REPRODUCTION
PIH-74
pork industry handbook
COOPERATIVE EXTENSION SERVICE • PURDUE UNIVERSITY • WEST LAFAYETTE, INDIANA
Management of Developing Gilts and Boars for Efficient Reproduction
Authors
Dwane R. Zimmerman, University of Nebraska
E. Dale Purkhiser, Michigan State University
Jack W. Parker, North Carolina State University
Reviewers
Kenneth L. Durrance, University of Florida
Vernon Leibbrandt, University of Wisconsin
Leif H. Thompson, University of Illinois
Successful introduction of replacement gilts and boars into the breeding herd is an important aspect of breeding herd efficiency. To attain herd efficiency, females must ovulate adequate numbers of fertile ova, show willingness to mate (estrus) and conceive readily. Males must produce adequate numbers of fertile spermatozoa, possess ability and show willingness to mate and produce high conception rates. If these traits are properly developed in both sexes, pregnancy rate and litter size will be maximized. The purpose of this fact sheet is to help producers better understand the important factors in the management of the developing gilt and boar for maximizing their reproductive efficiency.

Gilt Development
Gilts should reach puberty (exhibit first estrus and ovulate) at an early age, continue regular estrous cycles until bred and conceive readily at first breeding. Early puberty is necessary if replacement gilts are to be successfully bred during a limited breeding season and express their full potential for litter size. Gilts should express one or more estrous periods before the usual breeding age (7-9 mos.) since more eggs and larger litters are thus produced. Litter size in gilts receiving adequate energy will be increased approximately 2 pigs per litter by breeding at second estrus rather than at first estrus. Gilts that express first estrus at a young age (<6 mos.) can be bred earlier as long as one or more estrous periods have been expressed before breeding. This practice will materially reduce feed and other overhead costs associated with gilt maintenance without detracting from reproductive performance.

Genetic Factors
Most gilts reach puberty between 6 and 8 mos. (average 200 days). Crossbred gilts generally express first estrus earlier (1-4 wks.) than the average of the parent breeds represented in the cross. Thus, when bred at the usual breeding age, crossbred gilts will have experienced
more heat periods, will ovulate more eggs and will produce larger litters than purebreds. Inbreeding, on the other hand, tends to increase age at first estrus.
Producers should not routinely keep for breeding gilts that have not expressed first estrus by 1/2 months. Breeders should not keep replacements from dams that were late (old) in showing their first estrus or in conceiving their first litter because the heritability of age at puberty is relatively high (35-50%).

Level of Nutrition
Most gilts are developed to 175-200 lbs. (41/2-6 mos.) by self-feeding growing-finishing diets which allow maximum expression of their genetic potential for growth rate and fat deposition. Although this practice is necessary to obtain satisfactory individual growth and backfat records for use in selection, self-feeding should not be continued after the performance test is completed.

Energy intake can be restricted after 175-200 lbs. without delaying puberty. This restriction can be accomplished by daily hand feeding each gilt 4-5 lbs. of a 14% protein, well-balanced diet. Protein, vitamins and minerals should be supplied in the diet in amounts that provide the daily allowance recommended by the National Research Council when the diet is restricted-fed. Energy restriction not only saves on feed costs but prevents the accumulation of unneeded weight and body condition which may decrease longevity of production and be a contributing factor in the development of unsoundness in young as well as older breeding females.

Other factors which must be considered in the proper management of a limited feeding program are the influences of animal activity and cold temperature on the level of feed required. Gilts maintained in confinement require approximately 10% less feed (.5 lb./head daily) than gilts maintained outside in large pens. Requirements are approximately 25% greater (1 lb./head daily) during the extreme cold of winter as compared to other seasons.
Cooperative Extension Work in Agriculture and Home Economics, State of Indiana, Purdue University and U. S. Department of Agriculture Cooperating. H. G. Diesslin, Director, West Lafayette, Ind. Issued in furtherance of the Acts of May 8 and June 30, 1914. It is the policy of the Cooperative Extension Service of Purdue University that all persons shall have equal opportunity and access to its programs and facilities without regard to race, religion, color, sex or national origin.