Methods to Improve Profitability

Through Hyperprolificacy

Thomas E. Long

National Pig Development (USA)

 

Introduction

Reproductive traits (e.g., number of piglets born alive per litter and pre-weaning viability) are important components of the economic efficiency of a swine production unit (Tess et al., 1983; de Vries, 1989a; Rothschild and Bidanel, 1999). Some swine geneticists have noted that as genetic improvements are realized by a number of breeding organizations in the growth and carcass leanness of their products, reproductive efficiency will become more important as a selection objective (Ollivier et al., 1990; Webb, 1991; Schinckel et al., 1998). The magnitude of this shift in selection objectives for dam lines of an individual breeding organization will depend on that organization’s competitive position within the industry (de Vries, 1989b). However, evidence of the current importance of genetic improvement in reproductive efficiency in swine in the US can be seen in NPPC’s recent Maternal Line Genetic Evaluation Program (2000) and by NSIF including a symposium on hyperprolificacy at its 25^th anniversary meeting. While there are a number of traits, in addition to number born alive, that affect reproductive efficiency in sows (e.g., piglet birth weight, piglet survivability, farrowing interval, longevity), the focus of this paper will be on methods to increase number born alive per litter to “hyperprolific” levels.

 

“Traditional” Selection for Litter Size

Before discussing hyperprolific lines and schemes, one point needs to be made. Number born alive does have a relatively low heritability, and a review of studies investigating this trait found a pooled estimate of heritability for NBA of 0.10 (Hermesch, 1996). This does not mean that this trait cannot be substantially improved through selecting animals on a dam index over time. A number of groups have improved this trait using selection index and BLUP procedures in their lines (Lofgren et al., 1994, Short et al. 1994, Long, unpublished data). BLUP applications have been implemented in the US swine industry for genetic evaluation of reproductive traits (e.g., STAGES, Schinckel et al, 1986; PEST, Groeneveld et al. 1990; PIGBLUP, Long et al., 1990). Use of these tools, in a well-designed selection program, can improve number born alive, even though this trait is lowly heritable.

 

Component Trait Selection

Number born alive is a composite trait, made up of many component traits (e.g., ovulation rate, embryonic/fetal survival, uterine capacity). A great deal of work has been done in Nebraska to investigate whether selection for component traits of litter size would result in greater response than direct selection for litter size, both at the University of Nebraska-Lincoln and the ARS-USDA facility at Clay Center (Cunningham et al., 1979; Johnson et al., 1984; Bennett and Leymaster, 1989; Neal et al., 1989; Lamberson et al., 1991; Leymaster and Johnson, 1994; Johnson et al., 1999). Since one of the speakers at this symposium will be covering this area, it won’t be addressed directly here. However, results from the NPPC Maternal Line Genetic Evaluation Program (2000) would suggest that component trait selection has been successful in developing a hyperprolific dam line, the Nebraska Index Line.

 

Molecular Genetics Approaches

Given the low heritability of litter size in swine, the discovery of the estrogen receptor gene (Rothschild et al., 1996) and its effects on litter size were of great interest to swine breeders everywhere. Haley (1999) noted that the association between the ESR locus and litter size “has been identified in a number of populations, although the magnitude of the association varies between populations.” Since then other researchers have reported quantitative trait loci having affects on reproductive traits (Rathje et al., 1997; Southwood et al., 1998; Spiegel et al., 1999; Cassady et al., 1999). Use of genetic markers in developing hyperprolific dam lines in swine will become an important tool for swine breeders to use in the future. Use of these tools, in practical breeding programs, will depend on issues, such as: the availability of markers, the amount of variation a specific marker is accounting for, combining marker information with current quantitative genetic evaluation procedures, other genes in the genome that may have epistatic effects with the specific allele in question and the frequency of the allele of interest in the population being screened. As work continues in the molecular area, there may be many more opportunities to exploit this technology to enhance reproductive performance in dam lines.

 

Hyperprolific Schemes

Hyperprolific schemes have been described (Legault and Gruand, 1976; Bichard and Seidel, 1982) and involve screening multiplier units for hyperprolific sows such that their germplasm can be brought back into the nucleus selection program. These types of breeding schemes do afford higher levels of selection intensity than could be applied in the nucleus, but have generally not been adopted due to genetic lag in production traits that would be incurred in the process (Avalos and Smith, 1986). Bidanel et al. (1994), in reviewing 20 years of a hyperprolific scheme in France, noted that, while litter size was substantially improved and growth rate unaffected, these dam lines had higher backfat and poorer feed conversion than contemporary lines. Additionally, there are health risks that might be incurred in moving germplasm from multiplier units, as multiplier units tend to have poorer health levels than nucleus farms. However, as technology and swine populations change, should hyperprolific schemes be reconsidered? Information technology has improved such that estimated breeding values can be calculated on multiplier animals rather than relying on phenotypic data. As stated previously, many organizations have realized genetic improvements in dam lines for production traits, such that those organizations have favorable competitive positions for those traits. Could hyperprolific schemes be implemented in these systems, such that there was an enhancement of reproductive performance with smaller penalties for genetic lag in production traits than would have been incurred in the past? Finally, embryo transfer protocols may afford opportunities to move germplasm from multiplication hyperprolific sows to nucleus units in ways that minimize health risks. Although hyperprolific breeding schemes are probably not being used in most current dam line breeding programs, in a discussion of the development and use of hyperprolific lines, one should raise the question of whether these types of schemes should be revisited.

 

Use of Hyperprolific Lines

This paper has discussed methods of developing hyperprolific lines. Once these lines have been developed within a breeding organization or samples (animals, semen or embryos) from a hyperprolific line have been obtained, it needs to be determined how those lines will be used in breeding programs in combination with other dam and sire lines to produce market pigs, since many hyperprolific dam lines tend to have poorer growth and carcass characteristics than more “standard” dam lines (Bidanel, 1990; Bidanel et al., 1994; NPPC, 2000). The two obvious alternatives are: 1) use the line in a specific crossing program with other dam lines and superior terminal sire lines, or 2) develop new composite lines using the hyperprolific line as one of the foundation lines. Both these approaches have or are being implemented by breeding organizations. Using the hyperprolific line in a specific crossing program is relatively straightforward, but does rely on having superior terminal lines to use in breed combinations. Results from the NPPC Terminal Sire Line Genetic Evaluation Program (1995) and the NPPC Lean Growth Modeling Project (1998) clearly indicate that there are terminal sire line differences for production and carcass characteristics. Therefore, decisions on what type of terminal sire line to use in combination with hyperprolific lines are important for production and processing sectors to benefit from the use of these lines.

 

Examples of the development of hyperprolific composite lines can be found in breeding programs using Chinese breeds, such as the Meishan or Jiaxing, in combination with Western dam lines (Mercer and Hoste, 1994; Burlot et al., 1998). These programs depend on also using Western dam lines, as foundation lines, that are not only prolific, but also have excellent production characteristics. Selection objectives in these composite lines are distinct from usual dam line breeding objectives being applied in most current breeding programs, and the use of these composite lines in crossing programs also relies on superior terminal sire lines to produce market pigs.

 

The importance of reproduction to the economic efficiency of swine production has been previously stated. Trends over the last 5-7 years from USDA Pig Crop reports indicate an increase in the number of pigs being produced per sow. Industry analysts have provided a number of reasons for these trends, such as changing demographics of swine production, improved husbandry practices and better genetic lines. It is clear that what were once good reproductive output levels from sows may now only be ordinary. The use of hyperprolific dam lines in a breeding program is one way to improve reproductive output from a swine operation.

 

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