Pre-lambing preparation: feeding, metabolic profiles and minerals

Pre-lambing, for the purpose of this article, is considered as the final eight weeks of gestation. During this time, 75% of fetal growth takes place, with a corresponding increase in the ewe’s energy and protein requirements.

Why is nutritional management of pre-lambing ewes critical?

Poor nutritional management pre-lambing results in poor colostrum production; poor milk yield; low lamb birth weight; and increased risk of vaginal prolapse, mastitis, pregnancy toxaemia and hypocalcaemia (Phillips et al, 2014).

Mammary gland development takes place during the final month of pregnancy, although it is not until the week before parturition that the mammary gland increases in size, which coincides with colostrum synthesis. Poor pre-lambing nutrition can reduce colostrum and milk production, delay the onset of lactogenesis and increase the viscosity of colostrum produced (Banchero et al, 2015). Poor nutrition also predisposes ewes to mastitis through immunosuppression and damage to the teats from excessive lamb sucking in cases of lactation failure (Roger, 2009).

Lamb birth weight is correlated to survival rate. An optimum lamb birth weight for high survival in lowland sheep is 6kg, 5kg and 4.5kg for single, twins and triplets respectively (Povey et al, 2017). Dystocia is the main risk for heavier lambs, with an increased risk of starvation and exposure present for low birth weight lambs that are born with little brown fat, and are slow to rise and suckle (Figure 1).

Pregnancy toxaemia is characterised by low circulating blood glucose and high ketone body levels as a result of net energy deficiency at the time of increased fetal energy demand (Andrews, 1997). Prevention involves good feeding practices – from pre-tupping through to parturition – but becomes more critical in the final two months of pregnancy. About 20% of ewes with pregnancy toxaemia are also hypocalcaemic, but hypocalcaemia may occur independently of pregnancy toxaemia if the ewes are unable to absorb sufficient calcium from their diet, and skeletal calcium reserves are not sufficient to meet the increased requirements of pregnancy and lactation.

Minimising stress in late pregnancy, rather than feeding high levels of calcium to pregnant ewes, is advocated in the prevention of hypocalcaemia (Sargison, 2008), although it should be noted cereal-based diets and lush spring grass are particularly low in calcium.

What is good nutritional management?

Good nutritional management is a combination of what is fed and how it is fed. The two key components of ewe nutrition are energy and protein. Metabolisable energy (ME) is quoted in ration formulations – often as megajoules per kg of dry matter – and comprises fermentable ME (FME), such as carbohydrates and fibre, available to rumen microbes, and non-fermentable energy, such as oils and volatile fatty acids, which are a source of energy for the ruminant itself.

Similarly, metabolisable protein (MP) is a combination of digestible undegradable protein (DUP) fed directly to the animal and microbial protein produced by the rumen microbes from effective rumen degradable protein (ERDP) using FME. Microbial protein is the larger component of MP and is sufficient for most of the production cycle, except late pregnancy and early lactation, where additional DUP is required. Evidence suggests proteolysis in ewes fed insufficient MP is of detriment to future productivity of the ewe. Protein is essential for colostrum production.

The Agriculture and Food Research Council (AFRC) developed guidance on ME and MP requirements for ewes throughout pregnancy. Although a nutritionist may be required to formulate a ration, as a vet using the AFRC guidelines it is possible to check a ration if you know the ewe weight, litter size, stage of pregnancy and forge analysis (Panel 1).

Genetic improvement in terminal breeds has resulted in increased birth weights and milk demand. It has been questioned whether the 1993 AFRC MP recommendations are fit for purpose. However, feeding additional DUP over and above the AFRC recommendations has not proved to be of benefit unless the ewes are of poor body condition score (BCS). Research advocating the feeding of DUP pre-lambing, to reduce the effects of the periparturient relaxation of immunity, is also BCS-dependent (AHDB, 2017).

Formulating a perfect ration is only any good if the ewes can have access to it and are willing to eat what is put in front of them. Panel 2 is a checklist of the practical aspects of feeding.

How can we assess nutritional management in our sheep flocks?

Body condition scoring is a vital tool for sheep farmers and vets alike, and should be used at critical points in the year to assess nutrition. BCS should take place at weaning, six to eight weeks pre-tupping, mid-pregnancy and then fortnightly in the fourth and fifth month of pregnancy (Andrews, 1997). Target BCS for hill, upland and lowland flock are presented in Table 1. The aim is to have 90% of the flock at the target BCS for the time of year.

One BCS roughly equates to 10% to 13% bodyweight (Povey et al, 2017) and, as a guide, it takes eight weeks for a ewe to gain a BCS on good grazing alone. Good grazing alone may be sufficient for ewes to gain condition between weaning and tupping, but concentrate feeding may be required to achieve more rapid changes in condition score.

In late pregnancy, ewes should be grouped and fed based on BCS and litter size. For example, a thin twin-bearing ewe may be managed in a group with triplet-bearing ewes, as they have the same energy requirement. Some liveweight loss – up to 50g/day – is acceptable in late pregnancy and, to some degree, is inevitable. Unless the ewe is below target condition score, this rate of loss is unlikely to be detrimental.

One limitation of BCS as an assessment of nutrition is it is somewhat retrospective. In addition to BCS, metabolic profiling can be used at four weeks pre-lambing to monitor short-term nutrition. Generally speaking, three parameters are assessed – urea, albumin and beta-hydroxybutyrate (BHB) – in 5 to 10 representative animals from each separately managed group. While blood sampling and body condition scoring the selected animals, it gives an opportunity to visually assess how, and what, the ewes are fed, including the forage analysis and feed bag labels, if available (Figure 3).

BHB is a ketone made in the liver when the products of long-chain fatty acids cannot be metabolised in the tricarboxylic acid cycle. BHB is stable in clotted blood samples and is used as a measure of negative energy status in ruminants (Otter, 2013). A BHB greater than 3mmol/L is consistent with clinical pregnancy toxaemia. Levels greater than 1.6mmol/L indicate a serious energy deficiency (Phillips et al, 2014). Russel (1985) described the optimum target BHB to be 0.8mmol/L at four weeks pre-lambing, and provides guidance on feeding requirements should the ewes exceed this target value.

Low serum or plasma urea (BUN) can reflect insufficient ERDP in the diet, or low intakes of ERDP. To avoid post-prandial rises in urea, blood sampling should take place before concentrate feeding, or at least four hours after (Phillips et al, 2014).

Albumin is synthesised and degraded solely in the liver, and is a long-term measure of protein intake (Otter, 2013). Severe parasitism, liver pathology and Johne‘s disease are common causes of individual animals becoming hypoalbuminaemic. Plasma albumin levels will decrease in the final month of pregnancy due to mammary development and colostrum production. Levels exceeding 26g/L are acceptable (Phillips et al, 2014).

It is well recognised trace elements and vitamins – copper, cobalt, selenium, iodine and vitamin E – are required for good immune function, lamb vigour and viability. However, some evidence exists of the detrimental effects of over-supplementation in late pregnancy. Boland et al (2008) reported lamb IgG concentrations at 24 and 72 hours post-partum were reduced when ewes were fed excessive dietary iodine for three weeks immediately prepartum. It was also suggested high levels of cobalt supplementation pre-lambing had a negative effect on lamb serum vitamin E 72 hours post-partum. Although more research is required to substantiate these claims, the take home message may be to provide adequate, not excessive, levels of trace elements and vitamins to pregnant ewes.