lamb and pasture

Some feed for thought on performance

By Bruce McCorkindale

A challenging opportunity facing sheep farmers right now is how to lift your average lamb carcass weight by around 3kg – without changing your kill date profile. How could you do it and what changes would it require?

An increase of 3kg carcass weight is about 7kg liveweight. If you can pick up 2kg of this by weaning then the last 5kg needs to come from increasing post weaning lamb growth. If your average kill date is around 110 days post weaning then lamb average growth rates need to rise around 45g/day. Typically this increase will need to come from an increase in feed quality and in some cases, a combination of quality and quantity – in other words it is a “ME (metabolisable energy) story”, assuming that your lambs have the genetic capability to grow faster.

In low ME pasture, there is typically a low clover content and it has a higher than ideal proportion of grass stems and dead material.

In contrast, a high ME pasture will mean that the lamb could close its eyes and eat everything on offer. Every bit that goes in its mouth will be highly nutritious and it will be quickly digested to release lots of energy for growth.

A 35kg liveweight lamb needs around 9 megajoules (MJ) of energy each day to maintain itself on easy country and it takes around 4 MJ above maintenance for each 100g of lamb growth.

A lower quality pasture may have a ME of around 10MJ/kgDM but lambs will pick the best bits out of it, so their diet would be around 10.5 ME, but they use more energy searching. The poorer quality material slows down the passage of food through the gut, which means the animal doesn’t feel as hungry and consequently doesn’t eat as much.

The net result is lamb intake around 1.2kgDM per day. At 10.5 M.E., the animal is eating 12.6MJ of which 9MJ are needed for maintenance, leaving 3.6MJ for growth. That is enough for a growth rate of around 90g/day.

If we move the quality offering up a bit so that it is easier for the lamb to eat more and there is more clover in the diet, then the same lamb will readily eat 1.6 kgDM. If the ME of the diet has risen to 11.5, then energy available for growth rises to 8.6MJ, which would drive a growth rate of 215g/day. To get to 300g/ day, we need feed of 12.5 ME. A critical management effect is knowing when to move the lamb mob on before they need to start eating the less digestible material.

What does a 12.5MJ pasture look like? Basically, lots of legume, young grass leaves (they could be from older pastures) with low stem content, very low dead material, and other easily digestible leaves such as chicory and brassicas.