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# Genetic parameters of maturing rate index and asymptotic adult weight in French beef cattle

LEPERS A.<fnref n="1" />, AGUERRE S.<fnref n="1" />, PROMP J.<fnref n="1" />, TAUSSAT S.², VINET A.³, MARTIN P.³, PHILIBERT A.⁴, LARAMEE A.⁴, GRIFFON L.⁴

## INTRODUCTION

Working on the early development of French beef cows is interesting to:

* Reduce fattening time and added concentrates

* Reduce the environmental impact of finished products

* Facilitate 2-year-old calving through early development

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**Can early development be estimated from on-farm weighing? Is genetic selection for early development possible?**

## MATERIALS AND METHODS

**Phenotypes (5 beef breeds):** Use of Brody equation from birth weight declared by breeders, performance control weights and NORMABEV carcass weights of adult females.

**Brody equation:**

$$Weight(time) = Weight_{adult} - (Weight_{adult} - Weight_{birth})e^{-maturing\ rate\ index}*time$$

**Genetic parameters:**

* Estimated adult weight and maturing rate index: data corrected for fixed effects related to birth and slaughter.

* Birth weight and adjusted weights (AW) at 7m, 12m, 18m and 24m: corrected performances with a correction of genetic progress.

## RESULTS

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* Not enough data for the Aubrac breed

* Standard deviation of error between 0.01 and 0.03

* Average genetic variation coefficient equal to 6.16% for adult weight and 9.09% for maturing rate index

<table><caption>Genetic correlation of maturing rate index with bodyweight at different ages (average of the 4 breeds):</caption><thead><tr><th></th><th>Birth weight</th><th>AW at 7m</th><th>AW at 12m</th><th>AW at 18m</th><th>AW at 24m</th><th>Adult weight</th></tr></thead><tbody><tr><th>Maturing rate index</th><td>-0.12<br>(-0.18 to -0.07)</td><td>0.32<br>(0.29 to 0.37)</td><td>0.39<br>(0.32 to 0.45)</td><td>0.42<br>(0.27 to 0.69)</td><td>0.40<br>(-0.03 to 0.70)</td><td>-0.64<br>(-0,65 to -0,62)</td></tr></tbody></table>

## DISCUSSION

The maturing rate index seems to be a good phenotype for early development. It would be possible to genetically select the maturing rate index (due to its heritability and genetic variance) without increasing birth weight (average genetic correlation of -0.12). The maturing rate index seems to be positively correlated with adjusted weights between 7 and 24 months. However, the lack of animals with adjusted weights at 18 and 24 months limits the accuracy of the associated correlations and therefore their possible interpretation. The highly negative genetic correlation between maturing rate index and adult weight shows that selecting for heavier adult animals reduces their maturing speed. The disadvantage of the maturing rate index are that the phenotype arrives at the end of the animals' lives and is very dependent on the weaning and post-weaning weights. To obtain breeding values for young animals, a genomic prediction needs to be developed.

## CONCLUSION

**Maturing rate index is ...**

* Measurable from commercial data, although it is highly dependent on weighing at weaning and post-weaning.

* Heritable and genetically variable, so direct selection is possible.

* Genetically correlated with other traits, allowing indirect selection in particular via reduction in adult weight.

Many thanks to the 6 breed societies who participated.

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Funding :

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Research program

(co)funded by APIS-GENE

Avec la contribution financière du compte d'affectation spéciale développement agricole et rural CASDAR

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MINISTÈRE DE L'AGRICULTURE ET DE LA SOVERAINETÉ ALIMENTAIRE

Liberté Égalité Fraternité

La responsabilité du ministère en charge de l'agriculture ne saurait être engagée

<footnote n="1">¹Institut de l'Elevage, UMR 1313 GABI, F78350 Jouy-en-Josas, ²ELIANCE, UMR 1313 GABI, F78350 Jouy-en-Josas, ³INRAE, UMR 1313 GABI, F78350 Jouy-en-Josas, ⁴Institut de l'Elevage, F75595 Paris</footnote>