Proc. Assoc. Advmt. Anim. Breed. Genet. Vol 14

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Proc. Assoc. Advmt. Anim. Breed. Genet. Vol 14

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SELECTION FOR GROWTH RATE IN PIGS ON RESTRICTED FEEDING. GENETIC PARAMETERS AND RESPONSES IN GROWTH RATE AND BODY COMPOSITION

TRAITS FOR GROUP HOUSED AD LIBITUM FED PIGS.

C.P. McPhee¹, N.H. Nguyen² and L.J. Daniels³

1Queensland Department of Primary Industries, Animal Research Institute, Yeerongpilly, QLD 4105

2School of Veterinary Science, The University of Queensland, QLD 4072

3Queensland Department of Primary Industries, Research Station, Biloela, QLD 4715

SUMMARY

Estimation of genetic parameters for lifetime daily liveweight gain and carcass traits were carried out on 1892 records of pigs fed ad libitum in groups. Estimates of heritability (standard errors in parentheses) were 0.19 (0.04) for lifetime daily liveweight gain (LWDG), 0.13 (0.04) for daily carcass weight gain (CWDG), 0.35 (0.06) for carcass backfat (CF) and 0.33 (0.06) for predicted lean percentage in the carcass (LEAN). Genetic correlations among carcass traits were favourable.

Lifetime daily liveweight gain exhibited a negative genetic correlation with carcass fat. After four years of divergent selection for 6 week post-weaning growth rate on restricted feeding, pigs performance tested on ad libitum feeding in groups exhibited changes in estimated breeding values (EBV) of 9.97, -11.14 (g/d) for LWDG, 6.36, -8.79 (g/d) for CWDG, -1.82, 1.49 (mm) for CF and 3.2, -0.79 (%) for LEAN for the high and low lines, respectively.

Keywords: Pigs, genetic parameters, growth, body composition.

INTRODUCTION

Because of the economic importance of growth rate and carcass lean (McPhee and Macbeth 2000), it is necessary to evaluate in a production environment the responses in these traits to selection made in a performance testing environment. Production environments usually provide group housing and ad libitum feeding from weaning to slaughter. On the other hand, where selection is for growth, feed efficiency and carcass lean, performance testing has often been carried out on pigs penned separately over a limited post-weaning period and fed either ad libitum or on a restricted amount (McPhee et al.

1988). It is important to see if this affects the realisation of gains made in the selected generation by the production generation.

The current study examines genetic changes in growth and carcass traits in lines of Large White pigs selected divergently (high and low) for post-weaning growth in individual pens and fed on a restricted scale (80% of ad libitum) (McPhee et al. 2000). Testing of the lines occurred under a commercial production condition using group housing and ad libitum feeding. Genetic correlations between and genetic responses in the growth and carcass traits under these conditions are presented.

MATERIAL AND METHODS

Animals and performance testing. Details of the development of the two selection lines were described by Nguyen et al. (1999). Pedigree and data structure are given in Table 1. The lines, each of 36 sows and 6 boars were initially formed by sampling within Large White litters. They were

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divergently selected for high and for low post-weaning gain on a restricted scale (80% ad libitum) over a 6-week period starting at 50 kg. Pigs were also sampled from the lines within the same litters for growth testing on either individual or group ad libitum feeding. In this study, pigs of similar weight and sex were intermingled in group pens and fed ad libitum to an average live weight of 108 kg. All animals were fed a diet containing 14 MJ DE and 0.65 g/MJ available lysine. End weight was measured before sending the pigs to the abattoir. Lifetime daily liveweight gain (LWDG) was the ratio of end weight to days from birth to final liveweight.

At finish, pigs were tattooed and sent on an 8 hour journey from their grow-out farm in Central Queensland to an abattoir in Brisbane where they were lairaged overnight without food but with water. Hot carcass weight (HCW) and carcass fat depth (CF) were measured after slaughter.

Measurement of fat was made at the P2 position using Hennessy Grading Probe (Hennessy Grading Systems, Auckland, New Zealand). Hot carcass weight including fore and hind trotters, and leaf fat but without head. Average daily carcass gain (CWDG) was calculated as the hot carcass weight over days from birth to slaughter. Predicted lean meat percentage (LEAN) was derived basing on adjusted hot carcass weight and carcass fat measurements (Ferguson et al. 1994).

Table1: Number of sires, dams and their progeny in the data set

Years Animals Sires Dams

Base 255 15 70

1997 441 32 108

1998 491 45 133

1999 572 41 161

2000 133 22 72

Total 1892 113 379

Statistical analysis. Genetic and environmental variance components for all traits were estimated with animal model- restricted maximum likelihood method using the average information algorithm (ASREML, Gilmour et al. 1999). A series of univariate analyses was carried out to obtain estimates of heritability. Genetic and phenotypic correlations among the traits were derived from bivariate model analyses. The fixed effects of batch (33 batches) and sex (males and females), and the random effect of the individual animal was included in the model. Litter effect was found to be insignificant.

Lifetime age from birth to slaughter (AGE) and hot carcass weight (HCW) were fitted as linear covariates for carcass fat and AGE for lean percentage.

Estimation of breeding values (EBVs) for all the traits was carried out using the Best Linear Unbiased Prediction (BLUP) analysis of the PEST package (Groeneveld 1990). The additive genetic and residual (co) variances used in the multivariate model including the same fixed and random effects as described in estimation of genetic parameters. Selection lines means of EBVs for all traits were obtained after fitting the fixed effects of year and line and their interaction using REML analysis (Genstat 5 1997).

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217 RESULTS

Heritabilities and genetic correlations. Heritabilities and genetic and phenotypic correlations for carcass and production traits are presented in Table 2. Estimates of heritability for all the traits are generally moderate to high and fall within the literature ranges reviewed by Sellier (1998). Genetic correlations between growth rate and carcass traits, and among carcass traits are generally favourable.

Genetic correlations between CWDG and LWDG and LEAN are positive while those between CF and LWDG and LEAN are negative. The genetic correlation between LWDG and CWDG is strongly positive.

Table 2: Heritabilities (on the diagonal) and phenotypic and genetic (above and below the diagonal, respectively) correlations for carcass and production traits in ad libitum fed group housed pigs

Traits CWDG CF LEAN LWDG

CWDG 0.13 (0.04) 0.04 (0.04) -0.16 (0.03) 0.89 (0.02)

CFT -0.15 (0.18) 0.35 (0.06) -0.30 (0.00) -0.18 (0.03)

LEAN 0.15 (0.18) -0.29 (0.00) 0.33 (0.04) -0.18 (0.02)

LWDG 0.97 (0.02) -0.24 (0.16) 0.02 (0.15) 0.19 (0.04)

Standard error in parentheses

Genetic correlated responses. The genetic correlated responses in the high and low growth rate lines, evaluated as estimated breeding values (EBV) are shown in Table 3. At the end of selection, correlated responses in animals born in 2000 were 9.97, -11.14 (g/d) for LWDG, 6.36, -8.79 (g/d) for CWDG, -1.82, 1.49 (mm) for CF and 3.2, -0.79 (%) for LEAN for the high and low lines, respectively.

Table 3: Estimated breeding values for production and carcass traits in group housed, ad libitum fed pigs

Years CWDG CF LEAN LWDG

High Low High Low High Low High Low

Base -1.16 1.15 0.32 -0.62 -0.28 0.53 -3.89 6.18

1997 -3.20 6.11 -0.51 0.91 0.61 -0.90 -4.09 9.78

1998 0.58 0.08 -0.55 1.24 0.89 -0.95 1.58 -1.69

1999 3.37 -3.04 -0.57 0.53 0.73 -0.49 3.91 -3.94

2000 6.36 -8.79 -1.82 1.49 3.20 -0.79 9.97 -11.14

s.e.d. 6.39 0.50 0.49 7.90

DISCUSSION

The direction of the genetic correlations between the traits measured in group housed, ad libitum fed pigs was generally consistent with the literature results (Sellier 1998). Lifetime daily weight gain had a highly positive genetic correlation with daily carcass gain and moderately negative correlation with carcass backfat, suggesting that if selection for growth rate had been carried out under group housed,

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ad libitum feeding conditions, favourable improvements in these important traits would have occurred.

Although the genetic responses realised in the production environment were variable over years of selection, their changes over four years, particularly in carcass fat and lean percentage, were considerable. This indicates that the selection of breeding stock for increased post-weaning growth rate on individual restricted feeding is expected to bring about an economically beneficial increase in the rate of lean growth in their descendants raised in a production environment of group housing and ad libitum feeding.

This supports the conclusion from the review of Clutter and Brascamp (1998) that restricted feeding is an effective performance testing approach for seedstock lines supplying commercial systems that use either ad libitum or restricted feeding.

Even though food conversion efficiency was not measured in this study, an economically beneficial change in this trait would have been expected in the high growth line due to its increased rate and leanness of growth and from feed efficiency measurements on the same line fed ad libitum in individual pens (McPhee et al. 2000).

ACKNOWLEDGEMENTS

We acknowledge financial support from the Australian Center for International Agricultural Research.

REFERENCES

Clutter, A. C., and Brascamp, E.W. (1998) In “The genetics of the pig”, p. 427. CAB International.

Ferguson, D.M., Chandler, R.C., Mynard, P., Thomas, M. (1994) PDRC LMA6.P final report.

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Gilmour, A.R., Cullis, B.R., Welham, S.J., Thompson, R. (1999) “ASREML Reference Manual”.

Groeneveld, E. (1990) “PEST User's Manual”. Institute of Animal Husbandry and Animal Behaviour.

Federal Agricultural Research Centre, Germany.

McPhee, C.P., MacBeth, M. (2000) PDRC DAG58/1339 final report.

McPhee, C.P., Nguyen, N.H. and Daniels, L.J. (2000) Asian- Austr. Jour. Anim. Sci. 13 Suppl. July 2000 A: 222.

McPhee, C.P., Rathmell, G.A., Daniels, L.J. and Cameron, N.D. (1988) Anim. Prod. 47: 149.

Nguyen, N.H., McPhee, C.P., and Daniels, L.J. (1999) In “Manipulating Pig Production VI”, ed. P.D.

Cranwell. (Australian Pig Science Association: Werribee). p. 174.

Sellier, P. (1998). In “The genetics of the pig”, p. 463. CAB International.