Farm Tech

In The Paddock
Seasonal sheep flock management tips for August 2001

The late pregnancy countdown

Key points

• Late pregnancy is a critical time for breeding ewes
• Priority should be given to feed management so that ewe health is optimised and to ensure there is enough colostrum and early milk for lamb viability
• Shearing and ultrasound scanning in mid-pregnancy are useful tools for improving lamb birth weights and survival. They also lead to other improvements in lamb production
• After scanning, single- or multiple-carrying ewes can be run in separate mobs during late pregnancy for differential feeding
• Shearing crossbred ewes with winter combs and lifters in mid-pregnancy can result in increased lamb birth weights and improved lamb survival.

Good feed management of ewes in late pregnancy is a vital part of successful sheep farm management.

While more than 70 per cent of foetal growth occurs during late pregnancy, the factors that influence lamb birth weights are largely in place by this time. These include the development of the placenta - which occurs in the first third of pregnancy - and the condition of the ewe.

Nevertheless, ewes need to be fed at recommended levels (see below) to prevent sleepy sickness, milk fever and birth difficulties. Most udder development also occurs at this time, so adequate feed is needed to ensure there is enough colostrum and early milk for lamb viability.

It's a common misconception that tightening up the ewes in late pregnancy will significantly reduce the incidence of dystocia (difficult births) and bearings. It won't.

While deaths from dystocia occur in very big heavy lambs, especially singles, it is now too late to use reduced feeding levels to remedy the problem.

On the other hand, if ewes are under-fed, they are likely to produce insufficient colostrum and milk, resulting in poor lamb survival and growth. In ewes carrying twins, sleepy sickness may result.

Lamb survival is directly linked to birth weight, with starvation and exposure occurring most commonly among light lambs.

For example, in lambs weighing from 3.1 to 4.1 kg at birth, every 0.2 kg increase in birth weight reduces lamb mortality from starvation and exposure by 1.0 per cent.

Any increase in lamb birth weights is magnified several-fold by weaning and slaughter - resulting in improved overall lamb production. However, in flocks where ewes are reasonable condition, varying feed levels in late pregnancy will have minimal effects on lamb birth weights.

Only where ewes are in very poor condition, with no fat or energy reserves, will feeding levels have a significant effect on lamb birth weights and viability. Ewes in this situation should be given priority feeding.

Shearing in mid-pregnancy, because it has a direct influence on birth weights of twin lambs, is a useful tool for improving lamb survival.

How to make the most of scanning

Many farmers, especially those with high lambing percentages, ultrasound scan their ewes in mid-pregnancy between days 60 and 90 of pregnancy.

This enables ewes carrying more than one lamb to be identified and given preferential feeding in the last six weeks of pregnancy. This helps ewes carrying multiples to maintain their condition, leading to improved early lactation performance after lambing.

Ideally ewes should be at condition score 3-plus in the run-up to lambing. Seasonal conditions may make this hard to achieve, so monitoring feeding levels is of critical importance. Aim to have twin bearing ewes at the same live weight at weaning as single bearing ewes.

During the last six weeks of pregnancy, feed single-bearing ewes at an average of 1.5 times maintenance. Twin-bearing ewes need 1.75 times maintenance.

For a 60 kg ewe, this is an intake of good quality pasture dry matter of 1.8 and 2.1 kg DM/ewe/day. To achieve this, allow single-bearing ewes to graze pasture down to about 700 kg green DM/ha. Twin-bearing ewes should graze only to 950 kg green DM/ha.

In many seasons, these levels of pasture cover will be difficult to achieve. Thought should be given to the use of hay or silage to slow the rotation, so that the feed wedge can be maintained.

Shearing ewes in mid-pregnancy

Birth weights of twin lambs can be increased and lamb losses reduced by shearing crossbred ewes in mid-pregnancy.

In some trials at Massey University, birth weight was increased by up to 1.0 kg. In a large field trial involving more than 1000 twins lamb survival increased by 3.0 per cent.

The best response is likely to be between days 50 and 100 of pregnancy - with the optimum around day 70 - so it can be done at the same time as scanning. Genuine winter combs or winter combs plus lifters must be used.

Afterwards, put the shorn ewes in a paddock with plenty of shelter. Shelter, rather than increased feeding, is the key factor in reducing cold stress faced by sheep shorn with winter combs.

More information

• A Guide to improved lambing percentage, Chapters 4 & 5.
• A Guide to improved lamb growth, Chapter 3.
• Kenyon et al., 1999. Proceedings of the NZ Society of Animal Production 59:70-72.

Your local farm consultant or veterinarian will also be able to help. Alternatively, contact your local Meat & Wool Innovation extension specialist: