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The validation of the Sheep Ireland €uro-star Replacement index


Genetics are responsible for over 50% of production gains on farm. A four year study was carried out at Teagasc Athenry. The study aimed to investigate the effect of genetic merit on sheep performance traits to validate the Sheep Ireland €uro-star Replacement index and to compare Irish & NZ genetics

Teagasc & UCD staff & students  Nicola Fetherstone, Noirin McHugh, Tommy Boland, Henry Walsh, Tom Keane and Fiona McGovern give information on the Sheep Ireland €uro-star Replacement index through a comparison of Irish and New Zealand sheep genetics.

Main Findings

  • Genetic improvement results in production gains on farm.
  • Widespread use of high genetic merit animals can lead to an increase in the overall productivity and efficiency of the current Irish and ultimately, global sheep industry.
  • When selecting breeding rams, both commercial and pedigree farmers should consider using genetic indexes as a selection tool, as well as animal breed type and other physical attributes.

Introduction

Genetics are responsible for over 50% of production gains on farm. Although genetic gain is cumulative and permanent, the progress in the maternal index reported to date in Ireland has been low (€0.28/lamb/year) compared to the corresponding gains reported in New Zealand (€1.16/lamb/year). Previous research demonstrates the superiority of NZ genetics compared to Irish genetics but to date, Irish and NZ ewes of high genetic merit have not been compared in a common environment. Furthermore, the Sheep Ireland €uro-star replacement index, used to select maternal type ewes in Ireland, has not been validated. The aim of this study is to investigate the effect of genetic merit on sheep performance traits in order to validate the Sheep Ireland €uro-star Replacement index and to compare Irish and NZ genetics.

Validation of the Sheep Ireland €uro-star Replacement Index

A four year study was carried out at Teagasc Athenry, Co.Galway, Ireland from 2016 to 2019. Three treatment groups were established, containing 60 New Zealand ewes of high genetic merit (NZ), 60 Irish ewes of high genetic merit (High Irish) and 60 Irish ewes of low genetic merit (Low Irish), as selected from the New Zealand Maternal Worth Index (NZMW) and Sheep Ireland’s €uro-star Replacement Index, respectively. Each treatment group consisted of 30 Suffolk and 30 Texel ewes, all of which were pedigree animals.  Ewes classified as high and low animals were ranked within the top and bottom 20% of animals for maternal genetic merit, respectively, within breed, as recorded on each national database at the time they entered the study. In early October, ewes were artificially inseminated (laparoscopic AI) to a synchronised oestrus according to genetic merit and breed, e.g. high Suffolk ewes were mated to high Suffolk rams.

Lamb live-weight was measured at birth, at six weeks and fortnightly thereafter (kg) until drafting. Weaning occurred at 14 weeks of age and coincided with the first draft for slaughter. The number of lambs reared per ewe until weaning was calculated, where lambs that died, were artificially reared or were reared by another ewe were accounted for. The number of ewes that successfully reached the following breeding season was also recorded as ewe survival, where ewes that were culled, failed to rear any lamb or died were accounted for. Lambs are drafted from the treatment group at 43kg in June, i.e. at weaning, and an additional kilogram was added to the target per month thereafter to account for the decline in kill out percentages over time. The number of days from birth until drafting is calculated and reported as days to slaughter.

Results

Reproductive & Lambing performance

Ewe genetic merit had no significant impact on conception to first service, where rates between 77 and 84% were achieved. However, NZ ewes were more likely to hold to first service than Low Irish ewes (2.6 times, P< 0.05). Barren rates did not differ between treatment groups at approximately 8%, (P> 0.05). NZ ewes had a higher litter size that resulted in 26% more lambs being born than the Low Irish ewes (P> 0.05, Table 1). Ewe genetic merit significantly affected survival rates within the flock, i.e. a larger proportion of ewes of high genetic merit, regardless of country of origin, were retained in the flock from one year to the next (P< 0.05, Table 1). Ewes of Irish origin, whether of High or Low genetic merit were more likely to require significant assistance compared to NZ ewes (P< 0.001, Table 1). Lamb mortality within the first 24 h post-partum, did not differ between the three treatment groups, ranging from 8% to 11% (P> 0.05).

Lifetime performance

During the four year study, the NZ ewes gave birth to a higher total number of lambs than either Irish group (P< 0.05, Table 1). They also weaned a higher total number of lambs than the Low Irish ewes, although a similar rate to the High Irish ewes (P< 0.01, Table 1). Overall, ewes of high genetic merit, regardless of origin, successfully reared a greater number of lambs than the low genetic merit ewes who on average failed to rear 1.42 lambs over the period of the four year study (P< 0.01, Table 1).

Lamb performance

Ewe genetic merit did not have an impact on lamb birth weight, where lambs averaged 5.1kg (P> 0.05). However, at six weeks of age, lambs born from NZ ewes are significantly heavier than those born from Low Irish ewes (P<0.001, Table 1). Lambs born from NZ ewes then went on to reach slaughter five and fifteen days earlier that High or Low Irish lambs, respectively.

Table 1. Performance traits of ewes and lambs of various genetic merit and country of origin.

1Dystocia is classified as four on the scale of one to four for lambing difficulty where 1 = lambed without assistance to 4 =  considerable difficulty