Wright, Nerys
(2021)
Key Performance Indicators of ewe productivity: importance of ewe body condition score and liveweight on pregnancy outcomes and lamb performance to weaning.
PhD thesis, University of Nottingham.
Abstract
Body condition score (BCS) is a subjective assessment of the amount of subcutaneous fat reserves along the spinous and transverse processes of ruminants. It is an indicator of current and historical nutritional status and is considered vital for optimal ewe productivity. BCS in sheep has been documented since the early 1900’s. It can be considered to be the ratio of the amount of fat to the amount of non-fatty matter in the body of a living animal. A scale of 1 – 5 (1 being very thin and 5 being very fat) was developed during the 1960s.
Chapter 1 consists of a literature review of the published research relating to the impact of ewe BCS and liveweight from weaning of a production cycle to weaning of the subsequent production cycle on ewe fertility and lamb performance to weaning, The second chapter of this thesis analysed the quantitative data captured from the three study farms who collected ewe and lamb data between 2014 and 2016. The data was compared to national figures, where available. The generally accepted industry target of 3% or less barren ewes at scanning was achieved each year at two of the three study farms, and in two out of the three years at the third farm. In addition, between 2 and 4% of ewes scanned pregnant were not in possession of a lamb at tagging (48 h post-lambing). Furthermore, a 20 kg target (AHDB) for lamb weight at 8 weeks post-lambing was predominantly achieved on these commercial sheep flocks, with between 7 and 35 % of lambs below 17 kg at 8 weeks post-lambing (variation was between years and across farms). Data from these farms also indicated that a target of 25 to 28 kg lamb weight at weaning (at 12 weeks) is probably more realistic than the proposed 30 kg target (AHDB).
Two of the three study farms did not achieve the current BCS targets at every production point during the year. This is likely to be a reflection of what occurs on many farms in England. The farm that did achieve target BCS at every production point had the largest litter sizes at scanning, achieved the lamb 8-week target of 20 kg each year, had the lowest percentage of light lambs at 8 weeks and achieved the lamb weaning target of 30 kg in two out of the three years.
Chapters 3 to 5 considered the effects of ewe BCS and liveweight at various points of the production cycle. The effects on pregnancy establishment, that is the proportion ewes pregnant and litter size at scanning (Chapter 3); the effects on pregnancy outcomes, that is the proportion ewes lambed and litter size at lambing (Chapter 4); and the effects on lamb performance to weaning, specifically combined twin-lamb 8-week weight, combined twin-lamb weaning weight and weight gain between 8 weeks and weaning (Chapter 5). Lamb birthweight was not captured on every farm each year, therefore Chapter 5 focussed on performance at 8 weeks and between 8 weeks and weaning.
Ewe condition at weaning of the preceding production cycle and mating of the subsequent production cycle was associated with litter size at scanning, litter size at lambing, combined lamb 8-week weight and combined lamb weaning weight, but did not associate with proportion ewes pregnant, proportion ewes lambed or lamb weight gain between 8 weeks and weaning. Ewe condition change between weaning (of the preceding production cycle) and mating was not associated with ewe fertility or lamb weight at weaning. Ewe condition at scanning and gain in condition between mating and scanning were each positively associated with ewe fertility and lamb weight to weaning. Finally, ewe condition at 8-weeks, ewe condition at weaning and ewe BCS loss between lambing and 8-weeks were all positively associated with combined lamb weight gain to weaning. However, this relationship differed between farms, depending on ewe BCS at lambing. Ewes at target BCS at lambing (3 units) and mobilising condition during lactation produced heavier lambs at weaning. However, when BCS at lambing was below 3 units, ewes that mobilised less condition produced heavier lambs at weaning.
A survey sent to sheep farmers in England formed the basis of Chapter 6. Of the 384 English respondents, 97% agreed that ewe condition was important in determining flock performance. However, the level of importance they attached to condition, and how farmers assessed this parameter (i.e. BCS, weight, BCS and weight or visual) changed during the production cycle. Most farmers (99%) agreed that condition at mating was most likely to affect flock productivity with the fewest (70%) agreeing that condition at weaning was least likely to affect flock productivity. However, 46% did not record ewe condition data. The barriers to farmers assessing BCS were identified as time and the ability to manage multiple management groups. Finally, farmers confused the term BCS for breeding ewes with selecting lambs for slaughter.
In conclusion, ewe BCS and liveweight at key production stages and change between production stages have a long term association with ewe fertility and lamb performance to weaning.
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