The relationship between residual feed intake and feed intake ...

The relationship between residual feed intake and feed intake behavior in group-housed Duroc barrows1 W. M. Rauw,*2 J. Soler,† J. Tibau,† J. Reixach,‡ and L. Gomez Raya* *Department of Animal Biotechnology, University of Nevada, Reno 89557; †IRTA, Centro de Control Porcino, 17121 Monells (Girona), Spain; and ‡Seleccioń Batalle´ S.A., Riudarenes, Spain

were calculated for 5 periods of 14, 23, 28, 21, or 23 d in length (periods 1 through 5, respectively) on animals that were between 73 to 95 d of age at the start of the testing period. Barrows that grew faster consumed more feed (P < 0.001), and barrows that consumed more feed were fatter (P < 0.01). There were no correlations between VISITS and TIME, between VISITS and FI, or between VISITS and RFI. Barrows that spent more time at the feeder, however, consumed more feed (P < 0.05) and had greater RFI in periods 1, 3, and 5 (P < 0.05). As expected, FI and FCR were highly correlated with RFI (P < 0.001). These results suggest that a greater FI rather than greater feed intake activity resulted in greater RFI values.

ABSTRACT: Because feed is the major input in pork production, conversion of feed into lean tissue at minimum costs has been a focus for improvement. Several researchers have proposed using residual feed intake (RFI) rather than feed conversion ratio (FCR) for genetic improvement of feed efficiency. Little is known about the variation in RFI in pigs. As several studies suggest a greater RFI is related to greater animal activity levels, the current study investigated the phenotypic relationship between RFI and feed intake (FI) behavior of 104 group-housed growing Duroc barrows allowed ad libitum access to feed. Feed intake, BW gain, feeding time (TIME), feeding frequency (VISITS), RFI, and FCR

Key words: feed conversion ratio, feed intake behavior, pig, residual feed intake 2006 American Society of Animal Science. All rights reserved.

INTRODUCTION

J. Anim. Sci. 2006. 84:956–962

by an animal and its consumption as predicted from a model involving its production (growth in the case of pigs) and maintenance requirements, i.e., the error term of the prediction equation. Little is known about the causes of variation in RFI, and few papers have been published on this subject in pigs (De Haer et al., 1993; Von Felde et al., 1996). Background knowledge is especially relevant when RFI is to be used as a selection criterion in the breeding goal. Several studies indicate that greater RFI is related to greater animal activity levels. In a divergent selection experiment on RFI in laying hens, Luiting et al. (1991b) and Braastad and Katle (1989) observed that laying hens from the high RFI line were more active than those from the low line. Rauw et al. (2000b) indicated that female mice from a line selected for high litter size at birth had a greater RFI. Also, selected females were more active in several behavioral tests than females of a nonselected control line (Rauw et al., 2000a). In the study by De Haer et al. (1993), in pigs, variation in feed intake (FI) activity accounted for 44% of the variation in RFI. The objective of this study was to investigate the phenotypic relationship between RFI and FI behavior in group-housed growing Duroc barrows.

Feed is the major input to pork production and accounts for more than 65% of all production expenses. Therefore, conversion of feed into lean tissue at minimum cost has been a focus for improvement (Mrode and Kennedy, 1993; De Vries and Kanis, 1994). Direct selection for improved feed conversion ratio (FCR) is difficult and may even result in undesired correlated selection responses because of the ratio aspect (i.e., feed over gain). Therefore, several authors have proposed selecting for residual feed intake (RFI) instead (Luiting et al., 1991a; Kennedy et al., 1993). Residual feed intake is defined as the difference between the feed consumed

1

We are grateful to the Spanish Ministry of Science and Technology (MCYT) for financing this research through the Project AGL200204271-C03-02 entitled Arquitectura gene´tica de los componentes lipı´dicos de la carne porcina relacionados con la calidad y la salud humana. We are also very grateful to the company Seleccioń Batalle´ for providing the Duroc pigs and for making possible this research and to C. Beattie and the reviewers for their useful comments on the manuscript. 2 Corresponding author: wrauw@cabnr.unr.edu Received March 31, 2005. Accepted November 28, 2005.

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MATERIALS AND METHODS The data set was based on 104 Duroc barrows; in Spain, boars are castrated when the objective is the production of cured ham. The animals were born between August and September 2003 and were the sons of 5 sires (11, 16, 17, 27, and 33 sons per sire) and 104 dams distributed over 3 farms (30, 36, and 38 animals). At weaning (at 15 to 19 d of age), the pigs were moved to the test station Centre de Control Porci [CCP-IRTA, Monells (Girona), Spain] and were distributed over 16 pens in 4 rooms subjected to the same management. At this stage, animals were housed based on age; animals that belonged to this experiment were mixed with animals that did not take part in this experiment, possibly of different breeds and sexes. In the second half of November 2003, the pigs participating in the current study were moved to the fattening and control unit, where they were distributed over 10 pens (5 pens at each side of a central corridor in the same building) in groups of 8 to 12 animals. Beginning December 1, FI and FI behavior were recorded for a total of 106 d. Until d 72 of the study, barrows were fed ad libitum on a standard diet containing 18% CP, 3.8% fiber, 7.0% fat, 1.0% lysine, and 0.3% methionine on an as-fed basis. After d 72, the barrows were fed ad libitum on a standard diet containing 15.9% CP, 4.5% fiber, 5.2% fat, 0.7% lysine, and 0.2% methionine on an as-fed basis. The net energy concentrations of the diets were 2,450 and 2,375 kcal/kg on an as-fed basis, respectively. Body weight and backfat thickness (BFT) were measured on d 2, 16, 39, 67, 88, and 106 of the study. Backfat thickness was measured by the PIGLOG 105 A-mode apparatus (SFK Technology, Soborg, Denmark) as the average of 2 ultrasonic measurements taken on each side of the spinal column, 5 cm from the middorsal line at the last rib. Daily feed intake (DFI), daily feeding time (DTIME), and daily feeding frequency (DVISITS) were recorded automatically by means of an electronic identification system (HOKOFARM, IVO-G, Marknesse, The Netherlands). This was done during the entire test period, beginning on the first of December (total test period, 106 d); on this day, the animals were between 73 and 95 d of age. Because of electricity failures during the 106-d period, a total of 24 recordings were missing. Missing values of DFI were estimated for each individual by fitting a Von Bertalanffy growth equation to data on DFI against day on study (Lorenzo Bermejo et al., 2003): DFI = A × (1 − B(−k × t))3,

[1]

in which A = the asymptotic value of FI (kg), B = a biological constant, k = a rate constant, and t = day of the study (d 1 to 106). This equation was fit to the data with the sole purpose of estimating missing values. Therefore, no biological meaning was given to its parameters, and no limits were set to their estimation.

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Missing values of DVISITS were estimated for each individual by taking the average of the recorded values over the whole 106-d period. Missing values of DTIME were estimated by fitting a polynomial equation to data on DTIME against day on study: DTIME = a + (b × t) + (c × t2),

[2]

in which a, b, and c are regression coefficients and t is as in Equation 1. Daily (estimated) values of DFI, DVISITS, and DTIME were summed for each weight period and overall, giving FI, VISITS, and TIME, respectively. Feed conversion ratio (kilograms of feed per kilogram of BW gain) of each individual was estimated for each of the 6 periods by dividing FI by BW gain (BWG). Individual RFI was estimated for each of the 5 periods and for the total period from a multiple linear regression of FI on metabolic BW (BW0.75), BWG, BFT, and age: ) + (b2 × BWGi) FIi = b0 + (b1 × BW0.75 i

[3]

+ (b3 × BFTi) + (b4 × AGEi) + ei, in which FIi = feed intake (kilograms per period) for = average metabolic BW (kg0.75) of individual i; BW0.75 i individual i; BWGi = BW gain (kilograms per period) of individual i, BFTi = average backfat thickness (mm) of individual i; AGEi = test age (d) of individual i at the beginning of each period; b0 = the population intercept; b1, b2, b3, b4 = partial regression coefficients representing maintenance requirements per metabolic kilogram, feed requirements for growth, and feed requirements related to body composition and age, respectively; and ei = the error term, representing RFI (kilograms per period) of individual i. Metabolic BW and BFT for each period were estimated as the average of the corresponding values at the beginning and at the end of the period. The SAS program (SAS Inst. Inc., Cary, NC) was used for statistical analysis of all traits. The model used to describe the data was Yijkl = ␮ + PENi + FARMj + SIREk + eijkl,

[4]

in which ␮ = the population intercept, PENi = effect of pen i (1 to 10), FARMj = effect of farm of origin j (1 to 3), SIREk = effect of sire k (1 to 5), and eijkl = the error term of animal j, eijkl∼NID(0, σe2). All traits tested under this model (FI, BWG, BFT, VISITS, TIME, FCR, and RFI for each of the 5 periods and the total period) were denoted by Yijkl, as measured on animal j of pen i originating from farm j with sire k. Initially, the effects of age and the interaction of sire with farm of origin were also included. Because the effect of age was only significant for BFT in the sixth period (P < 0.05) and the interaction of sire with farm of origin was significant only for TIME in the second period (P < 0.05), these effects were excluded from further analysis. Phenotypic

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correlations were estimated after adjusting the traits for the effects of pen, farm of origin, and sire.

RESULTS AND DISCUSSION Means and Trends The average DFI, DTIME, and DVISITS from the first day until d 106 of the study and the estimated values according to Equations 1, 2, and the mean number of visits to the feed hopper are shown in Figures 1a, b, and c, respectively. The R2 of Equation 1 ranged from 2 to 81% (average 46%); that of Equation 2 ranged from 0 to 76% (average of 36%). The average number of visits to the feed hopper for the period of 106 d was 7.5 with a standard deviation of 2.7. Values for TIME (Figure 1b) were considerably greater than those reported by De Haer and De Vries (1993; 51.6 min for Great Yorkshire and 62.1 min for Dutch Landrace boars), Von Felde et al. (1996; about 50 min for Large White and Landrace boars) and Schulze et al. (2001; 63.5 min for boars). Values for VISITS were considerably greater than reported by Von Felde et al. (1996; about 5 visits/d), lower than those reported by De Haer and De Vries (1993; 18.8 and 15.4 visits/d for Great Yorkshire and Dutch Landrace boars, respectively) and similar to those reported by Schulze et al. (2001; 7.2 visits/d). Differences in breed, sex (boars vs. barrows), stocking rate, and feeding system used may all be responsible for differences found among these studies. Table 1 presents BW and BFT at the beginning of each period and at the end of the total period, and the average DFI, BWG, and FCR for each of the 5 periods and the total period. Body weight and BFT increased up to the end of the total period. Average DFI and BWG increased up to the fourth period and decreased in the fifth period. The FCR increased with age. Average DFI values were comparable with those reported by Von Felde et al. (1996; 2.0 kg/d at 100 d of age to 2.8 kg/d at 170 d of age) and those reported by Schulze et al. (2001; 2.58 kg/d), and greater than those reported by De Haer and De Vries (1993; 1.85 and 1.87 kg/d for Great Yorkshire and Dutch Landrace boars, respectively). Average BFT and FCR were greater than those reported by Von Felde et al. (1996; 11.7 and 2.32 mm, respectively, between 100 to 170 d of age), by De Haer and De Vries (1993; 11.2 and 2.59 mm for Great Yorkshire boars, and 11.9 and 2.98 mm for Dutch Landrace boars, respectively), and by Schulze et al. (2001; 10.2 and 2.68 mm, respectively). Animals in the current study grew slower than those reported by Von Felde et al. (1996; 1,025 g/d between 100 to 170 d of age) and by Schulze et al. (2001; 970 g/d), and faster than those reported by De Haer and De Vries (1993; 712.8 and 662.9 g/d for Great Yorkshire and Dutch Landrace boars, respectively). Compared with boars, barrows generally have greater FI, poorer feed efficiencies, fatter carcasses, and inferior growth rates (Xue et al.,

Figure 1. Average daily feed intake (DFI; a), feeding time (DTIME; b), feeding frequency (DVISITS; c), and the corresponding estimated values according to Equation 1 (a), Equation 2 (b), and the mean number of VISITS (c). Equation 1 is a Von Bertalanffy growth equation fitted to data on DFI against day on study. Equation 2 is a polynomial equation fitted to data on DTIME against day on study.

1997); compared with white pig breeds, an increasing proportion of Duroc genes increases voluntary feed intake, decreases feed conversion efficiencies, increases

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Table 1. Means (SE) of BW, backfat thickness (BFT), feed intake (FI), BW gain (BWG), and feed conversion ratio (FCR) for periods 1 to 5 and the total period1 Trait BW,2 kg BFT,2 mm FI,3 kg/d BWG,3 g/d FCR3

Period 1

Period 2

Period 3

Period 4

Period 5

Total period

31.6 (0.654) 7.04 (0.132) 1.66 (0.0455) 765 (24.8) 2.16 (0.0501)

42.3 (0.863) 8.70 (0.162) 2.25 (0.0502) 848 (21.5) 2.76 (0.0759)

61.8 (1.09) 12.2 (0.250) 2.83 (0.0496) 890 (15.3) 3.23 (0.0551)

86.8 (1.26) 16.0 (0.320) 3.19 (0.0454) 913 (16.0) 3.56 (0.0525)

105.9 (1.31) 20.0 (0.374) 2.50 (0.0455) 845 (25.3) 3.77 (0.0639)

121.1 (1.33) 23.6 (0.417) 2.63 (0.0366) 861 (10.4) 3.12 (0.0293)

1 BFT was measured by the PIGLOG 105 A-mode apparatus (SFK Technology, Soborg, Denmark) as the average of 2 ultrasonic measurements taken on each side of the spinal column, 5 cm from the middorsal line at the last rib; FI for each period was estimated by summing daily values of feed intake; and FCR was estimated by dividing feed intake by BW gain. 2 Value at the beginning of periods 1 to 5 and at the end of the total period. 3 Average value for periods 1 to 5 and the total period.

fatness, and results in similar growth rates (Edwards et al., 1992; Blanchard et al., 1999). Because RFI is the error term of the linear regression Equation 3, the average RFI for each period equals 0. The SE ranged from 0.022 to 0.028 in periods 1 to 5 and was 0.016 in the total period. Table 2 presents the estimates of intercept and the partial regression coefficients representing average maintenance requirements per metabolic kilogram, feed requirements for growth and feed requirements related to body composition and age (b0, b1, b2, b3, and b4, respectively) and their corresponding SE. Variation observed between animals in FI appeared to be related to variation in maintenance requirements per unit metabolic kilogram (b1) and BWG (b2) in all periods. Variation in FI appeared to be related to variation in BFT (b3) in all but the first period and to variation in age (b4) in periods 2 and 5 only. The R2-values of Equation 3 are given in Table 2. The variation observed between individual barrows in FI that can be attributed to variation in the prediction variables of Equation 3 is much greater than that reported by Foster et al. (1983; 7 to 40%), De Haer et al. (1993; adjusted R2 of 22%), Mrode and Kennedy (1993; 52%), and Nguyen et al. (2005; 20.7%). In the study of Foster et al. (1983), RFI was calculated for Landrace, Large White, and Welsh boars with a model including BW0.75, BWG, and a fat index over a period from 33.5

to 90 kg of live weight. In their study, animals were fed to appetite during a period of 30 min and once a day only. In the study of De Haer et al. (1993), RFI was calculated for ad libitum-fed Dutch Landrace and Great Yorkshire boars and sows with a model including BW0.75, BWG, and lean percentage over a period from 30 to 100 kg of live weight. Mrode and Kennedy (1993) calculated RFI for ad libitum-fed Yorkshire, Landrace, and Duroc boars with a model including BWG and BFT over a period between 30 to 90 kg of live weight. In the last study, Nguyen et al. (2005) calculated RFI for ad libitum-fed Large White boars and sows with a model that included BW0.75, BWG, and BFT over a 6-wk period from 50 kg of live weight on. In our study, adjusting the R2 for the degrees of freedom changed the values only about 1%. Inclusion of age in the model of the current study was not responsible for the greater R2 because age only influenced FI to a small extent and was significant only in periods 2 and 5; inclusion of age in the model improved the R2 by 1 and 2% only for periods 2 and 5, respectively. This might be expected when pigs are more similar in age than in BW; the usual case with pig data is that pigs are fed over a constant weight range with a varying number of days on test. Therefore, inclusion of some measure of age in the models used by Foster et al. (1983), De Haer et al. (1993), and Mrode and Kennedy (1993) could have explained a larger portion of variation in RFI in their

Table 2. Estimates (SE) and R2-values of Equation 3 of the intercept (b0) and partial regression coefficients representing maintenance requirements per metabolic kilogram (b1), feed requirements for growth (b2), feed requirements related to body composition (b3), and feed requirements related to age (b4) for period 1 to 5 and the total period1 Period

b0

b1

1 2 3 4 5 Total

−0.382 (0.376) 0.564 (0.427) −0.643 (0.643) −0.0679 (0.742) 1.04 (0.608)† −0.406 (0.488)

0.123 (0.0183)*** 0.108 (0.0114)*** 0.0774 (0.0115)*** 0.0623 (0.00988)*** 0.0563 (0.00767)*** 0.0285 (0.0103)**

b2 0.0482 0.0176 0.0321 0.0673 0.0651 0.0143

(0.0115) *** (0.00725)* (0.00869)*** (0.00961)*** (0.00588)*** (0.00226)***

b3

b4

R2-value (%)

−0.0652 (0.0654) 0.230 (0.0364)*** 0.170 (0.0398)*** 0.0915 (0.0370)* 0.114 (0.0260)*** 0.0570 (0.0109)***

−0.00260 (0.00486) −0.00922 (0.00397)* 0.00264 (0.00458) −0.0000701 (0.00444) −0.00900 (0.00341)** 0.00111 (0.00279)

71 80 68 61 78 81

1 Values are estimates for an average day within the given period; Equation 3 is a multiple linear regression of feed intake on metabolic BW, BW gain, backfat thickness, and age; periods 1 to 5 and the total period are 14, 23, 28, 21, 23, and 106 d in length, respectively. On the first day of the study (period 1), animals were between 73 and 95 d of age. ***P < 0.001; **P < 0.01; *P < 0.05; and †P < 0.10.

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studies. Because of missing values in the current study, RFI was estimated from partly estimated data on DFI. However, it might be expected that estimation of missing values would decrease the R2 of the model because it would be less effective in representing the actual relationship between FI and BW0.75, BWG, and BFT. A possible explanation for the high R2-values found in our study may be the fact that RFI was estimated for one sex and one breed only and pigs were born in the same year. In other studies, these variables might have likely caused heterogeneity in the relationship between the prediction variables and FI (Rauw et al., 2000b; Nguyen et al., 2005).

Phenotypic Correlations Among Periods There was no significant relationship between FI in the first, and the fourth and fifth periods, nor between the second and fifth periods. All other correlations for FI and time periods ranged from r = 0.36 to 0.89 (P < 0.001, data not shown). Body weight gains in the first 3 periods were positively correlated with each other (r = 0.34 to r = 0.38, P < 0.001, data not shown). Also BWG between periods 3 and 4 (r = 0.27, P < 0.01), and between 4 and 5 (r = 0.20, P < 0.05) were positively correlated. Body weight gains in all periods were positively correlated with BWG in the total period (r = 0.40 to 0.73, P < 0.001, data not shown). Krieter and Kalm (1989) estimated the age at the inflection point of the Parks (1982) growth curve to be 18.3 and 20.3 wk of age in Pietrain and Large White pigs, respectively. This corresponds approximately to the third period in the current study and may explain the lack of significant correlations between the periods in the beginning and at the end of the test period. Animals that were fatter at the end of one period were also fatter at the end of other periods (r = 0.27, P < 0.01 to r = 0.88, P < 0.001, data not shown). Residual feed intake between the first, and the fourth and fifth periods, and between the second and fifth periods were not significantly correlated. All other correlations ranged from r = 0.24 (P < 0.05) to 0.67 (P < 0.001). The FCR in the 5 periods were only significantly correlated with FCR in the total period (r = 0.26, P < 0.01 to r = 0.63, P < 0.001, data not shown); the correlations between the other periods were in the range of −0.15 to 0.16 (data not shown). This could be expected because both FI and BWG were not always correlated within trait between periods. All correlations for VISITS and time periods were positive and highly significant (r = 0.44 to 0.93, P < 0.001, data not shown). Also TIME was mostly highly correlated between the different periods (r = 0.21, P < 0.05 to r = 0.89, P < 0.001, data not shown).

Phenotypic Correlations Within Periods Table 3 presents phenotypic correlations calculated in the current study between FI, BWG, BFT, FI behav-

ior (TIME, VISITS), and feed efficiency traits (FCR, RFI) by period. The table also presents observations by other researchers. Phenotypic correlations between FI and BWG were positive and very highly significant, indicating that animals that grew faster consumed more feed. The correlations between FI and BFT (measured at the end of each period) were also very highly significant, indicating that animals that consumed more feed were fatter at the end of each period. Faster growing animals were fatter at the end of the period. These findings are in agreement with literature observations (Table 3). Animals with a greater feeding frequency (VISITS) did not spend more time eating each day (TIME; Table 3). This is in contrast with results by Von Felde et al. (1996) but similar to the value presented by Schulze et al. (2001; Table 3). Animals that spent more time eating each day also consumed more feed, but feeding frequency (VISITS) was not correlated to FI (Table 3). Results agree with those given by Von Felde et al. (1996) and Schulze et al. (2001) but not with those of De Haer et al. (1993; Table 3). Although FCR was estimated as a function of FI, the correlation between the 2 traits was significant only in the third period. This resulted from few animals that grew little on the feed consumed. These outliers were not the same animals in each period. Faster growing animals were more feed efficient as indicated by highly significant negative correlations of BWG with FCR (Table 3). Animals with high FCR in the first period were leaner at the end of this period. As during growth, lean is energetically less expensive to deposit than fat (Webster, 1985); this result is unexpected. In all other periods, correlations were not significant and in agreement with literature observations (Table 3). As expected, FCR was highly positively correlated with RFI (Table 3); animals with a greater FCR and those with a greater RFI are less feed efficient. In agreement with other authors, animals that consumed more feed had a greater RFI (Table 3). Residual feed intake was adjusted for BWG and average BFT for each period in its calculation. Therefore, the phenotypic correlations between RFI and BWG for each period were automatically zero (data not shown). The phenotypic correlation between RFI and BFT at the end of each period was nonsignificant, except for periods 3 and 4 (r = 0.20 and 0.22, respectively, P < 0.05). In general, RFI has been related to FI behavior as a measure of activity (De Haer et al., 1993; Von Felde et al., 1996). In this study, RFI was not correlated to VISITS. In periods 1, 3, and 5 only, animals with a greater TIME had a greater RFI (Table 3). Several authors reported a positive phenotypic correlation between RFI and TIME. Only De Haer et al. (1993) observed a positive correlation also between RFI and VISITS (Table 3). In the current study, there was a positive correlation between TIME and FI, whereas there was no correlation between VISITS and FI. This suggests that greater FI rather than greater FI activity resulted in greater RFI. In other words, assuming a cause and effect, a

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Table 3. Phenotypic correlations (adjusted for the effect of pen, farm of origin, and sire) between feed intake (FI), body weight gain (BWG), residual feed intake (RFI), feed conversion ratio (FCR), feeding frequency (VISITS), and feeding time (TIME) within period, and backfat thickness (BFT) at the end of each period Period 1

Period 2

Period 3

0.70*** 0.55*** 0.00 0.42***

0.77*** 0.77*** 0.04 0.33***

0.68*** 0.73*** 0.02 0.28**

0.58***

0.67***

0.50***

Trait BWG BFT VISITS TIME

−0.02

−0.12 −0.62*** −0.36*** 0.40***

0.01 −0.61*** −0.09 0.47***

0.24* −0.51*** 0.14 0.71***

0.83*** 0.71*** 0.06 0.65***

0.85*** 0.77*** −0.06 0.21*

0.50***

0.76***

0.47,1 0.35,1 0.38,1 0.55,1

0.74,2 0.33,2 0.07,3 0.40,3

0.28,3 0.724 0.24,3 0.574 0.015 0.345

0.41***

0.50,1 0.28,2 0.464

0.66*** −0.04 0.21*

0.48*** 0.09 0.07

0.59*** 0.06 0.31**

0.12

0.07

0.04

−0.08

0.38,3 0.155

FCR

Trait FI VISITS TIME

0.67*** 0.63*** −0.02 0.30**

Literature value

TIME −0.13

Trait FI BWG BFT RFI

Total period

BWG

Trait VISITS

Period 5

FI

Trait BFT

Period 4

0.18† −0.59*** 0.14 0.66***

−0.09 −0.64*** 0.07 0.44***

0.15 −0.37*** −0.07 0.77***

0.44*** 0.07 0.34***

0.39*** 0.17 0.08

0.223 −0.36,2 −0.53,3 −0.394 0.09,2 0.144 0.79,3 0.406

RFI 0.66*** 0.08 0.18†

0.983 0.51,1 0.133 0.64,1 0.373

1 De Haer et al. (1993): in Dutch Landrace and Great Yorkshire pigs between approximately 30 to 100 kg of live weight, including metabolic BW, BWG, and lean percentage in the RFI model. 2 Mrode and Kennedy (1993): in Yorkshire, Landrace, and Duroc boars between approximately 30 to 90 kg of live weight, including BWG and BFT in the RFI model. 3 Von Felde et al. (1996): in Landrace and Large White boars between 100 to 170 d of age, including BWG and BFT in the RFI model. 4 Johnson et al. (1999): in Large White boars between approximately 100 to 177 d of age. 5 Schulze et al. (2001): in boars between 98 d and 10 wk of age. 6 Nguyen et al. (2005): in Large White pigs over a 6-wk period from 50 kg of BW on, including metabolic BW, BWG, and BFT in the RFI model. ***P < 0.001; **P < 0.01; *P < 0.05; and †P < 0.10.

greater FI resulted in greater TIME and RFI, rather than that a greater TIME as such resulting in greater RFI. Further research on the relationship between RFI and other forms of activity is important if RFI is to be used as a selection criterion to genetically improve feed efficiency. According to Mrode and Kennedy (1993), variation in residual feed consumption should improve efficiency of energy use without reducing appetite required for productive purposes. However, RFI relates to the amount of resources available for processes other than maintenance and production: animals with high RFI have more buffer resources available for processes such as physical activity and the ability to cope with unexpected stresses (Luiting et al., 1997; Rauw et al. 2000a). As described by Koolhaas et al. (1999), less reactive animals have a different neuroendocrine reactivity and neurobiological make-up resulting in different types of stress-pathology than more reactive animals: more docile animals are not necessarily less susceptible to stress (Duncan and Filshie, 1979). Two studies showed that laying hens with low RFI were less able to cope with high temperatures (Bordas and Minvielle, 1997) and maintained elevated corticosterone levels for a longer period of time after injection

with ACTH (Luiting et al., 1994). Therefore, selection against RFI may actually result in animals that are more susceptible to stress. Duroc barrows with a greater feeding time, but not those with a greater feeding frequency, consumed more feed, had a greater residual feed intake, and grew faster. This suggests that greater FI rather than greater FI activity is responsible for greater RFI values. The relationship between RFI and other forms of activity remains to be investigated. This is of importance in assessing consequences of selection for more feed efficient animals.

LITERATURE CITED Blanchard, P. J., C. C. Warkup, M. Ellis, M. B. Willis, and P. Avery. 1999. The influence of the proportion of Duroc genes on growth, carcass and pork eating quality characteristics. Anim. Sci. 68:495–501. Bordas, A., and F. Minvielle. 1997. Re´ponse a` la chaleur de poules pondeuses issues de ligneés se´lectionneés pour une faible (R−) ou forte (R+) consommation alimetaire re´siduelle. Genet. Sel. Evol. 29:279–290. Braastad, B. O., and J. Katle. 1989. Behavioural differences between laying hen populations selected for high and low efficiency of feed utilisation. Br. Poult. Sci. 30:533–544.

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