Longitudinal assessment of the impact of aging on wastage, productivity and welfare of ewes

Retention of aging ewes is a strategy producers may use to maintain or increase flock size. Older ewes can have a higher risk of mortality and health issues, but these may be minimised with early intervention and culling. This retrospective study examined the incidence and causes of mortality and culling in a flock of Merino ewes as they aged from 4.5 to 5.5 until 9.5 to 10.5 years. The annual rate of ewe mortality ranged from 6.2% to 9.9%, with lambing complications being the major source of reported loss. Annual culling rates ranged from 1.6% to 42.8%, the larger rates reflecting opportunistic replacement with younger ewes rather than essential culling to maintain welfare and productivity. The major reasons for culling were low condition score, worn or missing teeth or faulty udders. The incidence of other faults was low. Only 18.3% of ewes remained when aged 9.5-10.5 years and these were fertile (89% pregnant) with a high proportion of multiple fetuses (52%) in their final year, although fleece weight had declined. Younger ewes born in 2004/2005 tended to have lower annual mortality rates than the aged ewes, but 8%-12% were still culled annually. It is concluded that an annual inspection pre-joining of all ewes and preventative culling for health issues is necessary to minimise any increase in mortality and risk to ewe welfare from retaining aging ewes. More frequent inspection will be required as potential health issues develop during the year, as these may not be evident pre-joining.

Ewe culling and retention strategies to increase reproductive rates in Merino sheep

The present empirical study of data from three Merino resource flocks of varying genetic backgrounds run in different environments investigated the impact of various selection strategies aimed at culling poor performers (‘passengers’) or retaining good doers (‘performers’) longer, or both, on the flock reproductive performance. Four strategies were investigated and applied either individually or in combination, to quantify their impact on the litter size, rearing type and breeding efficiency (number of lambs weaned per ewe per year) of the whole flock. The main benefit from implementing any of the strategies was a decrease in the proportion of dry ewes in each flock (1–7%). There was little difference between the single strategies of culling twice-dry ewes or culling ewes that twice lambed and lost their lambs. When retaining a cohort of older ewes, it was more efficient to select these on the basis of their lifetime reproductive performance. There was only a marginal benefit gained by combining strategies and, in some cases, a single strategy was still better in terms of breeding efficiency. Commercial producers can improve the reproductive rate of these flocks by culling twice-dry ewes on the basis of their pregnancy scan data.

Genetic and economic benefits of selection based on performance recording and genotyping in lower tiers of multi-tiered sheep breeding schemes

BACKGROUND

Performance recording and genotyping in the multiplier tier of multi-tiered sheep breeding schemes could potentially reduce the difference in the average genetic merit between nucleus and commercial flocks, and create additional economic benefits for the breeding structure.

METHODS

The genetic change in a multiple-trait breeding objective was predicted for various selection strategies that included performance recording, parentage testing and genomic selection. A deterministic simulation model was used to predict selection differentials and the flow of genetic superiority through the different tiers. Cumulative discounted economic benefits were calculated based on trait gains achieved in each of the tiers and considering the extra revenue and associated costs of applying recording, genotyping and selection practices in the multiplier tier of the breeding scheme.

RESULTS

Performance recording combined with genomic or parentage information in the multiplier tier reduced the genetic lag between the nucleus and commercial flock by 2 to 3 years. The overall economic benefits of improved performance in the commercial tier offset the costs of recording the multiplier. However, it took more than 18 years before the cumulative net present value of benefits offset the costs at current test prices. Strategies in which recorded multiplier ewes were selected as replacements for the nucleus flock did modestly increase profitability when compared to a closed nucleus structure. Applying genomic selection is the most beneficial strategy if testing costs can be reduced or by genotyping only a proportion of the selection candidates. When the cost of genotyping was reduced, scenarios that combine performance recording with genomic selection were more profitable and reached breakeven point about 10 years earlier.

CONCLUSIONS

Economic benefits can be generated in multiplier flocks by implementing performance recording in conjunction with either DNA pedigree recording or genomic technology. These recording practices reduce the long genetic lag between the nucleus and commercial flocks in multi-tiered breeding programs. Under current genotyping costs, the time to breakeven was found to be generally very long, although this varied between strategies. Strategies using either genomic selection or DNA pedigree verification were found to be economically viable provided the price paid for the tests is lower than current prices, in the long-term.

Using partial records to identify productive older ewes to retain in the breeding flock to increase the flock net reproduction rate

The reproductive performance of 7286 ewes from three Merino genetic resource flocks maintained in the central-western region of New South Wales was used to compare the use of fertility and fecundity information and net reproduction information whether recorded early (at 2 and 3 years of age) or later in life (at 4 and 5 years of age) with lifetime performance (at 2–6 years of age) as alternative selection criteria to identify productive older ewes to retain in the breeding flock. While the lifetime net reproduction rate is the best indicator of a ewe’s reproductive performance in later life, producers basing their selection decisions on pregnancy scanning data, which provides information on both fertility and fecundity, can improve the net reproduction rate of their flock by 2% regardless of the base reproduction rate. Despite little difference in likely flock reproduction rates between using early or later life scanning information as the selection criteria, early life performance is the best choice for commercial producers because it provides scope for eliminating twice-dry ewes from the breeding flock early in life, especially when combined with udder examination at marking.

Across population genetic parameters for wool, growth, and reproduction traits in Australian Merino sheep. 1. Data structure and non-genetic effects

Accurate estimates of adjustment factors for systematic environmental effects are required for genetic evaluation systems. This study combined data from 7 research resource flocks across Australia to estimate genetic parameters and investigate the significance of various environmental factors for production traits in Australian Merino sheep. The flocks were maintained for several generations and represented contemporary Australian Merino fine, medium, and broad wool bloodlines over the past 30 years. Over 110 000 records were available for analysis for each of the major wool traits, with over 2700 sires and 25 000 dams. Univariate linear mixed animal models were used to analyse 6 wool, 4 growth, and 4 reproduction traits. This first paper outlines the data structure and the non-genetic effects of age of the animal, age of dam, birth-rearing type, sex, flock, bloodline, and year, which were significant with few exceptions for all production traits. Age of dam was not significant for reproduction traits and fleece yield. Generally, wool, growth, and reproduction traits need to be adjusted for age, birth-rearing type, and age of dam before the estimation of breeding values for pragmatic and operational reasons. Adjustment for animal age in wool traits needs to be applied for clean fleece weight (CFW), greasy fleece weight (GFW), and fibre diameter (FD) with inclusion of 2 age groups (2 years old and >2 years old), but for reproduction traits, inclusion of all age groups is more appropriate. For GFW, CFW, and hogget weight (HWT), adjustment for only 2 dam age groups of maiden and mature ewes seems sufficient, whereas for birth (BWT), weaning (WWT), and yearling (YWT) weights, adjustments need to be applied for all dam age groups. Adjustment for birth-rearing type (single-single, multiple-single, multiple-multiple) is appropriate for wool, growth, and reproduction traits. The implications of adjustment for non-genetic effects are discussed.

Influence of body weight, age, and weight gain on fertility and prolificacy in four breeds of ewe lambs1

Breeding ewes to lamb at 1 yr of age can improve profitability for some production systems. The first objective of this study was to evaluate the effect of age and weight at breeding and total postweaning weight gain on reproductive performance of ewe lambs. The second objective was to compare the effects of weight and age variables in four major sheep breeds (Columbia, Polypay, Rambouillet, and Targhee). Weights, ages, and the binary traits of fertility (pregnant or nonpregnant) and prolificacy (one lamb born vs. two or more) were collected on 2,055 ewe lambs at the U.S. Sheep Experiment Station, Dubois, ID, from 1984 through 1988. The effects of age and weight at breeding and total weight gain from weaning to breeding on fertility and prolificacy were analyzed with a logit model in a maximum likelihood analyses. Differences (P < 0.001) among breeds for fertility were identified, with a 93% fertility rate for Polypay ewe lambs compared with lower fertility rates in Columbia, Targhee, and Rambouillet ewe lambs (50, 60, and 75%, respectively). The percentage of multiple births (prolificacy rate) also was higher (P < 0.001) in the Polypay (47%) than in Columbia, Targhee, and Rambouillet breeds (1, 13, and 14%, respectively). Averaged across breeds, weight at breeding had a positive effect on fertility and prolificacy (P < 0.001), whereas total weight gain from weaning to breeding had a positive effect only on fertility (P < 0.027). In separate analyses for each breed, increasing age (P < 0.001) and weight at breeding (P < 0.001) increased the probability of pregnancy in Rambouillet ewe lambs. The probability of pregnancy for Targhee ewe lambs increased (P < 0.005) with weight at breeding. Increasing weight at breeding increased (P < 0.004) the probability of multiple births in all breeds. Increasing total postweaning weight gain increased (P < 0.007) the probabilities of multiple births in Rambouillet and Targhee ewe lambs. In conclusion, Polypay ewe lambs were superior in fertility and prolificacy to Columbia, Rambouillet, and Targhee under Western range conditions. Improved reproductive performance of Columbia, Rambouillet, and Targhee ewe lambs may be achieved by increasing age and weight at breeding and postweaning gain.