Impact of Birth and Body Weight at Twenty Days on thePostweaning Growth of Pigs with Different Weaning Management

The Professional Animal Scientist 23 (2007):197–210

Impact of Birth and Body Weight at Twenty Days on the Postweaning Growth of Pigs with Different Weaning Management A. P. Schinckel,*1 PAS, R. Cabrera,†2 R. D. Boyd,†3 S. Jungst,† C. Booher,† M. Johnston,†3 and M. E. Einstein* *Purdue University, Department of Animal Sciences, West Lafayette, IN 47907-2054; and †PIC North America, Hendersonville, TN 37075

ABSTRACT Pigs from 112 litters were assigned to 3 different treatments and their BW growth to 125 kg was determined. The 3 treatments were as follows: sow-reared, weaning at 14 d of age, and weaning at 2 d of age. Piglets from litters weaned at 14 d of age were fed a milk replacer after weaning until the day the sowreared litters were weaned. Piglets from litters weaned 2 d after birth were fed the milk replacer for 19 d after weaning. Pigs were individually weighed at birth at about 20 d of age, at 32 kg BW, and every 2 wk thereafter. The grow-finish BW data were fitted to a generalized Michaelis-Menten equation. The relationships of 20-d BW to birth BW were different for each treatment. Postweaning BW and days to attain 125 kg had nonlinear relationships with birth and 20-d BW. A 0.1 kg increase in birth BW reduced predicted days to reach 125 kg

1

Corresponding author: aschinck@ purdue.edu 2 Present address: Ralco Nutrition, Marshall, MN 56258. 3 Present address: Hanor Company, Inc., Franklin, KY 42134

BW by 3.48, 0.92, and 0.18 d at 1.0, 1.5, and 2.0 kg birth BW, respectively. A 0.1 kg increase in 20-d BW decreased predicted days to reach 125 kg by 0.57, 0.38, and 0.26 d at 5.0, 6.5, and 8.0 kg 20-d BW, respectively. Increasing the BW of the lightest birth BW and 20-d BW pigs had a greater impact on subsequent BW than increasing the BW of pigs with average to above average birth and 20-d BW. Key words: pig growth, milk supplement, growth equation

INTRODUCTION Variability in the growth rate of pigs is important to the economic cost and return for both the producer and processor (King, 1999; Patience et al., 2004; Patience and Beaulieu, 2006). Pork processors have the objective to market lean pork products that are uniform in weight and composition (Boland et al., 1993). The optimization of pork production systems, including the evaluation of alternative management and marketing strategies, requires knowledge of the between-pig variation in BW and carcass composition (King, 1999; Le Dividich, 1999). Mixed model nonlinear

analysis software can be used to account for the underlying variance-covariance structure of serial BW data (Craig and Schinckel, 2001; Schinckel and Craig, 2002). This has led to the development of a stochastic pig growth model (Schinckel et al., 2003), which can be used to develop optimum marketing strategies. Some of the variation in the age that is required to achieve target market BW is due to variation in birth BW and growth from birth to weaning ages of 14 to 28 d of age (Le Dividich, 1999; Wolter and Ellis, 2001; Klindt, 2003). The effects of supplemental milk and weaning age may change the relationships between birth BW and weaning BW to postweaning growth to market BW. The objectives of this research were to evaluate 1) the effects of birth and 20-d BW on growth rates during the postweaning growth period, and 2) the impact of different pig rearing strategies involving milk supplementation on subsequent pig growth.

MATERIALS AND METHODS A total of 1,034 pigs of 3 commercial genetic crosses litters (PIC327MQ sires × C22 dams, PIC327MQ sires ×

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C24 dams, and PIC337 sires × C22 dams; PIC North America, Hendersonville, TN) from 112 litters were assigned to 3 different treatments during the lactation period and their BW growth was followed to a market weight of 125 kg. The 3 treatments were as follows: sow-reared (SR; standard weaning), weaning at 14 d of age (14W), and weaning at 2 d of age (2W). The purpose of the treatments was to produce subpopulations of pigs that had significant differences in the mean 20-d BW. Litters of pigs that were assigned to the 2W and 14W treatment groups were fed an acidified, medicated milk replacer (AMMR) for 19 and 6 d, respectively, after their dam was removed from the farrowing crate. Litters remained intact and in the crate in which they were reared. The crate was not cleaned after removal of the sows. A total of 700 pigs were used to model the effect of birth BW, 20-d BW, and rearing strategy on variance and days to reach 125 kg BW. This is due to birth to 125 kg BW death loss, and because approximately 95% of each subpopulation or treatment was used. Approximately 12 litters of pigs were farrowed each week. All piglets were processed at d 1 (24 h after birth). Each pig was individually weighed and identified with button ear tags placed into both ears. A limited amount of cross-fostering of piglets from litter to litter was done on d 1. Each litter was randomly allotted to 1 of the 3 treatment groups within parity block (litter 1, 2, or 3 to 6). Each room of 12 farrowing crates was equally represented by the 3 treatments with equal distribution to sow age being achieved over 2 rooms. Litters assigned to the SR treatment group remained with their sow until the piglets were weaned at an average age of 19.5 d. Milk supplement was not provided to litters assigned to this treatment group. Sows were fed ad libitum beginning d 3 postfarrowing. Piglets from litters assigned to the 14W treatment group had AMMR made available 10 d of lactation, were

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weaned at 14 d of age by removing their mother from the crate, and then were fed an AMMR until the day when their contemporary SR litters were weaned within each room. Litters assigned to this treatment group were on AMMR for an average of 6 d postweaning. The AMMR, medicated with 55 g/metric ton of oxytetracycline and 110 g/metric ton of neomycin, was formulated to contain 2.40% lysine, 24% CP, 18% crude fat, and was fortified with vitamins and trace minerals (Advanced Birthright Nutrition, Goliath Company, Saint Louis Park, MN). The AMMR was delivered using the Supp-Le-Mate C.S. semiautomated milk system (Soppe Systems, Manchester, IA). Fresh AMMR was mixed daily with 3.75 g Cl/L added at a central reservoir (cone-style) and then was continuously circulated in a pressurized line to a series of pig-activated drinkers by using a pneumatic pump and valve panel assembly. The 2W pigs were weaned 2 d after birth by removing the sow and then after weaning were fed the AMMR for 19 d. The purpose of this treatment group was to produce pigs with an average weight ≥ 9.0 kg on the day when the contemporary SR pigs were weaned. All pigs received creep feed during the last week they were in the farrowing room. Pigs from the SR treatment group were weaned at approximately 20 d of age. Pigs from the 3 treatment groups were individually weighed and were then moved to the nursery building. Pigs were allotted to pens by treatment, gender, and size. The pigs remained in the nursery facility for approximately 49 d and then were moved into the finish building. Pigs were phase-fed 4 nursery diets based on a feed budget irrespective of BW. The finish building consisted of 6 rooms of 8 pens each. Fifteen barrows or 15 gilts were allotted to each pen. Pigs from all 3 treatment groups were allotted to each pen. Differences in BW within a pen and between the lightest and heaviest pigs did not ex-

ceed 6.8 kg at placement. Temperatures in the finish building were maintained within a range of 19.4 to 21.7°C. Individual feed intake recording equipment (FIRE) feeders (Osborne Industries, Inc., Osborne, KS) were used to record feed intake for each pig. Each room had 4 FIRE feeders with 2 pens alternately sharing a FIRE feeder or conventional feeder. A swinging gate between the 2 pens controlled access to the FIRE feeder. The position of the gate was changed each Monday morning so the pigs in the adjacent pen could have access to the FIRE feeder that week. Each pig was individually weighed and ear-tagged with a transponder for feed intake evaluation with FIRE feeders. The target on-test BW was 32 kg. The pigs were fed using a 4-phase diet program (22 to 41 kg, 41 to 73 kg, 73 to 95 kg, and 95 kg to market) based on corn-soybean meal and previously validated to meet the amino acid needs of at least 95% of the population (PIC, 1999). Each pig was weighed every 2-wk period until the pigs reached 122.5 kg. The BW data taken when the pigs entered the nursery were adjusted for age using the PROC GLM procedure of SAS (SAS, 1999). The initial model included the effects of genetic cross, sex, weaning treatment, and age as a covariate, and 2-way interactions between the fixed effects and interactions of each fixed effect with age. Variables that had probability values greater than 0.05 were deleted from the model. The final model, which included the effect of weaning treatment, age, and weaning treatment by age, was used to adjust the nursery entry BW to 20 d of age. Body weight adjusted to 20 d of age (20-d BW) was fitted to a model including the main effects of weaning treatment, sex, and genetic cross, their 2-way and 3-way interactions and birth BW, (birth BW)2, and (birth BW)3 as covariates. The postweaning BW data were fitted to a generalized Michaelis-Menten (GMM) equation (Lopez et al., 2000).

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The equation has 2 alternative forms: WTi,t = (WT0 × KC) + (WF × tC)/(KC + tC) or WTi,t = WT0 + [(WF − WT0) × (t/K)C)]/[1 + (t/K)C)], where WT0 is the mean birth BW, i is for the ith pig, t is days of age, K is a parameter equal to the days of age in which one-half WF is achieved, and C is a unitless parameter related to changes in proportional growth and shape of the growth curves, and WF is mean mature BW (Lopez et al., 2000). In this function, each pig’s actual birth BW (WTi,0) was used. This function has an inflection point age (IP, in days) = K × [(C − 1)/(C + 1)](1/C) and the BW at the IP = {[1 + (1/C)] × WT0 + [1 − (1/C)] × WF}/2. In this function WF, K, and C could be considered as random effects. The inclusion of a single random effect for WF in this function produces a series of growth curves in which each individual pig is either an approximate percent (wfi/WF) heavier or lighter at each day of age and the growth curves of the pigs have either identical or nearly identical shape. The inclusion of a second random effect accounts for different patterns of growth among pigs (Schinckel and Craig, 2002).

Estimation The BW data for each sex of pig were fitted to the fixed effects GMM functions using the nonlinear mixed (NLMIXED) procedure of SAS. The 3 alternative single random effects models were evaluated based on the Akaike’s Information Criteria (AIC). Then additional random effects were added in a step-wise order based on AIC values. The significance of the additional parameter was evaluated by comparing the AIC values. The R2 values were calculated as squared correlations between the predicted and actual observations. The relative standard deviation (RSD) was calculated with the equation RSD = ⎡T I ⎢ ⎣ t=1 t=1

⎤

∑ ∑ (ei,t)2/(n − p)⎥⎦1/2 where ei,t is the

residual value of the ith pig at age t, n is the number of observations, and

p is the number of parameters in the model. The NLMIXED procedure provided predicted values for the random effect of each pig, variance estimates for each random effect, and the residual variance. Approximate standard errors of the function parameters, variance estimates, and covariance estimates are based on the second derivative matrix of the likelihood function. These approximate standard errors are based upon large sample inferences (Lindsey, 1996; Neter et al., 1996). Bootstrap estimates of precision and confidence intervals are better in these cases but require extensive computer calculations (Efron, 1982; Neter et al., 1996). The BW and ADG of each pig were predicted for the GMM function at 70, 84, 98, 126, 140, 154, and 168 d of age. Also, ADG at 35, 50, 65, 80, 95, 110, and 125 kg BW was predicted for each pig. The ages to achieve 105 and 125 kg BW were also predicted for each pig. The betweenpig variations in these target BW are important parameters when evaluating alternative marketing strategies. The biweekly BW from 70 to 168 d of age, the ADG at each biweekly age, and days to achieve 105 and 125 kg BW were analyzed via the PROC mixed procedure of SAS. The model for each set of BW, ADG, or ages to achieve 105 or 125 kg BW included the effect of age, sex, weaning treatment, genetic cross, and respective interactions. The optimal variance-covariance structure was evaluated for each analysis based on AIC values. The BW at 20, 70, 126, and 168 d of age and days to achieve 105 and 125 kg were fitted to models including line cross, sex, weaning treatment and either birth BW (birth BW)2 and (birth BW)3 or 20-d BW, (20-d BW)2 and (20-d BW)3. The interactions of the covariates with line cross, sex, and weaning treatment were evaluated. If the interactions of both the linear and quadratic effects of birth or 20-d BW with the fixed effect were significant (P < 0.05), then separate equations were developed for each level of the fixed effect.

RESULTS AND DISCUSSION Weaning Management Barrows had greater birth BW than gilts (Table 1). The 2W and 14W pigs were 2.58 and 1.26 kg heavier (P < 0.001) than the SR pigs at the time of movement into the nursery. When adjusted to 20 d of age, the 2W and 14W pigs were 2.09 and 1.10 kg heavier, respectively, (P < 0.001) than the SR pigs. Rearing treatments were successful in establishing 3 subpopulations, differing significantly in average weight, in order to study postweaning growth rates.

Modeling the Relationship of 20-d BW to Birth BW The relationship of 20-d BW to birth BW was evaluated separately for each weaning treatment by regression analyses (Table 2, Figure 1). An overall analysis resulted in significant (P < 0.001) weaning treatment by birth BW and weaning treatment by (birth BW)2 interactions. Thus, separate regression equations were developed for pigs of each weaning treatment. The prediction equation for the 2W pigs was different than for 14W and SR pigs. The regression coefficients for the (birth BW)2 and (birth BW)3 variables had different signs (negative vs. positive) for the 2W pigs than for the SR and 14W pigs. The lightest 2W pigs, with birth BW from 0.90 to 1.1 kg, had 20-d BW intermediate to SR and 14W pigs with the same birth BW. The 2W pigs with birth BW from 1.25 to 2.25 kg had a consistent 0.90 to 0.95 kg advantage over 14W pigs of the same birth BW. As birth BW increased from 2.25 to 2.70 kg in the SR pigs, the maximum 20-d BW achieved was 7.4 to 7.5 kg. The 20-d BW of the 2W and 14W pigs also continued to increase as birth BW increased but to a greater extent. Pigs of the 2W treatment with 1.25 and 2.25 kg birth BW had 1.83 and 2.60 kg (32.5 and 35.5%) greater 20-d BW than SR pigs at the same birth BW. Pigs of the 14W treatment with birth BW of 1.25 and 2.25 kg were 0.93

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Table 1. Birth weight and BW means and SD adjusted to 20 d of age Standard weaning

Number of pigs Barrows Gilts Birth BW, kg Barrows Gilts Age at first BW Barrows Gilts First BW, kg Barrows Gilts First BW, adjusted to 20 d, kg Barrows Gilts 1

2-d weaning

14-d weaning

P-value

Mean

SD

Mean

SD

Mean

SD

SE

TRT1

Sex

111 134

— —

107 121

— —

120 107

— —

— —

— —

— —

0.34 0.30

0.03 —

0.015 —

0.007 —

1.4 1.2

0.10 —

0.001 —

0.41 —

1.62 1.50 19.4 19.8

0.38 0.38

1.67 1.62

2.0 1.7

0.34 0.32

21.3 21.1

1.4 1.5

1.59 1.55 20.5 20.6

6.35 6.12

1.4 1.3

8.84 8.79

1.6 1.6

7.54 7.45

1.4 1.1

0.13 —

0.001 —

0.24 —

6.46 6.15

1.3 1.2

8.40 8.39

1.6 1.6

7.46 7.35

1.3 1.1

0.12 —

0.001 —

0.16 —

Weaning age treatment (TRT) by sex interactions were not significant (P > 0.30).

and 1.78 kg (16.1 and 24.3%) heavier than SR pigs of the same birth BW. The 2W treatment resulted in a more similar percentage increase in 20-d BW relative to the SR treatment at different birth BW than the 14W treatment.

Growth Modeling The parameter estimates of the GMM function are shown on Table 3. The WF and C parameters were the 2 parameters considered as random effects. The C parameter primarily determines the percentage of mature BW (WF) in which maximal

ADG is achieved (Lopez et al., 2000; Schinckel et al., 2005, 2006). The RSD were similar for 5 of the 6 sex-weaning treatment groups of pigs. The 2W barrows had a greater RSD (2.50 kg) than the SR pigs and 14W barrows (RSD = 2.02 to 2.10 kg). The RSD of the 14W gilts was intermediate (RSD = 2.21 kg). The predicted mean ADG for each weaning treatment-sex group of pigs at each day of age is shown in Figure 2. Analysis of the biweekly individual pig predicted ADG indicated that barrows had a greater overall ADG than gilts (946 vs 896 g/d, P < 0.001) with no differences between treatments

(923, 914, and 925 g/d for the 14W, 2W, and SR pigs, respectively, P = 0.34). However, weaning treatment by age, sex by age, and weaning treatment by sex by age interactions were significant (P < 0.001). The SR gilts had a lower ADG than the 2W or 14W gilts at 70, 84, and 98 d of age. After 126 d of age, the SR gilts had a greater ADG than the 14W and 2W gilts. The 14W barrows had a greater ADG than the 2W and SR barrows at 70 and 84 d of age (P < 0.001). By 98 and 112 d of age, the ADG of SR and 14W barrows was greater than the 2W barrows (P < 0.01). At 140, 154, and 168 d of age, the ADG of the SR

Table 2. Regression of 20-d BW on birth weight1 b1 Treatment Standard weaning 2-d weaning 14-d weaning

b2

b3

b0

Estimate

SE

P value

Estimate

SE

P-value

Estimate

SE

P-value

R2

RSD2

3.17 −13.48 8.20

1.68 32.3 −4.98

0.8 9.5 1.8

0.03 0.001 0.01

0.64 −16.1 3.84

0.34 5.5 1.6

0.07 0.004 0.01

−0.252 2.83 −0.644

0.11 1.0 0.30

0.03 0.006 0.04

0.290 0.422 0.307

1.07 1.20 1.04

Adjusted 20-d BW = b0 + b1 × (birth BW) + b2 × (birth BW)2 + b3 × (birth BW)3 where b0, b1, b2, and b3 are the regression coefficients. Significant weaning treatment by birth BW (P < 0.001) and weaning treatment by (birth BW)2 interactions (P < 0.01). 2 RSD = relative standard deviation. 1

Impact of Birth and Body Weight at Twenty Days

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124.7 d, P < 0.001) than barrows. Overall, pigs of the 2W and 14W weaning treatments were younger and lighter than the SR pigs (91.3, 91.8, and 94.4 kg, BW); 129.6, 131.0, and 136.0 d, respectively, (P < 0.001) at their maximum predicted ADG. The difference between the barrows’ and gilts’ age and BW at maximum ADG was least for 2W pigs, intermediate for the SR pigs, and greatest for the 14W pigs. Maximum ADG was numerically greater for SR progeny (both sexes) than either 2W or 14W pigs. There appeared to be a positive relationship between time nursed by the sow and maximum ADG achieved. This suggests that the sow influences lifetime ADG.

Figure 1. Relationship of 20-d BW on birth weight. Weaning treatments (trt): 14W = weaning at 14 d of age, 2W = weaning at 2 d of age, and SR = sow-reared (standard weaning).

barrows exceeded that of the 14W and 2W barrows. These observations are important in that they suggest that length of time spent nursing by the sow is important to realized potential for ADG. Further, increasing 20-d BW thru milk supplement does not override the effect of the sow (Cabrera et al., 2002). The predicted mean ADG for each weaning treatment by sex groups of pigs at each BW is shown in Figure 3. The analysis of each pig’s predicted ADG from 35 to 125 kg BW at 15 kg BW intervals indicated significant weaning treatment by BW, sex by BW, and sex by treatment by BW interactions (P < 0.001). Gilts of the 3 weaning treatments had similar ADG at 35, 50, and 65 kg BW. The effect of weaning treatment was significant (P < 0.05) for the gilts after 85 kg BW. After 100 kg BW, the SR gilts had greater ADG than the 14W gilts, which in turn had greater ADG than the 2W gilts. The 14W and SR barrows had greater ADG than the 2W barrows at 50, 65, and 80 kg BW. After 95 kg BW, the SR barrows had

greater (P < 0.01) ADG than the 14W and 2W barrows. The least-squares means for predicted BW at biweekly ages and predicted ages to achieve 105, 115, and 125 kg BW are shown in Table 4. At 70 d of age, the SR barrows were 2.6 kg lighter than the mean of the 2W and 14W barrows. The SR gilts were 3.85 kg lighter than the mean of the 2W and 14W gilts at 70 d of age. The difference in BW between the SR and other barrows slowly decreased to 0.90 kg at 168 d of age. The difference in BW between the SR and other gilts essentially remained constant as age increased. At 168 d of age, the SR gilts were 2.6 kg lighter than the 2W and 14W gilts. The means and standard deviations for the predicted maximum ADG and age at maximum ADG (inflection point) and BW at maximum ADG are presented in Table 5. Overall, the gilts’ maximum ADG was less than the barrows’ (0.956 vs. 1.006 kg/d, P < 0.001) and their maximum ADG was achieved at greater BW (97.3 vs. 87.8 kg, P < 0.001) and age (139.7 vs.

Modeling the Relationship of Subsequent Growth to Early BW The regression equations for describing the relationship of days to attain 105 and 125 kg BW on birth BW are shown in Table 6 and Figure 4. The interactions of birth BW with weaning treatment, genetic population, and sex were not significant (P > 0.40). This indicates that the relationships of days to attain 105 and 125 kg with birth BW were similar across weaning treatments, sexes, and genetic crosses. The derivatives of the regression equations predict the change in the dependent variable per unit change in the independent variable. A 0.1 kg increase in birth BW was predicted to reduce days to 105 kg BW by 2.86, 0.89, and 0.26 d at 1.0, 1.5, and 2.0 kg birth BW. Similarly, a 0.1 kg increase in BW was predicted to reduce days to 125 kg BW by 3.48, 0.92, and 0.18 d at 1.0, 1.5, and 2.0 kg birth BW. Increasing the birth BW of the lightest pigs had a much greater impact than increasing the birth BW of pigs with above average birth BW. The significance of the main effects of weaning treatment and sex indicate that at the same birth BW, pigs of different sexes or weaning treatments had different subsequent BW (Tables 7 and 8). Barrows required 5.7 fewer days to achieve 105 kg BW

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Table 3. Parameter estimates and approximate SE for generalized Michaelis-Menten equation1 Barrows

Standard weaning WF K C Var (e) Var (wfi) Var (ci) Cov (wfi, ci) 2-d weaning WF K C Var (e) Var (wfi) Var (ci) Cov (wfi, ci) 14-d weaning WF K C Var (e) Var (wfi) Var (ci) Cov (wfi, ci)

Gilts

Estimate

SE

RSD,2 kg

R2

Estimate

SE

RSD, kg

R2

361.3 227.3 1.964 4.92 936 0.0155 0.965

15.4 8.2 0.027 0.27 171 0.0025 0.46

2.08 — — — — — —

0.9961 — — — — — —

425.7 277.7 1.861 4.87 1981 0.018 2.28

21.3 11.7 0.024 0.24 387 0.0026 0.74

2.07 — — — — — —

0.9960 — — — — — —

389.3 246.3 1.838 7.05 1191 0.0132 1.64

22.6 12.7 0.031 0.39 269 0.0022 0.57

2.50 — — — — — —

0.9942 — — — — — —

426.0 279.2 1.762 4.41 1460 0.017 2.16

22.6 13.2 0.024 0.23 314 0.0025 0.66

2.02 — — — — — —

0.9962 — — — — — —

343.9 218.3 1.939 4.98 789 0.0164 0.387

13.0 7.2 0.026 0.26 131 0.003 0.40

2.10 — — — — — —

0.9960 — — — — — —

457.0 295.6 1.762 5.46 2022 0.0101 1.77

30.5 17.3 0.027 0.30 481 0.0016 0.65

2.21 — — — — — —

0.9954 — — — — — —

The equation has the form WTi,i = (WT0 × KC) + (WF × tC)/(KC + tC), where WTi,t is the BW of the ith pig at t days of age, WT0 is birth BW, WF is mature BW, and C and K are growth parameters. The Var (e) is the predicted residual variance, var (wfi) is the variance of the random effects for WF, and var (ci) is the variance of the random effects for C. The cov (wfi, ci) is the covariance between the 2 random effects. 2 RSD = relative standard deviation. 1

than gilts (P < 0.001) at the same birth BW. The 14W and 2W pigs required 2.7 and 1.9 fewer (P = 0.07) days to achieve 105 kg BW than SR pigs at the same birth BW. Barrows required 6.3 fewer (P < 0.001) days to achieve 125 kg BW at the same birth BW as gilts. The 14W and 2W pigs required 2.5 and 1.4 fewer days (P = 0.05) to achieve 125 kg BW than the SR pigs. Significant birth BW (linear) by treatment sex interactions (P = 0.015) were found for 70-d BW. The SR pigs were lighter at 70 d of age than the 2W and 14W pigs (Figure 5). As birth BW increased, the 14W and 2W pigs had similar 70-d BW. The advantage of the 2W and 14W pigs over the SR pigs increased as birth BW increased. The regression equation predicted

that the 70-d BW of the SR pigs only marginally increased as birth BW increased from 1.75 to 2.50 kg. The advantage of increasing birth BW from 1.25 to 2.25 kg BW on 70-d BW was similar for pigs of the 2W and 14W weaning treatments. Adjusted for birth weight, SR barrows were 1.54 kg heavier than the SR gilts at 70 d of age. The 2W and 14W barrows were only 0.34 and 0.54 kg heavier than the 2W and 14W gilts. The predicted relationships of 70-d BW to birth BW (Figure 5) were similar to the relationships of 20-d BW to birth BW (Figure 1). The regressions of 126 and 168 d of BW on birth BW did not interact with weaning treatment or sex (P > 0.40; Table 6, Figure 6). A 0.1 increase kg in birth BW was predicted to in-

crease 126-d BW by 2.10, 0.83, and 0.39 kg, and 168-d BW by 2.86, 0.86, and 0.26 kg at 1.0, 1.5, and 2.0 kg birth BW, respectively. As age increased, the R2 of the prediction equations decreased slightly and the RSD substantially increased. At the same birth BW, barrows were 4.5 kg heavier (P < 0.001) than gilts at 126 d of age (Table 8). The 14W and 2W pigs were 2.8 and 2.2 kg heavier than the SR pigs at 126 d of age (P < 0.001). At 168 d of age, barrows were predicted to be 5.8 kg heavier than gilts (P < 0.001). Pigs of the 14W and 2W treatments were 2.1 and 1.0 kg heavier, respectively, than the SR pigs (P = 0.05) at 168 d of age. The regressions of days to 105 and 125 kg BW on 20-d BW are shown in Table 7 and Figures 7 to 11. Prelimi-

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nary equations including only 20-d BW and (20-d BW)2 had significant (P < 0.05) 20-d BW and (20-d BW)2 by weaning treatment interactions. When (20-d BW) was included in the model, the interactions of 20-d BW, (20-d BW)2, and (20-d BW)3 with weaning treatment, sex, and genetic cross were not significant (P > 0.30). The regression equations predicted that a 0.1 kg increase in 20-d BW will decrease days to achieve 105 kg by 0.54, 0.36, and 0.25 d and decrease days to achieve 125 kg by 0.57, 0.38, and 0.26 d at 5.0, 6.5, and 8.0 kg 20d BW, respectively. At the same 20-d BW, barrows required 5.8 less days to achieve 105 kg BW (P < 0.001; Table 8). Pigs of the 14W and 2W treatments required 0.7 and 3.5 additional days to achieve 105 kg BW than SR pigs. Barrows required 6.6 less days to achieve 125 kg BW than gilts with the same 20-d BW. Pigs from 14W and 2W weaning treatments required 1.1 and 4.2 additional days to achieve 125 kg than SR pigs of the same 20-d BW.

Figure 2. The mean ADG (kg/d) at each day of age for barrows and gilts of each weaning treatment as predicted by the Michealis-Menten equation. Weaning treatments (trt): 14W = weaning at 14 d of age (B = barrows; G = gilts), 2W = weaning at 2 d of age, and SR = sow-reared (standard weaning).

Table 4. Least squares means for predicted BW and days to target market BW1 Standard weaning

2-d weaning

14-d weaning

P-value

Variable

Sex

Mean

SD

Mean

SD

Mean

SD

SE

TRT

Sex

TRT × sex

70-d BW, kg

Barrows Gilts Barrows Gilts Barrow Gilts Barrows Gilts Barrows Gilts Barrows Gilts Barrows Gilts Barrows Gilts Barrows Gilts Barrows Gilts Barrows Gilts

34.0 31.8 46.1 42.8 59.3 54.6 73.1 67.2 87.2 80.2 101.4 93.4 115.4 106.7 129.1 120.0 144.3 153.3 154.5 164.0 164.8 175.0

4.4 4.8 5.2 5.8 6.1 6.7 6.9 7.6 7.7 8.5 8.6 9.5 9.5 10.6 10.5 11.6 9.5 12.6 10.6 14.1 11.9 15.7

36.6 35.0 48.7 47.5 61.7 59.7 75.1 72.4 88.8 85.4 102.6 98.5 116.1 111.6 129.3 124.5 143.2 147.5 153.7 158.4 164.3 169.4

4.3 5.0 5.1 5.8 5.9 6.4 6.6 7.0 7.4 7.7 8.3 8.3 9.3 9.0 10.4 9.9 8.9 9.6 9.9 10.4 11.0 11.4

36.0 35.3 48.4 46.7 61.8 58.9 75.6 71.7 89.7 84.8 103.7 98.1 117.4 111.5 130.7 124.7 141.9 148.1 152.3 158.7 162.9 169.5

4.8 4.3 5.7 5.2 6.5 6.1 7.3 7.0 8.1 7.9 8.8 8.9 9.7 10.0 10.5 11.1 9.7 10.8 10.6 11.8 11.7 13.1

0.42 — 0.52 — 0.61 — 0.68 — 0.75 — 0.84 — 0.93 — 1.0 — 1.0 — 1.1 — 1.2 —

0.001 — 0.001 — 0.001 — 0.001 — 0.001 — 0.001 — 0.002 — 0.01 — 0.001 — 0.002 — 0.009 —

0.002 — 0.001 — 0.001 — 0.001 — 0.001 — 0.001 — 0.001 — 0.001 — 0.001 — 0.001 — 0.001 —

0.13

84-d BW, kg 98-d BW, kg 112-d BW, kg 126-d BW, kg 140-d BW, kg 154-d BW, kg 168-d BW, kg Days to 105 kg Days to 115 kg Days to 125 kg

1

Model included effects of weaning treatment (TRT), genetic cross, sex, and interactions.

0.10 0.12 0.05 0.05 0.05 0.06 0.09 0.05 0.07 0.09

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Table 5. Means and SD for age at maximum ADG, BW at maximum ADG, and maximum ADG predicted from the mixed model generalized Michealis-Menten function Standard weaning

2-d weaning

14-d weaning

SD

Mean

SD

Mean

127.6 144.5

9.6 14.4

126.0 133.2

10.9 15.8

89.6 99.2

9.8 14.6

89.7 92.9

11.0 13.5

Mean Age at maximum ADG, kg Barrows Gilts BW at maximum ADG, kg Barrows Gilts Maximum ADG, g/d Barrows Gilts 1

1,020 966

92 107

990 945

92 89

P-value

SD

SE

TRT1

Sex

TRT × Sex

120.5 141.4

9.8 12.8

1.2 —

0.001 —

0.001 —

0.001 —

84.1 99.6

8.8 13.2

1.1 —

0.013 —

0.001 —

0.001 —

84 94

8.5 —

0.013 —

0.001 —

0.83 —

1,000 956

TRT = weaning age treatment.

The regressions of 70-d BW on 20d BW predicted a 0.1 kg increase in 20-d BW will increase 70-d BW 0.29, 0.21, and 0.17 kg at 5.0, 6.5, and 8.0 kg 20-d BW, respectively (Table 7). These results agree with the regression equations of Schinckel et al. (2004), which predicted that a 0.1 kg increase in 21-d BW at 5.0, 6.5, and 8.0 kg will increase 62-d BW by 0.31, 0.24, and 0.18 kg, respectively. Thus any increase in 20-d BW would be ex-

pected to almost triple in lightweight pigs and double in average weight pigs by 70 d of age. When 70-d BW was adjusted for 20-d BW, 2W gilts were lighter than 14W and SR gilts (34.2, 35.3, and 35.2, respectively, P < 0.05). Finally adjusted for 20-d BW, barrows had greater 70-d BW than gilts (35.3 vs. 34.5 kg, P = 0.002). The regressions of 126 and 168 d BW on 20-d BW predicted that increasing 20-d BW at 5.0 kg BW will

increase 126 and 168 d BW to a greater extent than increasing 20-d BW at 6.0 kg or greater (Table 7). The equations predicted that a 0.1 kg increase in 20-d BW will increase 126 d BW by 0.47, 0.32, and 0.24 kg at 5.0, 6.5, and 8.0 kg 20-d BW, respectively. The equations predicted a 0.10 kg increase on 20-d BW will increase 168 d BW 0.51, 0.33, and 0.22 kg at 5.0, 6.5, and 8.0 kg 20-d BW, respectively. Thus, increasing 20-d BW of pigs

Table 6. Regression of days to achieve 105 kg, days to achieve 125 kg, 70-d BW, 126-d BW, and 168-d BW on birth weight1 b1 Variable Days to 105 kg Days to 125 kg 70-d BW,4 kg Treatment 14-d weaning 2-d weaning Standard weaning 126-d BW, kg 168-d BW, kg

b0

Estimate SE

220.5 −108 263.8 −141

34.9 −8.5 −5.6 34.4 50.0

−13.9 62.8 61.3 71.1 115.0

b2 P<

Estimate SE

26 32

0.001 0.001

53.1 71.4

7.0 32 19 19 27

0.05 0.05 0.001 0.001 0.001

13.7 −29.6 −31.1 −33.3 −58.4

P-value2

b3 P<

Estimate SE

P<

TRT

Sex Cross

R2

RSD3

15 19

0.001 −8.92 3.0 0.003 0.007 0.001 0.001 0.250 9.5 0.001 −12.2 3.7 0.001 0.05 0.001 0.001 0.212 11.8

5.2 14.0 11.0 12.0 16.0

0.01 0.03 0.005 0.001 0.001

−2.93 4.97 5.24 5.50 10.1

1.1 2.3 2.1 2.2 3.1

0.01 0.001 0.34 0.001 0.280 0.02 0.53 0.03 0.270 0.01 0.007 0.38 0.188 0.01 0.001 0.001 0.001 0.286 0.001 0.05 0.001 0.001 0.229

4.0 4.1 4.3 7.2 9.9

Equation has the form Y = b0 + b1 × (birth BW, kg) + b2 × (birth BW)2 + b3 × (birth BW)3 where b0, b1, b2, and b3 are the regression coefficients. 2 Significance of main effects of weaning treatment, sex, and cross in the overall analyses. 3 RSD = relative standard deviation. 4 Significant birth BW by weaning treatment interaction (P = 0.015), which indicated that separate equations should be developed for each treatment. 1

205

Impact of Birth and Body Weight at Twenty Days

Figure 4. The predicted days of age to achieve 105 (xd105) or 125 (xd125) kg BW relative to birth weight based on regression analyses.

with below average 20-d BW will increase finisher BW to a greater extent than increasing the 20-d BW of above average pigs. The least-squares means adjusted for 20-d BW need careful interpretation. These means are indicative of the expected subsequent growth of pigs at similar 20-d BW. These means are not indicative of the true treatment differences as the treatments greatly impacted 20-d BW. The use of

a covariate for 20-d BW adjusts the subsequent data to the overall mean 20-d BW of 6.82 kg. Pigs with 6.82 kg 20-d BW represent different populations of pigs within each treatment with respect to birth BW, percentile rank, and growth potential (Foxcroft et al., 2006). Pigs with 20-d BW of 6.82 kg are pigs with BW 0.41 SD greater than the mean for SR pigs, 0.49 SD less than the mean for the 14W pigs, and 1.0 SD less than the

mean for the 2W pigs. These means should be interpreted that close to the overall mean 20-d BW of 6.82 kg, pigs of the 2W treatment did not grow as well as pigs of the 14W and SR treatments. Some pork production contracts discount pigs below a specific weaning BW, typically 3.6 or 4.1 kg BW. The growth potential and subsequent value of pigs with similar weaning BW may not be comparable across the 3 treatments evaluated in this trial. Alternative discount systems may need to be developed for pigs of each weaning treatment. At 126 d of age, barrows were 4.6 kg heavier (P < 0.001) than gilts with the same 20-d BW (Table 8). Pigs of the 14W and 2W treatments were 0.4 and 2.9 kg lighter than SR pigs with the same 20-d BW. Barrows were 6.0 kg heavier (P < 0.001) than gilts at the same 20-d BW at 168 d of age. Pigs of the 14W and 2W weaning treatments were 1.3 and 4.3 kg lighter at 168 d of age than SR pigs with the same 20-d BW. This result continues to suggest that postnatal growth rate is influenced by some aspect of being reared by the sow (e.g., immunological acclimation to pathogens). One issue with this data analysis is the impact of survival on subsequent growth rate was not taken into account. The BW data used were from

Table 7. Regression of days to achieve 105 kg, days to achieve 125 kg, 70-d BW, 126-d BW, and 168-d BW on 20-d BW1 b1 Treatment

b0

Days to 105 kg 203.8 Days to 125 kg 229.5 70-d BW, kg 126-d BW, kg 168-d BW, kg

5.8 36.5 70.9

Estimate SE −16.1 −17.6 7.41 13.5 15.7

b2 P<

Estimate

SE

P-value2

b3 P<

Estimate

5.3 0.003 6.7 0.009

1.45 1.61

0.70 0.04 0.68 0.006

−0.050 −0.056

2.1 0.001 3.8 0.005 5.6 0.005

(0.62 −1.20 −1.43

0.28 0.001 0.51 0.02 0.74 0.08

0.022 0.042 0.049

SE

P<

TRT

Sex

Cross

R2

0.024 0.04 0.001 0.001 0.001 0.302 9.2 0.028 0.05 0.001 0.001 0.001 0.234 11.6 0.011 0.05 0.01 0.002 0.001 0.444 0.021 0.05 0.001 0.001 0.001 0.391 0.025 0.05 0.001 0.001 0.006 0.271

Equation has the form Y = b0 + b1 × (20-d BW) + b2 × (20-d BW)2 + b3 × (20-d BW)3 where b0, b1, b2, and b3 are the regression coefficients. 2 Significance of the main effects of weaning treatment (TRT), sex, and cross. 3 RSD = relative standard deviation. 1

RSD3

3.7 6.7 9.7

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Table 8. Least squares means for predicted days to achieve 105 kg or 125 kg BW and 126-d and 168-d BW, adjusted for either birth weight or 20-d BW Weaning treatment1 Variable Days to 105 kg Non-adjusted Adjusted for birth BW Adjusted for 20-d BW Days to 125 kg Non-adjusted Adjusted for birth BW Adjusted for 20-d BW 126-d BW, kg Non-adjusted Adjusted for birth BW Adjusted for 20-d BW 168-d BW, kg Non-adjusted Adjusted for birth BW Adjusted for 20-d BW

Sex

14W

2W

SR

SE

P<

Barrows

Gilts

SE

P<

145.0 145.1 145.6

145.3 145.9 148.4

148.8 147.8 144.9

0.70 0.66 0.67

0.001 0.007 0.001

143.1 143.4 143.4

149.6 149.1 149.2

0.57 0.54 0.50

0.001 0.001 0.001

166.2 166.3 166.9

166.9 167.5 170.0

169.9 168.9 165.8

0.85 0.80 0.83

0.01 0.06 0.004

164.0 164.4 164.3

171.3 170.7 170.9

0.70 0.64 0.64

0.001 0.001 0.001

87.3 87.2 86.7

87.1 86.6 84.2

83.7 84.4 87.1

0.55 0.53 0.49

0.001 0.001 0.001

88.6 88.3 88.3

83.5 83.8 83.7

0.44 0.42 0.37

0.001 0.001 0.001

127.7 127.6 127.0

126.9 126.4 124.0

124.5 125.4 128.3

0.74 0.72 0.70

0.01 0.05 0.001

129.7 129.4 129.4

123.0 123.6 123.4

0.60 0.58 0.53

0.001 0.001 0.001

1

Weaning treatments: 14W = weaning at 14 d of age, 2W = weaning at 2 d of age, and SR = sow-reared (standard weaning).

the pigs which survived from birth to end of the postweaning test period.

Past unpublished data indicates that the 2W treatment increased the sur-

Figure 5. The mean predicted 70-d BW relative to birth weight. Significant linear birth weight by weaning treatment interaction (P = 0.015). Weaning treatments (trt): 14W = weaning at 14 d of age, 2W = weaning at 2 d of age, and SR = sow-reared (standard weaning).

vival of the pigs with below average birth BW. Pigs with below average birth BW, especially those with birth BW below 1.1 kg, have lower survival rates to weaning and market (King, 1999; Le Dividich, 1999) and reduced growth potential (Foxcroft et al., 2006). In subsequent trials, the impact of the weaning management treatments to increase the survival rates of pigs with less than average birth BW should be taken into account. Increasing the survival of pigs with birth BW less than 1.1 kg will reduce the mean growth of the surviving pigs and increase the SD of the subsequent BW data. The relationships between preweaning survival, impact of weaning management to increase survival, and subsequent growth rates to birth BW are all likely nonlinear. Thus, future analyses may require larger datasets that take into account the impact of increasing survival rates of the light birth BW pigs. Past research trials have found that birth BW has an impact on subsequent postweaning BW. Wolter and Ellis (2001) found heavy birth BW

Impact of Birth and Body Weight at Twenty Days

Figure 6. The mean predicted 126-d (xpwt126) and 168-d (xpwt168) BW relative to birth weight.

pigs (mean = 1.8 kg) grew faster and required 7 fewer d to reach 110 kg BW than light birth BW pigs (mean = 1.3 kg). Schinckel et al. (2004) found that 35, 49, and 62-d BW had linearquadratic relationships with birth

BW. Klindt et al. (2003) found that 170-d BW had a linear-quadratic relationship with birth BW: (170-d BW, kg) = 54.8 + 59.25 (birth BW, kg) − 15.16 (birth BW)2. The results of the present trial indicate that increasing

Figure 7. Predicted days to achieve 105 kg relative 20-d BW based on regression analyses.

207

the birth BW of pigs with lesser than average birth BW had a much greater ability to increase subsequent BW and to decrease the age to achieve market BW than increasing the birth BW of pigs with greater than average birth BW. This illustrates that the effect of increasing litter size with declining average birth weight on 168-d BW can be partially offset by the use of milk replacers, especially for the smallest 25 to 30% of the pigs in the litter. If the proportion of small pigs increased, then the effect of birth BW on 168-d BW and days to 125 kg would increase. The present trial found that 20-d BW accounted for more of the variation in subsequent BW than birth BW. Klindt (2003), found that 17-d BW accounted for 19% of the variation in 170-d BW in comparison to 11% accounted for by birth BW. To reduce variation in postweaning growth, the preweaning growth of the pigs with below average birth BW must be increased. Other researchers have found that pigs with below average BW at 17 to 23 d of age required several additional days to achieve target market BW (Mahan and Lepine, 1991; Mahan, 1993; Cabrera et al., 2002; Wolter and Ellis, 2001), which may not be possible in a fixed time system. Mahan and Lepine (1991) sorted pigs at weaning into light, average, and heavy BW groups (4.7, 6.2, and 7.5 kg, respectively.) The 3 groups averaged 180.8, 172.8, and 165.7 d to 105 kg BW. Mahan (1993) sorted 23d old at weaning into 2 groups that averaged 4.83 and 7.97 kg BW and required 179.9 and 169.4 d to achieve 105 kg. Wolter and Ellis (2001) sorted pigs weaned from 18 to 22 d of age into heavy and light groups (5.4 vs 3.9 kg) that later required 161.6 and 170.2 d to achieve 110 kg BW. Pigs with birth BW under 1.0 kg required substantially greater days to achieve target market BW. Regression equations predict that pigs with birth BW of 1.0 kg require 11.4 additional days to achieve 105 kg BW and 12.6 additional days to achieve 125 kg

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Figure 8. The predicted days to achieve 125 kg relative to 20-d BW based on regression analyses.

than pigs with 2.0 kg birth BW. In a fixed time system (or for most profitable barn use), the alternative, less kilograms of market weight in 168 d of age illustrates the harshness of penalty. Birth BW had a nonlinear relationship such that increasing the

birth BW of the lightest 20% of the pigs could substantially increase subsequent BW and potentially reduce variation in BW. The lightest birth BW pigs may not be able to adequately compete with the other pigs for milk prior to weaning (Lepine et al., 1989;

Figure 9. The predicted 70-d BW for pigs relative to BW.

Le Dividich, 1999). Schinckel et al. (2004) found that 20% of the pigs with the lowest birth BW had substantially less ADG than other pigs at either similar age or similar BW. The use of milk supplement in the 2W and 14W treatments significantly increased 20-d BW. The 20-d BW reached a maximum of 7.5 kg achieved by even the heaviest birth BW pigs of the SR treatment. These results confirm the fact that sows do not produce enough milk to maximize pig growth from birth to 20 d of age, except under conditions of extraordinary stimulation (Harrell et al., 1993; Boyd et al., 1995). Postweaning growth relative to 20d BW differed for pigs of the 3 treatments. Based on the overall prediction equation, 2W and 14W pigs should have reached 125 kg in 6.42 and 3.81 less days, respectively, than SR pigs. The 14W pigs grew close to expectations, achieving 125 kg in 3.7 less days than the SR pigs. The 2W pigs grew slower than expected based on their 20-d BW and only reached 125 kg BW in 3.0 d less than the SR pigs. The 20-d BW alone can not be used to predict days to achieve desired market BW. Both the weaning management treatment and 20-d BW must be taken into account. It must also be realized that 2W pigs with the same 20-d BW of 6.82 kg, the overall mean, are pigs with substantially below average 20-d BW within the 2W treatment and below average birth BW. Relative to birth BW, especially for pigs with below average birth BW, pigs of the 14W treatment had greater 20-d and 70-d BW than SR pigs of the same below average birth BW. Alternative management of the lightest birth BW pigs including early weaning and use of milk replacers should be evaluated in relation to their ability to stimulate growth in pigs of low birth BW and reduce variation in days to target market BW. As litter size increases, the percentage of pigs with below average birth and weaning BW increases (Le Dividich, 1999). With increased litter

209

Impact of Birth and Body Weight at Twenty Days

growth of the pigs from large litters is expected to be less than that predicted using average birth BW of the litter. Sampling pigs from the mean of the litter at birth or weaning may not represent the mean performance of the entire litter when litter size is over 13 pigs (Le Dividich, 1999).

IMPLICATIONS

Figure 10. The predicted 126-d BW for pigs relative to 20-d BW.

sizes, producers should consider nutritional changes during late gestation to increase pig birth BW, or greater pressure will be placed on a change in the management of the light birth BW pigs. Because milk production limits preweaning pig growth, crossfostering smaller birth BW pigs from

several litters into one nurse litter is not likely to be a complete solution to the light 20-d BW problem unless that litter also receives a milk supplement. Due to the nonlinear relationships between birth and 20-d BW with subsequent postweaning BW, the average

The nonlinear relationships of birth and weaning BW with postweaning BW make the modeling of postweaning growth complex. Increasing the BW of the lightest birth and 20-d BW pigs could increase the mean and decrease the variance in BW at subsequent ages and days to achieve target market BW. Continued selection for litter size will likely result in an increased percentage of light pigs and increase the variation in age to achieve market weight. Further modeling of the sources of variation from birth to market BW could be used to evaluate and refine alternative strategies to reduce the magnitude of between pig variations on growth rate.

ACKNOWLEDGMENTS Research was conducted at the PIC research farm, Franklin, KY. Authors recognize the technical assistance of Eldon Wilson and the partial financial support of Ralco Animal Nutrition, Inc.

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Figure 11. The predicted 168-d BW for pigs relative to 20-d BW.

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