you see the letters ME – it’s the energy in the feed that the sheep can put to full use.
  • Using energy is a big advance from just thinking about DM – you now can find out how much energy is in each Kg of the DM as shown in the table below. But be warned about the limitations of these general figures and get a proper analysis done on a representative sample of feed from a registered laboratory.
RG = ryegrass WC = White clover

This material is provided in good faith for information purposes only, and the author does not accept any liability to any person for actions taken as a result of the information or advice (or the use of such information or advice) provided in these pages.

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