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Restricted-fed sows lose more energy than those under non-restriction. However, their body energy mobilization, generally is not sufficient to supply litter energy requirements for maintenance and growth and therefore, the restricted-fed sows will produce less milk on a daily basis, resulting in lower litter daily weight gain, and consequently, in lower piglet weight and lighter litters at weaning. Although restricted-fed sows mobilize more energy on a total basis when compared to control sows, they lose less energy per kg of BW when compared to non-restricted sows. The same is observed for lipid contribution to BW loss, which decreases from 62 to 42%, respectively when comparing ad libitum and restricted-fed sows. These observations indicate that restricted sows are more efficient in using energy from feed and body mass when compared to sows fed ad libitum. Bergsma et al. (2009) mentioned that lactation efficiency expresses the ratio between sow energy input (feed intake and body mobilization) and output for litter growth.
The definition applied to the restriction model used in our study clearly indicates that restricted-fed sows were more efficient (+13%) than the ad libitum fed sows, i.e., for each kg of feed intake, restricted-fed sows produced 1.49 kg of milk per kg of feed intake, whereas the control sows produced 1.13 kg. Patterson et al. (2011) also observed that restricted-fed sows were more efficient (+14%) relative to those fed ad libitum (controls). This suggests that the ad libitum fed sows are able to produce more milk while protecting themselves from catabolism and its negative impacts, and therefore have improved longevity. Based on our findings and literature reports, it is evident that genetic improvement programs should seek the enhancement of energy efficiency of lactating sows in order to obtain dams with good maternal ability, lean carcasses, and higher milk production at a determined feed intake and body reserve mobilization.
Literature studies have shown the relationship between weaning-to-estrus interval and energy intake during lactation (Foxcroft et al., 1995; Patterson et al., 2011). However, independently of feed intake pattern, 90% of the current sows tend to present estrus within 3 to 5 days after weaning (Patterson et al., 2010; Schenkel et al., 2010; Patterson et al., 2011). Our findings confirm that hyper-prolific sows, despite experimentally restricted-fed to 50% of the expected voluntary intake and extensive body tissue catabolism, presented estrus almost immediately after weaning and acceptable farrowing rates after breeding. This may be partially due to the genetic selection against weaning-to-estrus interval (Vinsky et al., 2006; Bergsma et al., 2013). However, although energy balance may become positive after weaning, sows with rapid return to estrus may not have fully recovered from the damages caused by catabolism during lactation, with possible negative impacts on follicle and embryo quality, and consequently on the size and quality of the next litter (Quesnel et al., 2007; Quesnel, 2009).
Final considerations and implications
Modern lactating sows are very productive and show a reduced feed intake capacity as a result of the genetic selection for higher feed efficiency. Therefore, lactating sows suffer from excessive body weight mobilisation. It appears that the intense catabolism of body tissues can negatively impact on their milk production, litter performance and next litter size. On the other hand, restricted-fed sows are more efficient, producing more milk per amount of feed intake. The higher lactation efficiency observed in these sows indicate that there is more available energy through mobilization from body reserves above maintenance of the sow used for piglet growth and maintenance than from feed intake. The intense selection pressure to which modern sows have been submitted changed their biology and metabolism. Therefore, there is a need for developing new nutritional strategies to attend the modern lactating sow’s daily needs.
