Lactation, a ubiquitous feature of all mammals, involves secretion of milk from mammary glands and is an essential process in mammalian reproduction. Energy requirements during lactation are very high, as the female has to meet maintenance and milk production requirements simultaneously.
Today’s sow has additional challenges as a result of genetic and management changes that have occurred in the past few decades. Litter size in pigs has increased during this period, and larger litters mean more demand for milk, which results in increased milk output from the lactating sow. Poor body condition of the sow at weaning can result in a longer wean-to-service interval. Studies have suggested that selection for increased feed efficiency and leanness during the grow-finish stage can result in a reduction of sow appetite, as there exists a negative genetic correlation between leanness and appetite.
Because of this, sows with heavy selection emphasis on increased grow-finish feed efficiency are prone to reduced voluntary feed intake during lactation. All these factors result in a pig that excels in lean growth but has reduced potential for lactation traits. Poor lactation traits can lead to decreased longevity (early culling) and lifetime production, which in turn reduces profitability. To improve lactation performance, along with other traits of economic importance, we need a better understanding of traits associated with lactation and their interactions with other traits. Our objective is to explain the complex genetic relationships among traits associated with lactation and reproduction, and how these can be improved genetically using available genomic tools.
Factors affecting nutrient requirements
Farrowing marks the end of gestation and starts the lactation phase. The nutrient requirement of sows during lactation is difficult to quantify and evaluate, as it changes daily due to changes in feed consumption; milk composition; and volume, body weight loss and the composition of this weight loss.
Litter size is one of the most important factors that drives milk production in the sow. As a result of genetic selection, litter size has increased during the past decades and will remain as an important goal in pig breeding programs. In response to greater suckling intensity resulting from larger litters, sows that nurse more piglets produce more milk. An increase in litter size is directly correlated to an increase in mammary gland tissue, which results in higher milk production and, thereby, higher energy requirements for the sow.
Parity also plays a major role in the nutrient requirements of lactating sows. As body weight increases with parity, maintenance requirements also increase. Nutrient requirements for first parity sows during lactation are higher than for older sows, as young sows are still growing during lactation. They require additional nutrients for growth, structural formation and mammary gland development, along with the requirements for replenishing body reserves that are mobilized during lactation. Studies conducted on first parity sows by overfeeding them during lactation concluded that first parity sows partition the extra energy to body growth rather than to milk production.
Stage of lactation is another factor that determines the nutrient requirements of sows during lactation. Under modern management, the average lactation length in sows is around 21 days. Energy requirements during this three-week period are much higher than during gestation, as the metabolism of a lactating sow is much higher than that of a gestating sow. Sows lose body reserves (measured as backfat and loin depth) during the first two to three weeks of lactation to support milk production, and thereafter start to recover the lost body reserves. Reducing the lactation length hampers this recovery process.
Availability of body reserves at the time of farrowing is another element that plays a vital role in regulating the energy requirements of sows during lactation. Fat sows do not require as much feed during lactation, while sows with low body reserves at farrowing cannot mobilize many resources, and need more feed during lactation to compensate for this reduced mobilization. Studies indicate that sows with more backfat at farrowing consumed 30% less feed during lactation compared to lean sows. Body weight and fat deposits influence feed intake by modulating long-term regulatory mechanisms.
Feed consumed and body reserves mobilized during lactation are the two sources of energy for a lactating sow (energy input). Factors that positively affect appetite of lactating sows are increasing stage of lactation, litter size (or milk production), growth rate of piglets, parity number and feeding frequency.
Lactation feed intake is low immediately post-farrowing and attains a maximum by the second or third week of lactation. The lower feed intake during early lactation is likely due to gastrointestinal limitations, as the GI tract may require time to adapt to the high daily feed requirement. Feed intake during lactation also depends on factors such as breed, feeding system, housing, management and temperature. Table 1 shows the total feed consumed by purebred Yorkshire and Landrace sows, during a 20-day lactation period, in different parities.
Energy requirements of a lactating sow are usually not met by voluntary feed intake alone. Therefore, to meet energy demands, sows mobilize body reserves, and thus lose weight during lactation. The physiological drive of a lactating sow to produce milk at the expense of other body functions is a key component of the metabolic state of the lactating sow and is controlled by factors such as genetics, parity, stage of lactation and litter size. Mobilization of body reserves during lactation can be measured in different ways, such as body weight loss, backfat loss, loin depth loss, fat mass loss, protein mass loss, etc. On average, sows lose around 7 to 12 kilograms of body weight during lactation (after accounting for weight loss due to piglets and placental fluids, and for the water content in mammary glands) and 2 to 3 millimeters of back fat and loin depth.
Energy output during lactation
Energy available from feed intake and mobilization of body reserves is used for growth and maintenance of the sow and for producing milk. That part of the energy that is utilized for producing milk, which in turn is used for the growth and maintenance of piglets, is considered as the output from the sow during lactation. Unlike in dairy cattle, direct measurement of milk yield is not practical in pigs. Experimental methods such as weigh-suckle-weigh or isotope dilution are complicated, labor-intensive and expensive, and hence cannot be implemented on a routine basis in a nucleus herd.
Routine evaluation of milk yield needs a simple, more straightforward measurement. The increase in body weight of piglets nursed by the sow from birth to weaning is a simple and useful indicator trait for the milk production of a sow.
The entire energy flow/energy metabolism during lactation is summarized in Figure 1.
Relationships between traits
■ Body resource mobilization and litter weight gain. Different studies have shown that genetic correlations of litter weight gain (an indicator trait for milk yield) with body tissue mobilization traits are significantly positive (ranging from 0.24 to 0.57), i.e., sows with a high genetic predisposition to use body reserves during lactation also have the ability to wean heavier piglets at the end of lactation.
■ Feed intake, body resource mobilization and litter weight gain. Studies indicate that the genetic correlation between feed intake and litter weight gain is positive, but not very high (ranging from 0.06 to 0.31). At the same time, genetic correlations of lactation feed intake with traits associated with body tissue loss during lactation are strongly negative (-0.35 to -0.70), indicating that sows that have the genetic ability to eat more during lactation show significantly smaller body tissue loss. This pattern of association — i.e., strong negative genetic correlations of feed intake with tissue mobilization traits and a relatively weak but positive genetic correlation of feed intake with litter weight gain — reflects how the dietary energy is partitioned in the sow’s body. This suggests that the feed consumed by the sow during lactation is predominantly used for reducing sow body tissue losses rather than for milk production. These results also suggest that the milk production potential of a sow is genetically more driven by body resource mobilization than by lactation feed intake.
■ Body reserves accumulated during gestation and litter weight gain. During gestation, dramatic changes occur in the metabolism of mammals, as the dam has to accommodate the energetic demand for the developing fetuses and for its own maintenance, and also has to allow accumulation of energy stores in anticipation of lactation. These accumulated energy sources, along with the energy depots that were replenished during the latter part of the previous lactation (by increased feed intake), act as a source of energy during lactation. Studies have shown that the genetic correlation of body reserves at the beginning of lactation with body tissue mobilization during lactation is negative, i.e., sows with genetically greater body reserves at farrowing mobilize fewer body reserves during lactation.
As discussed before, body reserve mobilization traits are positively correlated (genetically) with litter weight gain, i.e., sows that genetically mobilize less body reserves wean lighter litters. These two results suggest that “heavy/fat sows” at farrowing may not be “good mothers.” Fat sows have fewer protein reserves to supply substrates for milk production, compared to lean sows of similar weight, which may be a reason for their lower milk production.
Studies suggest that traits associated with lactation in pigs are heritable and have sufficient genetic variation, and hence these traits can be improved by means of selection. Pedigree-based genetic evaluation methods have been very successful for selecting animals for easy-to-measure traits. However, most of the traits associated with lactation and reproduction in pigs are either less heritable, appear later in life or are difficult to measure on a routine basis. So for these traits, genomic selection can be an attractive alternative to pedigree-based evaluation methods.
At Genesus, in collaboration with the University of Alberta and Iowa State University and with funding from Genome Alberta and Alberta Livestock and Meat Agency, we have conducted a detailed study of more than 1,500 Yorkshire and Landrace sows for traits associated with lactation and reproduction. The sows were weighed and scanned for backfat and loin depth at around five days before farrowing and at weaning. The piglets were individually weighed immediately after birth, at fostering and at death or weaning. Daily feed consumption of each sow was measured using automatic feed recording equipment. All sows were genotyped using the Illumina Porcine 60k SNP Chip.
Genomic prediction methods involve estimating marker effects on a training data set and then testing the effects on a validation group. Within each breed, a subset (~ [approximately] 15%) of younger animals was allocated to the validation group. Estimated breeding values using genomic prediction for the validation group of animals were arrived at by summing the marker effects estimated in the training data set for the marker values of the genotype. Accuracies of the predicted breeding values (both genomic estimated breeding value and estimated breeding value from pedigree-based methods not using genomic data) were estimated as the correlation between the breeding values and phenotypes corrected for fixed effects of the animals in the validation data set, divided by the square root of heritability of that trait. The accuracies of GEBV and EBV from pedigree-based genetic evaluations are detailed in Table 2.
Accuracies of the GEBV and EBV for the Yorkshire and Landrace sows show that there are differences between breeds. Interestingly, feed intake during lactation showed only small differences in accuracies of GEBV and EBV for either breed. For most traits in both breeds, the accuracies for the genomic prediction methods were, on average, 60% higher than pedigree-based estimates. However, it should be noted that it is probably necessary to do the analysis for each breed and combining data across breeds should be done with care. The results show promise for routine use of genomic prediction methods to predict the genetic merit of animals at a young age, especially for traits with low heritability and those that are difficult to measure.
In conclusion, traits associated with lactation in sows have a sizable genetic component and potential for genetic improvement. Including traits associated with lactation in a maternal selection index should allow concurrent improvement of sow lactation performance along with grow-finish performance. For many of these traits, there are practical challenges associated with their routine measurements. Utilization of genomic tools can be an option to overcome this problem.