Breeding for feed intake capacity in pigs

This thesis deals with feed intake capacity of pigs. By selection, breeding organizations try to achieve genetic improvement in production and reproduction efficiency. Future genetic improvement may become constrained by a limited feed intake capacity of growing pigs and lactating sows, respectively. The aim of this thesis is to study the actual feed intake capacity of growing pigs and lactating sows in relation to their potential for production and reproduction in order to get a better understanding and to develop breeding strategies for feed intake capacity in dam and sire lines. Furthermore, the use of computerized feeding stations (i.e. feeding stations that are used for the recording of feed intake of group-housed growing pigs) is evaluated in order to reduce costs of recording feed intake and to increase benefits of investing in feeding stations for breeding organizations.<strong>Computerized feeding stations</strong>Chapter 2 describes algorithms to identify errors in feed intake data of growing pigs recorded with computerized feeding stations. The numbers of errors need to be kept small, as it is impossible to adjust feed intake data without bias and adjusting data is time consuming and, consequently, expensive. The objective of this study was to develop algorithms to monitor the operation of feeding stations, by frequently checking recorded feed intake data for errors. Results indicated several instances where feeding stations functioned sub-optimally during a period of days or weeks. Frequent checking and correction of the functioning of a feeding station would, therefore, reduce the incidence of errors. Expanding a feeding station's software with the editing system described in Chapter 2 will allow a daily check for errors of recorded data.Computerized feeding stations record data of each visit to the station, which means that a large number of feed intake traits can be derived per pig. In practice, average daily feed intake during the test period (from 25 to 110 kg) is the only information systematically used by pig breeding organizations. Average daily feed intake is usually computed after adjustments for errors in the recorded data. Instead of adjusting, it might be better to eliminate incorrect daily feed intake records and to consider these as missing values. The objective of Chapter 3 was to study the effect of such missing values on the estimate of average daily feed intake during the test period. In this study, average daily feed intake was estimated by fitting curves to the feed intake data per pig. Results indicated that pigs can have missing values for up 70% of the test days (randomly spread over the test period) and still end up with an accurate estimate of average daily feed intake. This implies that considering incorrect feed intake per day records as missing is a good alternative for adjusting incorrect data. Moreover, the use of functions to estimate average daily feed intake enables a more efficient use of feeding stations by recording feed intake data during only parts of the test period.The fit of a linear regression to daily feed intake records results per pig in a record for intercept (feed intake at the start of the test period), slope (increase during the test period), and residual standard deviation of the fit. Genetic aspects of these traits were studied in Chapter 4. Heritabilities estimated for intercept, slope, and residual standard deviation were .32, .32, and .46, respectively. Genetic correlations of these characteristics with performance and carcass traits ranged from .12 to .17 for feed conversion ratio, .36 to .75 for daily gain, and -.61 to .25 for lean content. The intercept seems the most promising candidate to improve the selection for an efficient lean growth as it was positively correlated with both average daily gain and lean content.<strong>Feed intake capacity in relation to production and reproduction performance</strong>For pig breeding organizations, it is important that end product growing pigs have a feed intake capacity that is close to their economic optimum. An experiment was set up to elucidate the desirable direction of selection for feed intake capacity of growing pigs (Chapter 5). A linear plateau relationship between protein deposition and feed intake was assumed to derive the optimum feed intake for the early (25-65 kg), middle (65-95 kg), and late phase (95-125 kg) of the growing period, separately. Actual feed intake capacity was below the optimum more often for gilts and boars than for castrates (within genotype), and more often for end product genotypes than for dam line genotypes. Feed intake capacity of end product gilts was below or close to optimum during the total growing period from 25-125 kg. Results indicated that it may be beneficial to select for a higher ad libitum feed intake in combination with selection for other production traits.Chapters 6 and 7 deal with feed intake capacity of lactating sows. In Chapter 6, it is argued on base of a literature study that voluntary feed intake of lactating sows should be increased by selection, especially if litter size, and consequently energy and nutrient requirements of sows continue to increase. The objective of the experiment described in Chapter 7 was to investigate whether nursing a large number of piglets has negative effects on lactation and post-weaning performance of primiparous sows and if a greater lactation feed intake can prevent these possible negative effects. Sows nursing a large litter during lactation had a feed intake similar as sows nursing a small litter, however, mobilized more body tissues and had a higher probability of a prolonged weaning-to-estrus interval. A higher daily feed intake during lactation (for a given litter size) reduced tissue losses of the sow and reduced the probability of a prolonged weaning-to-estrus interval. There were some indications that sows nursing a large litter may suffer more often from heat stress than sows nursing small litters. It is therefore not only recommended to increase lactation feed intake by selection but also to simultaneously improve environmental and dietal factors to reduce occurrence of heat stress.<strong>Breeding for feed intake capacity in dam and sire lines</strong>In sire lines, selection is mainly for production traits, whereas selection in dam lines is for production as well as for reproduction traits. Due to several tendencies in pig breeding, the relative importance of reproduction performance in future dam line breeding will increase compared to production performance. Therefore, the implications of the desired increase in feed intake capacity for dam line breeding are discussed from a reproduction point of view.Lactation feed intake capacity may be increased genetically by including lactation feed intake in the breeding goal and selection index (direct selection) or by including lactation feed intake only in the breeding goal (indirect selection). Direct selection requires recording of ad libitum feed intake during lactation, which is an expensive trait to measure. The most suitable way of increasing lactation feed intake seems therefore to be by indirect selection, possibly in combination with an aggravation of test circumstances for potential breeding sows (for example by standardizing litter size during lactation to 12 or 13 piglets). The latter would force sows to express their full genetic potential for reproduction. Candidate traits that can be used in a selection index to increase lactation feed intake by indirect selection are for example backfat and body weight losses during lactation, interval weaning-first-estrus, and pre-pubertal feed intake and daily gain. Which traits may be used best will mainly depend on genetic relationships among the various traits and lactation feed intake, and on additional costs of recording the required data.Though genetic relationships between lactation feed intake and production performance still have to be established, it seems likely that traits like pre-pubertal daily feed intake and daily gain will also increase when lactation feed intake increases. An increase in pre-pubertal feed intake capacity in dam line genotypes will be passed on to end product genotypes. This means that the desired increase in feed intake capacity of end product growing pigs will likely be accomplished at least partly via dam line breeding, which diminishes the need to increase feed intake capacity via selection in sire lines. For sire lines, the key to selection is to find the optimum balance between selection for carcass leanness, daily gain and feed intake in order to achieve the optimum feed intake in the end product pig.The development of computerized feeding stations brings forward the question whether or not breeding organizations should invest in these stations. This is first of all an economic question, i.e. a comparison of costs and benefits for selection. However, it seems justified to invest in feeding stations in order to monitor feed intake capacity of growing pigs.<strong>Main conclusions</strong>From this thesis (combined with some related studies), the following main conclusions can be drawn for future pig breeding programs:<UL>Feed intake capacity during lactation should be increased, especially because litter size, and consequently energy and nutrient requirements of the sow, continue to increase in the future. The most suitable way of genetically increasing lactation feed intake capacity in dam line genotypes seems to be by indirect selection, possibly in combination with an aggravation of lactation circumstances.Feed intake capacity during the entire growth period should be increased as feed intake capacity is lower than or close to optimum particularly for end product gilts. This increase will be accomplished, at least partly, via future dam line selection for feed intake capacity during lactation, which diminishes the need to increase feed intake capacity via selection in sire lines.The efficiency of the use of feeding stations will be increased by using computerized methods to check recorded data for errors and to summarize data per visit to the desired traits (e.g., average daily feed intake during the test period). Furthermore, it is sufficient to record feed intake during only parts of the test period (e.g., every second week) to end up with an accurate record for average daily feed intake. Inclusion of traits in the index describing the course of feed intake capacity results in a small increase in accuracy of selection. The question whether a breeding organization should invest in computerized feeding stations is primarily an economic question. However, it seems justified to invest in a number of feeding stations in order to monitor feed intake capacity of growing pigs.