Body condition score (BCS, on a 1–5 scale) for sheep was developed in the 1960s as a management tool to quickly assess body reserves.
The benefits of using body condition score include:
1. controlling the fat cover of the animal
2. detecting small differences in body condition
3. allowing farmers to be immediately aware of major body condition losses
4. following trends in feed levels and weight
The current industry BCS target is between 3.0 to 3.5 at mating. However, some farmers are feeding their sheep an amount that results in greater flock average BCS and this may not necessarily be the most economic use of feed.
The feed requirements increase at greater BCS.
A BCS increase from a low or high BCS is related to a similar increase in live weight.
It is inefficient to have a greater BCS than 3.5.
A study quantified how live weight, chemical body composition and energy partitioning changes as BCS increases in mature ewes. There were a total of 28 mixed-age Romney-cross ewes at different BCS (1.5, 2.0, 2.5, 3.0, 3.5, 4.0 and 4.5). The whole body chemical compositions were analysed. The ewes started off at a similar live weight and BCS. They were fed to obtain different BCS. The greater the BCS target, the greater the feed intake required to reach the target. It also took a longer period of time to reach the higher BCS.
The live weight increased with BCS, as shown in graph 1, there was a slight drop in the ewes from BCS 3.0 to BCS 3.5 that could be explained by the fact that there were only 4 ewes in each BCS group. Overall, an extra unit of BCS required on average a 7.7 kg increase in live weight. So to increase a 60 kg ewe from a BCS 3 to BCS of 4, she would have to put on 7.7 kg of weight, increasing to 67.7 kg.
The concentration of water, fat and energy in the whole body increased as the BCS increased. However, the concentration of protein and inorganic matter in the whole body did not differ between BCS groups. On a whole body kg basis the weight of fat increased with BCS while the total weight of protein did not.
The researchers predicted that the amount of energy required to gain one unit of BCS would increase at a non-linear rate, such that more energy is required at higher BCS to gain an additional unit of BCS.
Graph 2 below shows the partitioning of energy between energy retained (ER), energy lost as heat (HI) and energy required for maintenance (MEm). The energy retained increases up to a BCS of 3.5 (black in graph) and then is constant at BCS greater than 3.5. The energy lost as heat increases substantially for each increase in BCS (dark grey in graph) and the energy for maintenance (light grey in graph) increases at a similar rate.
As BCS increases, less of the metabolisable energy intake is retained in the whole body and more is lost in the form of heat production. Therefore, a greater intake of energy is required to gain BCS in ewes of higher BCS. This means that increases in BCS above 3.5 are associated with increased heat energy loss with only a small proportion of energy being retained. From an efficiency perspective there is little advantage in gaining BCS above a BCS of 3.5
Although there were only 4 ewes in each BCS group, this study provides good evidence that at a low BCS, yes the extra feed is worth it to gain an extra BCS, up to a BCS of 3.5. However, ewes above a BCS of 3.5 should have their feed intake restricted to ensure their maintenance feed requirements do not increase to a point where a large amount of energy is being used for maintenance and lost as heat.
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Morel PCH, Schreurs NM, Corner-Thomas RA, Greer AW, Jenkinson CMC, Ridler AL, Kenyon PR. 2016. Live weight and body composition associated with an increase in body condition score of mature ewes and the relationship to dietary energy requirements. Small Ruminant Research 143: 8-14.