Managing and feeding the pregnant ewe

Alastair Macrae

Senior Lecturer in Farm Animal Health and Production

BVM&S, PhD, CertSHP, DCHP, DipECBHM, DipECSRHM (Non-Practising), MRCVS

Dairy Herd Health and Productivity Service, Division of Veterinary Clinical Sciences, Royal (Dick) School of Veterinary Studies and the Roslin Institute, University of Edinburgh, EBVC, Easter Bush, Roslin, Midlothian EH25 9RG

Tel – 0131 651 7474

Email

Word count: 2638 words

Abstract

Issues with nutritional management in late pregnancy can have potentially harmful effects on ewe body condition, ewe survival, ewe health, perinatal lamb mortality and lamb growth rates. Use of ultrasound scanning to determine foetal number and body condition scoring are key practical tools that can used to feed pregnant ewes appropriately. Blood sampling for the assessment of nutritional status 3-4 weeks prior to the start of lambing can be used to rapidly identify problems, and correct them before the start to impact on flock profitability.

Introduction

Problems with nutritional management in late pregnancy can have potentially harmful effects on ewe body condition, ewe survival, ewe health (such as pregnancy toxaemia and hypocalcaemia), perinatal lamb mortality and lamb growth rates (due to low lamb birthweights, disease and low milk yields in the ewes)(Phythian et al. 2014). Understanding the basics of flock management and assessment of nutrition is therefore critical for sheep flock profitability.

How do you know there are issues with nutritional management during pregnancy?

The main point is to identify problems with the ration as early as possible. This provides an opportunity to change nutritional management before clinical problems occur and/or money is lost, and to see if any changes in the ration in response to problems are working as anticipated.

Table 1. Author’s suggested production targets for a typical UK lowland sheep flock (based on top 25% performance in flocks recorded using QMS and EBLEX), and potential relationship to nutrition.

Production target / Suggested target / Nutritional influence / Other influences
Ewe mortality / < 2% / All stages / Disease
Barren rate / < 4% / Ewe nutrition at tupping and 1st trimester / Disease, infectious agents, stress, ram
No. of lambs born/ 100 ewes / 180 - 200 / Ewe nutrition at tupping and 1st trimester / Disease, abortion
Lamb birthweights / Twins > 4.5kg
Triplets > 3.5 kg / Ewe nutrition in 2nd and 3rd trimester / Disease, abortion
Perinatal mortality / Below 7.5% / Poor lamb birthweights
Ewe nutrition in 3rd trimester (colostrum) / Dystocia, infectious disease, skilled labour, colostrum
No. of lambs sold/ 100 ewes / 160 - 180 / Ewe nutrition throughout pregnancy / All of the above! Disease in lambs
Lamb growth rates (to weaning) / 200-300g/day / Ewe nutrition in 3rd trimester and lactation / Disease,
lamb birthweights
Lamb growth rates
(to finishing) / 300g/day
Depends on system / Lamb nutrition
Specific trace elements / PGE, other disease

Table 1 gives some suggested targets for UK lowland flocks, and require to be adapted to individual flocks (taking into account breed differences, management, climate etc.). Comparison to current EBLEX and QMS data would imply that most UK sheep flocks are significantly underperforming compared to these targets. Other parameters such as stocking rates, concentrate usage per ewe/lamb and efficiency of liveweight gain may also be available and give an indication of flock performance. The major problem with most of these measures is that they are retrospective, and poor production will already be causing economic loss before it is identified.

Foetal development during pregnancy in sheep

Pregnancy in the ewe can be divided into three stages (see Figure 1):

  • First trimesterImplantation10 – 42 days
  • Second trimesterPlacental development42 – 90 days
  • Third trimesterFoetal growth90 days – lambing

Implantation (first trimester)

The aim at this stage is to avoid any unnecessary changes in either management or nutrition, which may result in early embryonic loss. On most farms, this is performed by keeping the ewes on the same pasture during tupping and for the 4-6 weeks afterwards, to minimise any such changes.

Placental development (second trimester)

Although the nutritional requirements of the placenta are relatively small (Figure 1), and easily met under most management systems, nutrition can influence the size and surface area of the placenta and thus have a major effect on foetal size and thus lamb birthweights. Both severe undernutrition (losses of over ¾ unit of body condition score) as well as overfeeding during this stage has been shown experimentally to reduce lamb birthweights. Provided that the ewes are in good body condition (over BCS 3.0), they can be allowed to drop up to ½ unit of body condition score in mid pregnancy (Russel 1984; SAC 2010). However thinner ewes should be fed to maintain body condition.

Real-time B mode ultrasound scanning at 50 – 90 days of gestation to determine foetal number will help with grouping of ewes at lambing, and nutrition in the last trimester (White and Russel 1984).

Foetal growth (third trimester)

75% of the foetal growth occurs in the last 6 weeks of pregnancy (Figure 1), and thus nutrition of the ewe at this critical stage has a major influence on lamb birthweights. This, combined with the requirements for colostrum production and ewe survival, means that energy supply to the ewe in late pregnancy is crucial to the reduction in lamb perinatal mortality. Numerous studies (reviewed by Dwyer et al. 2016) have shown that underfeeding of ewes in late pregnancy (in particular twin and triplet bearing ewes) results in increased levels of perinatal lamb mortality.

Protein nutrition in the pregnant ewe

Work from SAC/SRUC and others has highlighted the importance of protein nutrition in late pregnancy. Like all aspects, balance is the key to protein nutrition in late pregnancy, and sheep need a balance of rumen degradable protein (ERDP) and bypass protein (DUP). ERDP is important for the rumen microbes to work efficiently and produce protein, and supply can be limiting with diets based on poor quality hay or straw.

However SAC work also highlighted the importance of bypass protein (DUP), which is necessary to meet any shortfall in late pregnancy when the rumen is working at full capacity. In SAC trials, increasing protein supplementation via DUP by roughly 20% above existing Agricultural and Food Research Council (AFRC) recommendations resulted in heaver lambs at birth, increased milk production, reduced worm burden and worm egg output (SAC 2010). In the author’s opinion, there is debate about whether this increased protein supply was above requirements (ie. sheep were being fed excess protein), or whether AFRC recommendations are insufficient and so this supplementation was correcting a deficit already identified by reviews such as Robinson 2002 (ie. sheep were being fed to meet their higher protein requirements).

The SAC work looked at feeding twin-bearing Greyface or Mule ewes 200 grams of soya per head per day for the last 3-4 weeks of pregnancy (the rule of thumb advised was 100 grams of soya per day per lamb carried). This was then increased to 400 grams per head per day in early lactation to maintain the benefits (especially for worm control).One of the key findings was the reduction in worm burdens and egg outputs (Figure 2). As twin-bearing ewes excrete more eggs than singles, one recommendation was to separate off the twin bearing ewes in late pregnancy and target only these ewes for protein supplementation. The take-home message was that we need to look beyond the crude protein content of the concentrate (which is crude – as it implies), and feed diets in late pregnancy that are correctly balanced for protein supply.

Practical nutrition of the pregnant ewe

  1. Grouping of ewes

Ideally ewes should be grouped according to requirements, making feeding and management easier. For housed ewes about 30-40 ewes per group is the target, whereas 100-150 per group is normal for ewes kept outside. This enables the ewes to be checked efficiently, and regularly condition scored to see if adjustments are required. Considerations for grouping include:

Lambing date. This can be accomplished by changing raddle/keel colours at 7-9 day intervals. Important in stepped rate feeding systems, but less critical on flat rate systems.

Body condition score. This is particularly helpful for thin ewes, which can be separated and given extra feeding. May also help prevent problems in over-fat ewes.

Number of lambs. Scanning results can be used to closely match feeding with requirements. It can also be used in conjunction with body condition scores, so that lean ewes bearing twins can be fed as per triplets.

Age. Older ewes can be given extra feeding, especially if thin or broken mouthed. Separation of younger ewes (especially ewe lambs) will also prevent bullying and help DM intakes.

  1. Estimation of DM intakes

Pregnant sheep can eat 2 – 2.5% of their bodyweight during late pregnancy: this equates to 1.4 – 1.8 kg Dry Matter (DM) per day. Approximately 50% of the diet DM should come from forages, and similar to guidelines for dairy cow nutrition, diets that are greater than 60% concentrates : 40% forages on a DM basis will be a risk for the development of rumen acidosis.

DM intakes are one of the critical components of ewe nutrition. Most farmers will know what the intakes of concentrates are (eg. 3 buckets per pen), making it relatively easily to determine DM intake of concentrates. However intakes of forages are much more difficult to quantify as they are often provided ad-lib, and there are a host of reasons for variations (Figure 3). Forage intakes can be monitored by:

Working out how many bales of hay/loads of silage are used per pen per day.

Weighing bales/loads. There is tremendous variation in weights according to DM content of the material, compacting, storage etc. Therefore it is best to individually weight bales if at all possible. If not, book values can give rough estimates.

Allow for wastage of forage (eg. hay on bedding, waste forage removed from troughs).

Examine the ewes feeding. Is there enough trough space, especially when concentrates and/or fresh forages are put in the troughs?

Are there any other constraints on forage intakes (eg. processing of forages, diseases such as lameness)?

Average figures need to be looked at, so it is worth measuring intakes over a couple of days (get the farmer to record how many bales he goes through in that period). The ration can then be adjusted if forage intakes are not as expected.

  1. Condition scoring

Determining body condition score (BCS) is the simplest, easiest and cheapest method of monitoring nutrition (Russel 1984). Ideally it should be performed on all sheep on a regular basis, but practical application on farm will involve condition scoring of representative groups at times when ewes/rams are gathered for other management tasks (vaccination, anthelmintic treatments etc.). The critical points are prior to mating (BCS will influence ovulation rates) and prior to lambing (BCS will influence ewe health, lamb perinatal mortality and subsequent lamb growth rates). Suggested times are:

8 weeks prior to mating, which allows sufficient time to alter BCS before mating. Assessment can be done at the same time as sorting ewes for culling. If problems have been identified, BCS may be checked again 4 weeks prior to mating.

4 - 6 weeks prior to the start of lambing, allowing thin ewes to be separated and fed preferentially prior to lambing. Can be combined with blood sampling for metabolic profile analysis at 3-4 weeks prior to lambing.

Check of BCS at lambing will assess adequacy of nutrition prior to lambing, and need for supplementation. Will enable changes for late lambing ewes.

Other times (eg. at weaning) may be helpful if problems are anticipated.

  1. Occurrence of disease. In an ideal situation, problems should be identified before overt clinical disease is seen. However the appearance of certain diseases can be regarded as the “tip of the iceberg” as far as flock nutrition is concerned, and indicate an underlying problem that requires attention. Examples include:
  1. Energy deficits.Pregnancy toxaemia.
  2. Metabolic disease.Hypocalcaemia, Hypomagnesaemia
  3. Specific deficiencies.Trace element deficiencies
  4. Nutritional disorders.Rumen acidosis, Urolithiasis
  1. Metabolic profiles. These should not be regarded as a “quick fix”, and instead should be utilised as a pre-planned method of checking nutrition along with other monitoring techniques (eg. condition scoring). They can help identify problems before losses in production occur.

Metabolic profiles in pregnant ewes use a range of metabolites to check on:

Energy status (-hydroxybutyrate or BHB)

Protein status (urea-N as a measure of ERDP intakes, albumin as a long-term indicator of protein status and liver function).

Mineral status (magnesium and phosphate).

Blood sampling of pregnant ewes enables immediate assessment of current nutritional status. The critical time for sampling is during late pregnancy, as 75% of the lamb growth occurs in the last 6 weeks of pregnancy. Thus ewes due to lamb in the first week should be sampled 3-4 weeks prior to the start of lambing, allowing sufficient time to make any dietary changes. At the same time, the ewes should be condition scored and the ration assessed.

If ewes are not fed according to foetal numbers, then a random sample of 15-20 ewes should be sampled. However if the ewes have been ultrasound scanned and fed according to foetal number, then at least 5 ewes from each group (singles, twins and triplets) should be sampled. Ewe lambs and sheep with disease problems should not be sampled, as they will give results that are not representative of the group. Optimum values for sheep in late pregnancy under UK conditions are given in Table 2.

Table 2. Optimum blood values for sheep in late pregnancy under UK conditions

Metabolite / Assessment of / Optimum value
β-hydroxybutyrate (BHB) / Energy status / Ultrasound scanned – below 1.1mmol/l Unscanned – below 0.9mmol/l
Urea-N / Protein intake (ERDP) / Over 1.7mmol/l
Albumin / Long-term protein status and liver health / Serum: over 26g/l
Plasma: over 30g/l
Magnesium / Magnesium / Over 0.7mmol/l
Phosphate / Phosphate / Over 1.0 mmol/l

Dietary energy supply relative to metabolic demands can be accurately determined during late gestation by measuring serum or plasma BHB concentrations. Increased BHB values (a ketone body) reflect inefficient fatty acid utilisation caused by high glucose demand from the developing foetuses, not matched by dietary propionate or glucogenic amino acid supply. The authoritative article on interpretation of serum BHB concentrations in relation to dam energy requirements is given by Russel (1985).

A range of BHB concentrations is often encountered in a flock test, largely in relation to foetal number. Thus a more reliable interpretation of results can be made in those flocks which have determined foetal number by prior ultrasound scanning. The target mean BHB concentration is below 1.0 mmol/l. BHB concentrations above 1.6 mmol/l in individual ewes represents severe energy underfeeding, with the likelihood of pregnancy toxaemia developing as pregnancy advances and foetal energy requirements increase, unless dietary changes are implemented. BHB concentrations greater than 3.0 mmol/l are consistent with a diagnosis of ovine pregnancy toxaemia. (Note that ewes treated for hypocalcaemia also often have high BHB concentrations due to inappetance over the previous 24 hours, but respond to intravenous injection of calcium borogluconate alone without recourse to oral dextrose plus electrolyte solution etc.)

Once the mean BHB concentration has been determined, any alteration in the ration can be made with reference to Figure 4, and the farmer advised immediately by telephone and in writing, regarding any dietary or managemental changes. The flock should be re-visited and blood samples collected two weeks later to check on progress, and further monitor any changes in ewe body condition scores.

Blood samples can also be analysed for urea-N (which indicates short-term protein intake), and albumin which reflects longer term protein status. Care must be exercised with the interpretation of these parameters, as recent feeding can greatly influence urea-N concentrations. Blood samples should be collected either before concentrate feeding, or at least four hours later to avoid post-prandial increases. Low urea-N concentrations usually indicate a shortage of effective rumen degradable protein (ERDP). High urea-N levels are normal in sheep at grass, but in housed sheep indicate an excess of ERDP relative to energy supply in the rumen. This situation can arise when insufficient energy is supplied by poor quality concentrates, especially when these ewes are also showing poor energy status.

Serum albumin concentrations fall during the last month of gestation as immunoglobulins are manufactured and accumulate in the udder, thus serum albumin concentrations in the region of 26 to 30 g/l are ‘normal’ during the last month of gestation. Low albumin levels are also commonly associated with parasitic disease. Note that plasma protein concentrations are often 10-20 % higher than corresponding serum protein concentrations, therefore it is essential to be aware of what samples were submitted to the laboratory before interpretation.