Balanced nutrition holds the key to flock performance

High-quality silage combined with appropriate concentrate supplementation in late pregnancy delivers higher lamb birth and weaning weights, writes Tim Keady

APPROPRIATE nutrition and management during late pregnancy is one of the key factors influencing flock productivity and profitability.

Indeed, each additional 0.1 lamb reared per ewe joined is worth approximately €9.50 per ewe.

This article, the eighth in the current series, summarises results from studies at Athenry on the effects of the plane of nutrition offered to ewes during late pregnancy on ewe and lamb performance.

Lamb birth weight

Previous studies at Athenry have shown that each 0.5kg increase in lamb birth weight increases subsequent weaning weight by around 1.7kg.

The effect of lamb birth weight on lamb mortality is shown in Figure 1.

Regardless of litter size, as lamb weight increases mortality declines initially but levels out around the optimum birth weight, which varies by litter size.

Subsequently, as birth weight increases above the optimum, lamb mortality increases again — probably reflecting difficulties immediately prior to and during delivery.

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.

Concentrate protein

For prolific flocks the concentrate should be formulated to contain 19pc crude protein (i.e., 190gr of crude protein per kg as