What does this even mean? I am talking about mature sheep size. Ontario is home to many sheep breeds of many different sizes. There can also be significant variation in size within breeds. It is important to consider a number of factors when thinking about what mature size will work best for your operation.
Environmental Concerns
Concerns have been raised in the past few years about the effect of livestock farming on our environment. One of the primary concerns is related to the contribution of ruminants to greenhouse gas. Methane is produced by sheep during the fermentation of feed in the rumen. The amount of methane produced is related to genetics, feed quality and feed intake. This means that larger animals that eat more will produce more methane.
Production Factors
There are efficiencies in meat production with larger animals. Larger animals will have larger birth weights, faster growth and can produce lean carcasses at heavier weights than smaller animals. In the early 2000s, Chris Logan from the University of Lincoln, New Zealand presented data collected from their research farm showing the difference in performance between small and large commercial ewes. These results are shown in Table 1.
Table 1. Small and large twin bearing ewes sourced from commercial farms and lambed together at research farm over a 3-year period.
| Trait | Small | Large |
| Ewe Weight (kg) | 59.9 | 69.5 |
| Lambs born/ewe joined | 1.67 | 1.77 |
| Lambs weaned/lamb alive | .90 | .91 |
| Birth weight (kg) | 4.8 | 5.0 |
| Weaning weight (kg) | 24.3 | 26.3 |
| Litter weaning weight (kg) | 40.3 | 45.2 |
Source: Chris Logan, Lincoln University, Ewe Efficiency Handout.
The larger ewes had more lambs that were larger and grew faster resulting in a litter weaning weight of 45.2kg compared to a litter weaning weight of 40.3kg for the small ewes.
Creating a System
The production system for an individual farm is determined by farm specific factors such as: access to feed sources, climate, topography, buildings, labour and market opportunities. This means that there is not one right answer for every farm. Each farm must assess its own resources and determine the best choice for their operation. Smaller ewes require less feed for maintenance and take up less space in buildings. But this must be balanced against production capacity, growth rates and market weights. Crossbreeding programs can utilize smaller ewes and larger terminal sires to save on maintenance feed costs and still capture some of the advantages of faster growth and carcass size.
Ewe Efficiency
One method of balancing the benefits and challenges with size is to calculate a ewe efficiency number. A common calculation is average total weight of lamb weaned divided by average mature ewe weight. Looking at table 1, if you do that calculation, the small ewes are weaning 67% of their weight and the large ewes are weaning 65% of their weight. Chris Logan also presented the overall performance of the small and large ewes shown in Table 2.
Table 2. Overall performance per hectare of the ewes shown in table 1.
| Trait | Small | Large |
| Stocking rate (ewes/ha) | 10 | 8.6 |
| Lambs weaned | 15 | 13 |
| Weaning weight (kg) | 403 | 390 |
| Carcass weight (tonnes) | 245 | 217 |
| Wool (kg) | 34 | 31 |
Source: Chris Logan, Lincoln University, Ewe Efficiency Handout.
In this second table, feed is balanced against production by showing the performance per hectare. When you look at performance per hectare, the small ewes produced more weaning weight and carcass weight than the large ewes. As a result, even though the larger ewes produced more, they were not as efficient as the smaller ewes. It is worthwhile to evaluate your ewes based on efficiency.
Conclusions
These are not all the factors that might need to be considered. You need to decide what traits will work best in your operation. It is important to monitor production against costs. Ewe efficiency is something that needs to be considered as an indicator of profitability. One way of starting to benchmark this for a meat operation is to monitor the kilograms of feed required to produce a kilogram of meat on your farm.
