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The Genetics of shoulder height

The main selection criteria for the Dohne Merino breed is among others to increase yearling live weight, maintain fleece weight and to reduce mean fibre diameter. It is known that heavier and leggier animals tend to be less hardy and possibly less reproductive. It is hypothesised that selection for growth (weaning and post weaning) will lead to a leggier type of animal with a negative impact on reproduction rate.

To estimate genetic parameters for shoulder height, live weight, wool and reproduction traits in a Dohne Merino flock.

Data on yearling shoulder height (SH), weaning weight (WWT), yearling weight (YWT), yearling clean fleece weight (CFW), yearling fibre diameter (FD), number of lambs weaned at first lambing (NLW1) and number of lambs weaned at second lambing (NLW2) were recorded in the University of Stellenbosch Dohne Merino flock between 2002 and 2015. The number of available records ranged from 926 for NLW2 to 7434 for WWT. ASREML was used to estimate genetic parameters for these traits.

Derived heritability estimates (±SE) amounted to 0.40±0.05 for SH, 0.18±0.03 for WWT, 0.41±0.06 for YWT, 0.40±0.03 for CFW, 0.69±0.03 for FD, 0.13±0.05 for NLW1 and 0.10±0.06 for NLW2. Significant maternal genetic effects were found for WWT (0.07±0.02) and YWT (0.08±0.02), while WWT was also affected by dam permanent environment (0.06±0.01). Genetic correlations with SH amounted to 0.58±0.10 for WWT, 0.54±0.10 for YWT, 0.10±0.11 for CFW, 0.13±0.09 for FD, 0.17±0.19 for NLW1 and 0.39±0.22 for NLW2. The genetic correlations of SH with maternal genetic variance ratios for WWT and YWT amounted to respectively 0.38±0.14 and 0.08±0.16.

Shoulder height is highly heritable and correlated with growth traits (WWT and YWT). Selection for these traits, as currently in practise, will increase SH in the breed. No data is available to support the assumption that it will lead to a leggier type of animal. The direction of genetic correlations of SH and reproduction traits was positive but these correlations were not significant as reflected by estimates below twice the corresponding standard error. SH was lowly correlated with YFW and YFD.

The current Dohne Merino breeding objective will result in animals with a higher shoulder height. It is recommended that additional data be collected on condition scores at weaning and yearling age to quantify the impact of the current breeding objective on all traits of economic importance.