Food intake capacity in relation to breeding and feeding of growing pigs

The production of an animal depends on its genotype and its environment. Therefore, in general, two ways exist to improve production traits: improvement of the genotype and improvement of the environment. In growing pigs, the first is often done by selection for a combination of economically important traits, such as average daily growth rate, food conversion ratio and proportion of lean meat in the carcass. The amount of food offered to a pig can be considered a major environmental factor, particularly with restricted feeding. With ad libitum feeding, food intake is assumed to be equal to food intake capacity, which contains a genetic component. High food intake may result in unfavourable food conversion ratio and high proportion of fat in the carcass. Therefore, growing pigs are often fed at a restricted feeding level during part of the growing period.
There is evidence that selection programmes giving high emphasis to food conversion ratio and lean proportion, at the expense of growth, can lead to reduction in food intake capacity, especially with testing under ad libitum feeding. However, a low food intake capacity can become a constraint for further improvement of body-weight gain and lean tissue growth rate.

In this thesis, relationships of food intake and food intake capacity with production traits were studied. Implications of variation in food intake capacity for breeding and feeding strategies were discussed. Production traits dealt with were: average daily body weight gain (DG), food conversion ratio (FCR), ultrasonically measured backfat thickness (BF), percentage lean parts in the carcass, percentage fatty parts in the carcass, lean tissue growth rate (LTGR), fatty tissue growth rate (FTGR) and lean tissue food conversion (LTFC).
Results were based on a series of six experiments with growing pigs, fed according to different strategies. For each experiment, a fixed number of animals per litter (Dutch Yorkshire * Dutch Landrace crossbreds) was purchased. Animals were housed and fed individually. On a total of 687 barrows (experiments 1 to 6) and 98 gilts (only in experiment 6), live body weight and food intake were recorded weekly during a growing period from about 27 to 108 kg live weight. At the end of the growing period, backfat thickness was measured ultrasonically. Carcasses were dissected and amounts of lean and fatty tissue in the carcasses were determined.

In chapter 1, relationships of average daily food intake (FI) with production traits were investigated for each experiment by sex combination. Because animals were fed on different feeding levels, FI ranged from about 1.7 kg.d-1 to 3.2 kg-d-1 (22 to 42 MJ digestible energy per day). Body composition appeared to be linearly related to FI, showing fatter animals at higher FI. FTGR had a high linear correlation with FI (0.85 to 0.95), indicating that in the present range of FI a rather fixed proportion of food for production was used to deposit fatty tissue. The response of DG and of LTGR on increasing FI was not always significantly different from linear. However, quadratic polynomials indicated diminishing returns in each experiment. Therefore, a nonlinear regression model of the type a(FI- f _o ) ^b, with parameters a, fo and b, was fitted to the data on DG and LTGR. For FCR and LTFC the corresponding model was: FI/(a(FI-f _o ) ^b). These non-linear models were preferred over quadratic polynomials because of good fit and better biologically interpretable parameters.
FCR and LTFC showed minima at a daily food intake of about 2.6 and 2.2 kg per day, respectively. At a high feeding level, the increase of FCR with increasing FI was low. Compared to most literature, the minimum FCR was at higher F. Results were not consistent in demonstrating or refuting a plateau in LTGR, which, in any event, appears to lie near to or beyond ad libitum FI for most pigs.

In each experiment, each animal had two to five littermates of the same sex. In each litter, one to three animals were fed ad libitum and two or three were fed at a restricted feeding level. In experiments 1 to 5, animals fed at a restricted feeding level received a constant proportion of ad libitum intake of animals in the same experiment at similar weight. In experiments 6 and 7, animals on restricted feeding were fed ad libitum to 48 kg live weight and according to scale afterwards. Food intake capacity (FIC) of animals fed at a restricted feeding level was estimated by two methods. The first method consisted of assigning the average daily food intake of ad libitum fed littermates of the same sex to each animal fed at a restricted feeding level. The second method could only be applied to experiment 6. It consisted of estimating FIC with multiple regression based on individual ad libitum performance till 48 kg live weight.

Effect of FIC on production traits was discussed in chapter 2. It was shown that FIC had a significant effect on body composition traits but not on DG and FCR. Irrespective of the method used to estimate FIC, animals with higher FIC produced more fatty tissue and less lean tissue from the same amount of food than animals with lower FIC. It was suggested that the partition of metabolizable energy between energy for maintenance, protein deposition and fat deposition is associated with FIC. The findings confirmed that selection for leaner and more efficient pigs may result in animals with lower FIC, irrespective of the feeding strategy during performance testing.

From recent literature, it is known that estimated genetic correlations of production traits (especially growth) measured in test- stations with similar traits measured in commercial environments, are lower than expected. This type of genotype by environment interaction has a negative influence on efficiency of breeding programmes because the breeding goal is generally defined in the commercial environment. Variation in degree of food intake restriction (DFR) may be a possible reason for these interactions because often in one or even in both environments, restricted feeding is applied according to scale. This means that animals receive an amount of food irrespective of their FIC and thus variation in DFR occurs.
In chapter 3 this hypothesis was tested by investigating the effect of DFR on litter by feeding regimen (L*F) interaction in animals fed at a restricted feeding level. It appeared that L*F interactions were significant for FI, DG, LTGR and FTGR in experiments 4 and 5. In experiment 6, sex by feeding regimen (S*F) interactions were significant for the same traits. L*F or S*F interactions were not significant for body composition and food conversion traits. Experiments 4 to 6 were also the experiments where not all animals received food according to their FIC. Correction of FI and production traits for differences in DFR resulted in disappearance of L*F and S*F interactions. it was concluded, therefore, that the poor relationships often found between teststation results of boars and results of their progeny in commercial environments may be caused, to a large extent, by variation in DFR in one or both environments. The easiest way to prevent these genotype by environment interactions is to feed animals ad libitum in test and in commercial environments.

In chapter 4, courses of daily gain, food intake and food efficiency (FE, defined as daily gain over food intake) during the normal growing period were described. Knowledge of these courses is necessary to optimize feeding strategies. The non-linear model y=a*exp(-b*W-c/W) was fitted to weekly recorded DG, FI and FE of 653 barrows and gilts fed at ad libitum or restricted feeding levels. In this model, y is DG, FI or FE, W is live body weight and a, b and c are parameters. The model had attractive mathematical properties and fitted well to the expected course of the traits investigated. The accuracy was similar to that of quadratic polynomials. Coefficients of determination in barrows averaged 0.29 for DG, 0.88 for FI and 0.45 for FE. In gilts these values were somewhat lower. For each trait, four types of curves could be distinguished, depending on the signs of b and c. With ad libitum feeding, 83% of the barrows and 61% of the gilts had a curve for DG with a maximum (b and c both positive). The predicted maximum DG was at an average live weight of 64 kg for barrows and 77 kg for gilts. In 60% of the barrows and 39% of the gilts, a FI curve with a maximum was found. Curves for DG and FI in gilts were flatter than in barrows. FE curves had a maximum in 59% of the barrows and in 52% of the gilts fed ad libitum. This predicted maximum FE, however, was, on average, before the start of the growing period. With ad libitum feeding, gilts had a higher FE than barrows from 35 kg body weight onwards. The difference between gilts and barrows increased with increasing live weight. Differences in FE between pigs fed ad libitum and pigs fed at restricted feeding levels were small, with a tendency for restrictedly fed pigs to be more efficient at the end of the growing period.
For each combination of experiment, feeding level and sex, average FE curves and individual FI or DG curves were used for indirect prediction of individual DG or FI curves, respectively. The correlation between directly and indirectly predicted values of DG and of FI at different weights was about 0.7 in ad libitum fed barrows and gilts, and over 0.8 in pigs fed at a restricted level. This indicates that the model is suitable to predict and control the course of individual daily gain of growing pigs by influencing the course of food intake.

In the literature, some pragmatic solutions are given to prevent further decline of FIC, such as more selection emphasis on DG or restriction of the genetic change of FIC to zero. In chapter 5, a method is presented to optimize selection for FIC by means of a biological growth model based on the linear/plateau relationship between protein deposition and food intake. Production costs were calculated with input variables: minimum fat to protein deposition ratio (R), maximum protein deposition rate (Pd _max ) and FI. Economic values were estimated for breeding goal traits R, Pd _max and FIC at three alternative levels of FIC. If FIC was too low to realize Pd _max , the economic value of FIC was about 100 Dfl. per kg.d ^-1and optimal selection emphasis
should be mainly on FIC, resulting in a rapid increase of daily weight gain. If FIC was higher than necessary to realize Pdmax, the economic value of FIC was about -40 Dfl. per kg.d ^-1and short-term selection resulted in increase of carcass leanness but decrease of FIC and DG. If FIC was just sufficient to realize Pd _max , the lowest production costs occurred and selection should be for R and Pd _max . In this third alternative, the gain in FIC should follow the gain in R and Pd _max in an optimal way and selection should best be carried out with a desired gains index, which will result in improvement of DG and of carcass leanness. It was shown that, in cases where FIC was higher than necessary to realize Pd _max , selection with a desired gains index should be preferred because this is more profitable in the long term. From the model calculations, it followed that future profit from selection of growing pigs for production traits is likely to decline because of the necessity to increase food intake capacity.
In the second part of chapter 5, the relationship between the shape of the food intake curve and production traits was investigated. After correction for variation in average daily FI, more curvature of the food intake curve appeared to be associated with a lower DG and a higher food conversion ratio. A high food intake at the end of the growing period (with the same FI) was favourable for DG and for carcass leanness.
To achieve optimal results in pig meat production, accurate tuning of selection procedures and feeding regimens on the biological possibilities of the pig will be required. More knowledge is necessary concerning the genetic background of protein deposition and lean tissue growth in pigs.