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Pregnancy Scanning - using the results

With the optimum time to scan ewes being 80 to 90 days post ram turnout, scanners are currently in the thick of scanning March lambing flocks. Damian Costello, Teagasc Sheep Specialist outlines how scanning information can be used as a valuable tool to increase flock productivity & profitability.

Scanning day is eagerly awaited by the flockowner, albeit with some degree of apprehension, hoping for favourable results. On assessing your scanning results it’s a good time to ask yourself if you are happy with your scan. Do you need to introduce a more prolific breed of ewe to the flock to increase output? Do you need to pay more attention to building ewe body condition score (BCS) in the lead up to mating? Do you have an unusually high number of barren ewes? This can be a ram fertility issue but could also be an underlying abortion issue which would need further investigation. These are some issues to consider for future breeding seasons but for now the important thing is to use the current scanning information to your advantage. I will now focus on two main areas - producing lambs of optimum birth weight and ensuring ewes have adequate quantity and quality of colostrum for their new-born lambs. In broad terms, this is achieved by grouping ewes by predicted litter size after scanning and offering the appropriate late pregnancy nutrition based on the number of lambs being carried by ewes in each group.

Optimum lamb birth weight

The optimum birth weight per lamb varies by litter size. It can be defined as the weight at which lambs have the best chance of survival balanced with the weight that most lambs can be born naturally with minimal assistance.

Figure 1: Relationship between lamb birth weights and mortality

(Hanrahan and Keady, 2013)

As shown in Figure 1, studies indicate that increasing lamb birth weight reduces mortality initially but it plateaus around the optimum birth weight. As birth weight goes above the optimum mortality begins to rise again mainly due to dystocia associated with delivery of oversized lambs. Achieving optimum birth weight not only reduces lamb mortality but also reduces the labour associated with assisting delivery of big lambs and the additional attention needed to care for lambs that are small and weak at birth. The target optimum birth weights set for lowland flocks in the Teagasc Sheep Better Farm programme are 6kg, 5kg and 4kg for lambs born singles, twins and triplets respectively (based on 75kg mature weight lowland ewes). At farm level recording the birth weights of a selection of lambs from different birth types and comparing them to these targets is a fair indicator of how successful the late pregnancy feeding regime has been on the farm. The experience from the Better Farm Programme recorded birth weights is that, particularly on farms making high quality silage, there is a tendency towards exceeding the optimum birth weight targets. This represents an opportunity to reduce the level of concentrate fed pre lambing on these farms. The optimum birth weight target for ewe genotypes with lower mature bodyweights is adjusted down accordingly (e.g. typical Scottish Blackface ewe in lamb to Scottish Blackface ram target birth weight would be 4.5kg single and 3.5kg twin).

Feeding the ewe to litter size in late pregnancy

The key benefits of getting it right in terms of late pregnancy nutrition of the ewe flock based on litter size include:

  • Lambs being born at optimum birth weight
  • Ewes with adequate supply of colostrum
  • Avoiding metabolic related conditions such as pregnancy toxaemia etc.

In developing a nutrition plan for housed ewes in late gestation it is essential to carry out a representative forage analysis to determine its feed value, in particular DMD as it is positively correlated with energy concentration. The forage feed value combined with expected litter size determines the required amount of concentrate supplementation per ewe. The concentrate should be made up to 19-20% crude protein using high quality ingredients with soya bean meal as the main protein source. Good feeding management of both forage and concentrates is advised. In summary:

  • After scanning group ewes in pens based on expected litter size.
  • Implement a stepped concentrate feeding plan based on forage quality (consult your adviser). Commence supplementation in time, in particular for triplets (or greater) - 8 to 10 weeks from expected lambing date.
  • Where raddle colour has been changed regularly during mating expected lambing date can be used alongside litter size to group ewes in late pregnancy. In practical terms this may sound like a lot of different groups. However, there is the option to pen week 3 lambing triplets with week one lambing twins and week 3 lambing twins with week 1 lambing singles for example. As well as saving on feed costs it helps achieve optimum birth weights throughout the lambing season.
  • It is important to monitor body condition score (BCS) throughout the year. After scanning if there are twin bearing ewes for example that are behind target on BCS they could be penned with triplets and put on a higher plain of nutrition to help increase body condition.
  • In high prolificacy flocks where cross fostering is widely practised it is important to plan for single bearing ewes having adequate colostrum to potentially rear two lambs. In a scenario where single bearing ewes have access to high quality forage and are being supplemented with relatively low concentrate levels there may be a shortfall in crude protein intake. The solution is adding soya bean meal to the diet in the last 2 weeks before lambing aiming for a total protein intake from all feed (silage, concentrates and supplementary soya bean meal) of 200g/hd/day.