The optimum birth weight, based on lamb mortality, for crossbred lambs born as singles, twins and triplets is 6.0kg, 5.6kg and 4.5kg respectively. Thus the optimum birthweight for lambs born as twins is 0.93 that of singles: the corresponding proportion for triplets is 0.78. Lamb mortality is influence by litter size.
Variability in the feed value of grass silage
Digestibility (DMD) is the most important characteristic of grass silage from the viewpoint of animal performance because it is positively correlated with energy concentration and intake. As silage DMD varies from 52pc to 82pc on Irish farms, it is essential to know the feed value of the silage (forage) (as determined by laboratory analysis) that will be offered to ewes when developing a nutritional plan for housed ewes in late pregnancy.
Impact of grass silage feed value
The results from studies undertaken at Athenry on the impact of silage digestibility on the performance of pregnant ewes, and that of their progeny, are summarised in Table 1.
Improving silage DMD increased ewe live weight immediately after lambing and increased lamb birth and weaning weights by 0.3kg and 1.2kg respectively. Each 5pc increase in silage DMD increases ewe weight post lambing by 6.5kg and increases lamb birth weight by 0.25kg.
An alternative way to evaluate silage feed value is to determine how much concentrate is required to yield lambs of a similar birth weight.
At Athenry (Table 2) ewes offered a high feed value (high DMD) grass silage and supplemented with 5kg concentrate (soya bean meal plus minerals and vitamins) during late pregnancy produced lambs that were heavier than the lambs from equivalent ewes offered a medium feed value silage supplemented with 20kg concentrate.
Therefore, the high feed-value grass silage enabled concentrate supplementation to be reduced by at least 75pc.
Silage feed value and concentrate requirement
The effects of concentrate feed level and silage feed value on lamb birth weight and ewe body condition score (BCS) at lambing are presented in Table 3. Note for ewes offered silage with 70pc DMD, increasing concentrate feed level allowance above 25kg yields no real gain in lamb birth weight but the ewes were clearly fatter (higher BCS).
The same is true for ewes on 75pc DMD silage - concentrate allowances of 15kg and 25kg yielded an increase in BCS but a minimum change in lamb birth weight.
Therefore the increased energy intake from feeding excess concentrate to ewes during late pregnancy is converted to body fat.
The effects of silage feed value on the concentrate requirement of twin-bearing ewes in late pregnancy are presented in Table 4.
It is assumed that the silage is being offered using good feeding management - for example, ewes have access to fresh silage 24 hours per day and that any silage residue is removed twice weekly. Concentrate requirement is influenced by both silage DMD and harvest system (chop length).
The main factor influencing concentrate requirement during late pregnancy is silage DMD. For example, for silages at 79pc and 64pc DMD an additional 4-10kg concentrate, respectively, are required for long chop silages, compared to precision chop silages, respectively.
The concentrate requirements per ewe presented in Table 4 can be reduced by 5kg in the case of single-bearing ewes, while concentrate supplementation should be increased by 8kg for ewes carrying triplets.
For prolific flocks the concentrate should be formulated to contain 19pc crude protein (i.e., 190gr of crude protein per kg as fed) as the grass silage on many sheep farms has a low protein concentration.
Some people within the industry suggest formulating low and high protein concentrates for feeding to ewes during weeks 6 to 4 prior to lambing and weeks 3 to lambing, respectively.
However, the savings from using two different concentrates is, at best, marginal. This assertion is based on:
The small size of most sheep flocks in Ireland;
The fact that ewes require low levels of concentrate during the first two to three weeks of supplementation;
The low protein concentration of grass silage on most sheep farms.
For example, a 14pc crude protein concentrate is approximately €30/t cheaper than a 19pc crude protein concentrate.
This equates to only 1 cent per ewe daily during the first few weeks of supplementation, when ewes are offered between 0.2kg and 0.4kg per day. At these feeding rates, one tonne of concentrate will last for 50 and 25 days respectively for a flock of 100 ewes.
Therefore, for most farms there is no benefit to offering a low protein concentrate during the first weeks of feeding.
Where maize silage is offered as the forage during late pregnancy then concentrate crude protein concentration should be increased to 23pc and mineral and vitamin supplementation should be increased by approximately 50pc.
The effects of concentrate protein source offered during late pregnancy on the performance of ewes and their progeny was evaluated at Athenry.
Two concentrates were formulated to have the same metabolisable energy (12.4 MJ/kg DM) and protein concentrations (18pc as offered). The protein sources in the concentrates were either soya-bean meal or a mixture of by-products (rapeseed, maize distillers and maize gluten).
Lambs born to ewes that had been offered the soyabean-based concentrate were 0.3kg and 0.9kg heavier at birth and weaning, respectively, than lambs born to ewes offered concentrate that contained by-products as the protein source.
The increase in the weaning weight of lambs from ewes offered the soyabean-based concentrate in late pregnancy (extra cost ~ €0.50/ewe) is similar to the response obtained from offering each lamb 6kg of creep concentrate until weaning (cost ~ €3/ewe per set of twins).
The ingredient composition of the concentrate being offered to ewes during late pregnancy at Athenry is presented in Table 5.
The concentrate was formulated to contain 19pc protein using good protein (soya, rapeseed), energy (maize, barley) and fibre (beet pulp, soya hulls) sources.
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When offering similar levels of concentrate to ewes during late pregnancy as is offered at Athenry, a reduction in concentrate price of €20/t equates to a saving equivalent of only 45c per ewe.
Therefore, when purchasing concentrate it is important to be aware of its ingredient composition rather than basing the decision on which concentrate to purchase solely on price alone.
Concentrate feeding management
To optimise the use of concentrate ewes should be grouped according to predicted litter size (based on ultrasonic scanning) and expected lambing date (mating date - raddle colour).
As lamb weight increases by 70pc during the last six weeks of pregnancy the demand for nutrients increases substantially.
Consequently, supplementation should be stepped up weekly over the weeks immediately prior to lambing.
The objective is to produce lambs at the optimum birth weight (that will not require assistance during delivery), and ewes with adequate supplies of colostrum.
The feed schedules required to deliver different concentrate feed levels, varying from 10-45kg per ewe in late pregnancy, are given in Table 6.
During the week prior to lambing ewes receive up to 1kg daily, clearly illustrating the benefits of penning ewes according to expected lambing date as well as expected litter size.
For example, for each extra week ewes are on the high level of concentrate supplementation they would consume around 7kg concentrate - thus dramatically increasing feed usage.
Dr Tim Keady is principal research scientist at the Animal and Grassland Research and Innovation Centre, Teagasc, Athenry, Co Galway