New Zealand sheep farms tend to have a high level of lamb losses. Wairarapa vet Sara Sutherland looks at what can be done.

Lamb survival at lambing is a major production and welfare issue for commercial sheep farmers in New Zealand. Losses range from 5% to 50% per farm, with an average of 20-30%. This is considerably higher than in other sheep-producing countries, even those with extensive systems. Most losses occur in the first 48 hours. A number of factors influence lamb survival, some we can control and some we can’t.

Wind and rain have the most visible effect. Little can be done apart from putting our most susceptible ewes in the most sheltered paddocks.

Lamb survival is also influenced by maternal behaviour. This can be selected for genetically between breeds and within a breed. Surprisingly, this is also influenced by pasture cover – ewes take better care of their lambs on 4cm of grass than 2cm of grass.

Lambs that stand and nurse quickly are more likely to survive. The energy level of the lamb depends to some extent on the amount of “brown fat” the lamb is born with. This can be influenced by making sure ewes are in BCS 3 before lambing, and on good feed for the last month before lambing. Twins and triplets have less brown fat each than singles do, so good feeding is even more important for multiple-bearing ewes.

Fat ewes are no more likely to get a bearing than skinny ewes. Don’t limit feed to ewes close to lambing in an effort to reduce bearings – it won’t, and will potentially have a major detrimental effect on lamb survival.

Once the brown fat has been used, lambs need colostrum. Ewes on low covers will give less colostrum, and less milk. Making sure ewes are on good covers for the last month before lambing will improve milk production. Ewes with crook udders or mastitis give less milk. We are looking forward to the results of a recent comprehensive study by Massey University on udders and udder abnormalities.

Death losses of ewes on commercial hill-country farms are also variable and higher than losses in other countries. Studies trying to quantify the reasons for losses haven’t had a lot of success.

Dystocia is lambs getting stuck during lambing, either because they are too big or because several are trying to come out at once. This is another significant cause of death in ewes that we can’t do much about. Breeders are talking about an EBV (breeding value) for small size at birth, for rams used for hogget mating, as is done in beef cows, but nobody has used the method yet. Feeding level of ewes in the last month before lambing will not make the lambs bigger – by that stage lamb size has been set.

We know that bearings (vaginal prolapse) are a major cause of ewe death on some farms in some years. A number of possible causes have been studied. There is some suggestion that feeding levels between mating and scanning can have a big effect. Apart from that, the only factor that reliably prevents bearings is to set up a trial to study bearings on a farm – this will guarantee that fewer bearings happen.

Fat ewes are no more likely to get a bearing than skinny ewes. Don’t limit feed to ewes close to lambing in an effort to reduce bearings – it won’t, and will potentially have a major detrimental effect on lamb survival.

Another cause of ewe death is metabolic conditions (“down ewes”). This includes low energy (ketosis, also called “sleepy sickness” or “twin lamb disease”), and low calcium (“milk fever”). The balance of calcium, magnesium, vitamin D and phosphorus, and their influence on energy, is extremely complicated.

First, ketosis. Before the lamb is born, the ewe needs to provide it with energy in the form of glucose. That foetus, or foetuses, will take glucose every day, even if the ewe is not eating. Glucose can come from feed intake, or from fat. However, making glucose from fat requires energy, and puts a load on the liver. The liver has hundreds of important roles in the body and can get overwhelmed.

If the ewe is on limited feed, or stops eating because of illness or a spell of bad weather, the liver may just not be able to cope. The outcome is either subclinical ketosis (the ewe survives but the lamb is born dopey), or clinical ketosis (aka sleepy sickness) (the ewe goes down, and dies if not treated).

A ewe that has had facial eczema will have limited liver function, as will a ewe that has been too fat (commonly seen on lifestyle blocks, but also keep an eye on the kids’ pets!). Older ewes will have livers that just aren’t quite as good at handling glucose (just like old people don’t process alcohol quite the same way they did when they were young!). Two foetuses need more glucose than one – so older twin and triplet-bearing ewes are more susceptible to sleepy sickness.

Secondly, milk fever. Calcium is used for all sorts of things in the body, including keeping the heart beating, rumen moving, muscles contracting, and temperature regulated. In late pregnancy the requirement increases dramatically as calcium is needed to form the skeleton(s) of the lamb(s).

In a normal, healthy pregnancy, the ewe gets calcium from the grass until late in pregnancy when that intake from grass isn’t enough and she starts to pull it out of her bones. At some point in the pregnancy this changeover from gut to bones has to happen. Unfortunately, it takes a couple of days for the changeover to take place.

Spring pasture in NZ is high in calcium, so ewes are used to absorbing calcium from the diet in high quantities every day and don’t become efficient at absorbing it. When they are off feed (weather event, or prelamb shearing for example), they still have this demand for calcium. They try to draw it from their gut but have nothing in their gut. They can’t switch over straight away to drawing it from their bones, so their blood calcium levels drop and they get milk fever.

A ewe that is low on calcium looks identical to a ewe with sleepy sickness – she will be lying down and reluctant to stand up, “dopey” or “sleepy”, and may be bloated. Milk fever and sleepy sickness often happen together, and telling them apart isn’t usually important.

Appropriate feeding close to lambing will mean ewes are less likely to get sleepy sickness, ewes are less likely to get milk fever, lambs will be born with more brown fat and be less likely to be dopey because of subclinical ketosis, ewes will have more milk and have better maternal behaviour. They won’t have more oversize lambs or get more bearings.

The only downside is you might have to call in a couple of extra teenagers to help with docking because of all the extra lambs!

  • Sara Sutherland is a vet with Vet Services Wairarapa