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Effects of birth-rearing type on weaning weights in meat sheep are systematically associated with differences in mean performance among flocks.

Notter DR, Brown DJ - Genet. Sel. Evol. (2015)

Bottom Line: However, increasing use of multibreed genetic evaluations that incorporate data from both purebred and commercial flocks has generated concerns regarding the ability of simple additive or multiplicative adjustment factors to properly correct for environmental effects in flocks that differ widely in mean performance.Thus, consistency of adjustment factors across flocks and systematic effects of the level of flock performance on these factors were evaluated using data from the US National Sheep Improvement Program.Interactions between these effects and flock were assessed.

View Article: PubMed Central - PubMed

Affiliation: Department of Animal and Poultry Sciences, Virginia Tech, Blacksburg, VA, 24061, USA. drnotter@vt.edu.

ABSTRACT

Background: Adjustment of body weights for systematic environmental effects such as dam age and litter size is essential for accurate prediction of breeding values in meat sheep and often accomplished by pre-adjusting records using simple multiplicative adjustment factors, which are derived as ratios of least-squares means of weights of lambs in target and reference classes. However, increasing use of multibreed genetic evaluations that incorporate data from both purebred and commercial flocks has generated concerns regarding the ability of simple additive or multiplicative adjustment factors to properly correct for environmental effects in flocks that differ widely in mean performance. Thus, consistency of adjustment factors across flocks and systematic effects of the level of flock performance on these factors were evaluated using data from the US National Sheep Improvement Program.

Methods: We used birth and weaning weights of lambs from 29 flocks that had at least 500 records per flock and represented several terminal-sire sheep breeds. Effects of lamb sex, dam age class and litter size on birth weights, and of dam age class and combined effects of type of birth and rearing on weaning weights were evaluated. Interactions between these effects and flock were assessed. Bias associated with different adjustment protocols was evaluated for high- and low-performance flocks.

Results: Effects of litter size and differences between yearling and adult dams varied (P < 0.001) among flocks. For weaning weights, additive adjustment factors were not associated with the level of flock performance, but multiplicative adjustment factors were strongly and inversely related to flock means for weaning weights (W). Flock-specific adjustment factors (F = αW(β)) reduced bias in adjusted weaning weights associated with differences in flock performance. By contrast, simple multiplicative adjustment factors were appropriate to adjust birth weights.

Conclusions: Differences in weaning weights among single, twin, and triplet lambs were inversely related to the level of flock performance. Use of simple multiplicative adjustment factors led to adjustment bias when applied across flocks with large differences in mean performance. This bias was reduced by using additive adjustment factors or multiplicative factors that were derived as simple exponential functions of flock means for weaning weight.

No MeSH data available.


Related in: MedlinePlus

Relationship between adjustment factors and flock means for birth weight. Additive or multiplicative factors (Y) that are required to adjust birth weights of single lambs to a twin-lamb basis are plotted against flock means for birth weights of twin lambs (W). Prediction equations were derived by weighting each observation by the number of single-born lambs in the flock
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Fig1: Relationship between adjustment factors and flock means for birth weight. Additive or multiplicative factors (Y) that are required to adjust birth weights of single lambs to a twin-lamb basis are plotted against flock means for birth weights of twin lambs (W). Prediction equations were derived by weighting each observation by the number of single-born lambs in the flock

Mentions: Flock means for birth weights of twin lambs in the core data ranged from 4.3 to 6.7 kg (Fig. 1). Effects of birth type differed among flocks (P < 0.001) in all datasets. Effects of flock × dam age class interaction were likewise significant in all datasets (P ≤ 0.04) and most significant (P < 0.001) in the dataset that included yearling dams. Effects of flock × lamb sex interaction were significant (P ≤ 0.01) in core and aged-dam datasets and approached significance (P = 0.08) in the dataset that included yearling dams. In general, the additive factors that were necessary to adjust for effects of birth type were proportional to flock means. For example (Fig. 1), differences in birth weights between single- and twin-born lambs were proportional to the 0.91 ± 0.51 power of the flock mean for twin-lamb birth weight, whereas multiplicative factors that were necessary to adjust birth weights of single lambs to a twin-lamb equivalent were independent of flock means for birth weight (P = 0.92). Differences in birth weights between twin and triplet lambs (not shown) were likewise approximately proportional (1.15 ± 0.33) to flock means for twin-lamb birth weights whereas corresponding multiplicative factors were independent of flock means (P = 0.56). A somewhat different pattern (not shown) was observed for lambs from yearling dams: differences in birth weights between yearling and adult dams were proportional to the 2.12 ± 0.74 power of the flock mean for birth weights of lambs from adult dams. The corresponding proportionality constant for multiplicative factors was positive (0.26 ± 0.16) but not significantly different from zero (P = 0.13). However, the smaller additive and multiplicative factors that were required to adjust birth weights of lambs from 2-year-old or aged dams to an adult-dam equivalent or to adjust weights of ram lambs to a ewe-lamb basis (not shown) were not associated with flock means for birth weight.Fig. 1


Effects of birth-rearing type on weaning weights in meat sheep are systematically associated with differences in mean performance among flocks.

Notter DR, Brown DJ - Genet. Sel. Evol. (2015)

Relationship between adjustment factors and flock means for birth weight. Additive or multiplicative factors (Y) that are required to adjust birth weights of single lambs to a twin-lamb basis are plotted against flock means for birth weights of twin lambs (W). Prediction equations were derived by weighting each observation by the number of single-born lambs in the flock
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4489108&req=5

Fig1: Relationship between adjustment factors and flock means for birth weight. Additive or multiplicative factors (Y) that are required to adjust birth weights of single lambs to a twin-lamb basis are plotted against flock means for birth weights of twin lambs (W). Prediction equations were derived by weighting each observation by the number of single-born lambs in the flock
Mentions: Flock means for birth weights of twin lambs in the core data ranged from 4.3 to 6.7 kg (Fig. 1). Effects of birth type differed among flocks (P < 0.001) in all datasets. Effects of flock × dam age class interaction were likewise significant in all datasets (P ≤ 0.04) and most significant (P < 0.001) in the dataset that included yearling dams. Effects of flock × lamb sex interaction were significant (P ≤ 0.01) in core and aged-dam datasets and approached significance (P = 0.08) in the dataset that included yearling dams. In general, the additive factors that were necessary to adjust for effects of birth type were proportional to flock means. For example (Fig. 1), differences in birth weights between single- and twin-born lambs were proportional to the 0.91 ± 0.51 power of the flock mean for twin-lamb birth weight, whereas multiplicative factors that were necessary to adjust birth weights of single lambs to a twin-lamb equivalent were independent of flock means for birth weight (P = 0.92). Differences in birth weights between twin and triplet lambs (not shown) were likewise approximately proportional (1.15 ± 0.33) to flock means for twin-lamb birth weights whereas corresponding multiplicative factors were independent of flock means (P = 0.56). A somewhat different pattern (not shown) was observed for lambs from yearling dams: differences in birth weights between yearling and adult dams were proportional to the 2.12 ± 0.74 power of the flock mean for birth weights of lambs from adult dams. The corresponding proportionality constant for multiplicative factors was positive (0.26 ± 0.16) but not significantly different from zero (P = 0.13). However, the smaller additive and multiplicative factors that were required to adjust birth weights of lambs from 2-year-old or aged dams to an adult-dam equivalent or to adjust weights of ram lambs to a ewe-lamb basis (not shown) were not associated with flock means for birth weight.Fig. 1

Bottom Line: However, increasing use of multibreed genetic evaluations that incorporate data from both purebred and commercial flocks has generated concerns regarding the ability of simple additive or multiplicative adjustment factors to properly correct for environmental effects in flocks that differ widely in mean performance.Thus, consistency of adjustment factors across flocks and systematic effects of the level of flock performance on these factors were evaluated using data from the US National Sheep Improvement Program.Interactions between these effects and flock were assessed.

View Article: PubMed Central - PubMed

Affiliation: Department of Animal and Poultry Sciences, Virginia Tech, Blacksburg, VA, 24061, USA. drnotter@vt.edu.

ABSTRACT

Background: Adjustment of body weights for systematic environmental effects such as dam age and litter size is essential for accurate prediction of breeding values in meat sheep and often accomplished by pre-adjusting records using simple multiplicative adjustment factors, which are derived as ratios of least-squares means of weights of lambs in target and reference classes. However, increasing use of multibreed genetic evaluations that incorporate data from both purebred and commercial flocks has generated concerns regarding the ability of simple additive or multiplicative adjustment factors to properly correct for environmental effects in flocks that differ widely in mean performance. Thus, consistency of adjustment factors across flocks and systematic effects of the level of flock performance on these factors were evaluated using data from the US National Sheep Improvement Program.

Methods: We used birth and weaning weights of lambs from 29 flocks that had at least 500 records per flock and represented several terminal-sire sheep breeds. Effects of lamb sex, dam age class and litter size on birth weights, and of dam age class and combined effects of type of birth and rearing on weaning weights were evaluated. Interactions between these effects and flock were assessed. Bias associated with different adjustment protocols was evaluated for high- and low-performance flocks.

Results: Effects of litter size and differences between yearling and adult dams varied (P < 0.001) among flocks. For weaning weights, additive adjustment factors were not associated with the level of flock performance, but multiplicative adjustment factors were strongly and inversely related to flock means for weaning weights (W). Flock-specific adjustment factors (F = αW(β)) reduced bias in adjusted weaning weights associated with differences in flock performance. By contrast, simple multiplicative adjustment factors were appropriate to adjust birth weights.

Conclusions: Differences in weaning weights among single, twin, and triplet lambs were inversely related to the level of flock performance. Use of simple multiplicative adjustment factors led to adjustment bias when applied across flocks with large differences in mean performance. This bias was reduced by using additive adjustment factors or multiplicative factors that were derived as simple exponential functions of flock means for weaning weight.

No MeSH data available.


Related in: MedlinePlus