Selecting replacements born to hoggets

Updated: Mar 16, 2020

Emma Pettigrew is looking at selecting replacements that are born to ewe hoggets for her PhD studies. She has written this article to promote a pilot study she published, selecting singles and twins born to hoggets or mature ewes.

Farmers are increasingly breeding their hoggets to increase ewe and farm productivity, and reduce the non-productive period of the ewe’s lifetime. Hoggets are commonly bred later than the mature ewes, to increase their likelihood of attaining puberty prior to breeding, and to ensure lambing occurs during the spring pasture flush.

Lambs born to hoggets are smaller at birth, and have lower growth rates than lambs born to mature ewes. Therefore, lambs born to hoggets are smaller at the time of replacement selection. These lambs are less likely to achieve breeding weights in time to be bred as hoggets themselves, so aren’t commonly selected. Thus, there is little information on the productive performance of replacements that are born to hoggets.

This was a pilot study, looking at the performance of replacement ewes born to hoggets.

Key points:

Selecting lambs born to hoggets can increase rates of genetic gain
Ewes born to hoggets have lighter live weights than ewes born to mature ewes for their lifetime
Ewes born to hoggets have similar rates of reproductive performance as ewes born to mature ewes.
Farmers shouldn’t discount lambs born to hoggets at selection based on their dam age alone. If they are heavy enough at selection, they should be considered for replacement selection.

This study used 115 ewe lambs all born during the same lambing period from the same sires, forming four groups:

Singles born to mature ewes (E1)
Twins born to mature ewes (E2)
Singles born to hoggets (H1)
Twins born to hoggets (H2)

The ewes were selected at weaning, and were run as one mob for their lifetimes, and culled for age at 8 years of age. Their live weights and body condition scores were recorded monthly from weaning until culling.

The E1 (purple-solid) ewes were the heaviest throughout their lifetime, with the E2 (purple-dash) and H1 (blue-solid) ewes being similar and 5 kg lighter than the E1 ewes, as seen in Figure 1 below. The H2 (blue-dash) ewes were the lightest for their lifetimes, being at least 5 kg lighter than the E2 and H1 ewes, and 10 kg lighter than the E1 ewes.

Figure 1. Predicted conceptus-free live weight of ewes born as singles to mature ewes (E1), born as twins to mature ewes (E2), born as singles to hoggets (H1), or born as twins to hoggets (H2).

Interestingly, there were no differences in body condition score between the groups throughout their lifetime. The average body condition score ranged between 2.7 to 3.0 at breeding for all groups.

There was no difference between the ewe groups for the average number of lambs born (1.47-1.56), or weaned (1.37-1.49) per ewe mated, or the weight of lambs at birth, and docking. There was a small, but non-significant difference between the groups for the litter weight of lambs at weaning, with the E2 (purple-dash) and H1 (blue-solid) ewes producing slightly more kilograms of lamb at weaning than the E1 (purple-solid) and H2 (blue-dash) groups, as seen in Figure 2 below.

Figure 2. The litter weight of lambs at birth, docking, and weaning, and the weight of ewes at breeding for ewes born as singles to mature ewes (E1), born as twins to mature ewes (E2), born as singles to hoggets (H1), or born as twins to hoggets (H2).

This is interesting, because there are strong positive relationships between the live weight of a ewe at breeding and her reproductive performance, including ovulation rate, conception rate, lamb birth weight, lamb survival rates, and lamb growth rates to weaning.

While the H2 ewes were lighter and had slightly lighter litters at weaning, the E1 ewes had similar rates of production to the H2 ewes, but were 10 kg heavier. This indicates that the H2 ewes are more efficient than the E1 ewes, as they are lighter (lower maintenance requirements), but produce similar amounts of lamb.

We measured the efficiency of the ewes by predicting their daily energy requirements by using (the list below) to calculate a total energy requirement for the lifetime of the ewe:

maintenance requirements from a predicted daily live weight
daily liveweight gain/loss requirements
pregnancy requirements based on day of pregnancy and litter birth weight
lactation requirements
additional lamb requirements from pasture

The total lifetime weight of lambs produced was also calculated, along with the total lifetime number of lambs weaned. This was used to calculate the total lifetime efficiency as the total lamb weaning weight divided by the predicted pasture eaten. There was no statistical difference in the efficiencies of these ewe groups, but there were small numbers of animals in each group.

The initial study only culled animals for ill-thrift, but not for non-production. The survival of the ewe groups are shown in Figure 3.

Figure 3. Survival of ewes born as singles to mature ewes (E1), born as twins to mature ewes (E2), born as singles to hoggets (H1), or born as twins to hoggets (H2), when only culled for ill-thrift, and not for non-production.

We were then able to go back and retrospectively impose culling based on non-production (i.e. diagnosed dry at pregnancy scanning, or identified as wet-dry at docking or weaning), culling the ewe as per commercial farming standards, with survival shown in Figure 4. As expected, when we imposed culling, the survival of all groups decreased.

Figure 4. Survival of ewes born as singles to mature ewes (E1), born as twins to mature ewes (E2), born as singles to hoggets (H1), or born as twins to hoggets (H2), when retrospectively culled for non-production at pregnancy detection, docking, or weaning.

Interestingly, the E1 ewes had no deaths until their 4th year of life, however, when culling was imposed the E1 ewes were culled at two years of age, showing there were some ewes being unproductive during that time. The H2 ewes had the lowest rates of actual and imposed survival to eight years of age, especially their actual survival.

The conclusions from this study are that lambs born to hoggets can be selected as replacements. They are lighter in live weight, than replacements born to mature ewes, but their decreased mature weights don’t appear to affect their production.

Caution needs to be taken when selecting these animals though, as they are lighter at birth, and born a month later. Therefore, only lambs that are heavy enough at selection should be considered, but farmers should not discount lambs that are heavy enough at selection, based on the age of their dam alone.

Further research is required, with larger numbers of animals, to determine if lambs born to hoggets can be selected and successfully bred as hoggets themselves.

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