Productivity of sows has increased worldwide, especially during the last decade. Sows have been changed genetically to produce larger litters. It was hypothesized that including feed intake or feed efficiency during lactation or both in the breeding objective for dam lines is necessary to facilitate sow’s future increase of unproblematic production of grower-finishers that efficiently convert feed into meat. Increasing feed intake of sows is one solution to prevent excessive mobilization from body stores. As a result of selection for leaner pigs with higher feed efficiency, however, feed intake tends to decrease because high leanness and high feed efficiency are genetically associated with low appetite. There is a risk, therefore, that feed intake during lactation reduces due to selection for lean and efficient finishing pigs. In this thesis a model was developed to estimate the energy efficiency of a lactating sow based on on farm observations enabling large scale data recording. Increasing energy efficiency during lactation might be a solution to overcome the apparent contradiction of the desired direction of selection for feed intake during growing-finishing and lactation. Increased energy efficiency during lactation will yield more milk output given the feed intake and mobilization from body stores. To study the consequences of selection, heritabilities and genetic correlations were estimated for fertility, lactation performance and growing-finishing characteristics. For growing-finishing characteristics the genetic models contained social interactions and for lactation feed intake, environmental sensitivity was studied as well. The main conclusion of a simulation of a breeding program in pigs was that it is possible to achieve a balanced genetic progress in fertility, lactation performance and growing-finishing characteristics. Genetic regulation of feed intake during growing-finishing is to a large extend different from genetic regulation of feed intake during lactation. Results of this thesis show that feed intake of sows during lactation is not an immediate risk for further improvement of more and heavier piglets. Higher piglet production is still on its way via the genetic pipeline and will continue to increase by selection for more and heavier piglets. Selection for increased milk production or litter weight gain is preferred; this will lead to increased protein and energy demands as well. At all events, sows need to eat more and be more efficient at the same time to keep up with this increased demand. It is a question of tuning the breeding objective in order to optimize the relation between feed intake and body weight losses during lactation.
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