ABSTRACT: This study investigated follicular de- velopment during and after postweaning altrenogest treatment of primiparous sows in relation to subse- quent reproductive performance. Primiparous sows (n = 259) were randomly assigned at weaning (d 0) to 1 of 4 groups: control (no altrenogest, n = 71), RU4 (20 mg of altrenogest from d -1 to 2, n = 62), RU8 (20 mg of altrenogest from d -1 to 6, n = 65), or RU15 (20 mg of altrenogest from d -1 to 13, n = 61). Average follicular size (measured by ultrasound) increased dur- ing altrenogest treatment and resulted in larger follicles at the start of the follicular phase for RU4, RU8, and RU15 compared with controls (5.3 ± 0.9, 5.5 ± 1.3, 5.1 ± 1.2, and 3.4 ± 0.6 mm, respectively; P < 0.0001). Farrowing rate was greater in RU15 (95%) than in RU8 (76%; P = 0.04). The RU15 group also had more pig- lets (2 to 3 more piglets total born and born alive; P < 0.05) than the other treatment groups. Follicular development at weaning clearly affected reproductive performance. At weaning, average follicular size: small (<3.5 mm), medium (3.5 to 4.5 mm), or large (≥4.5 mm), was associated with farrowing rates of 86, 78, and 48%, respectively (P < 0.001). Sows with large fol- licles at weaning had low farrowing rates (71%) in RU4, very low farrowing rates (22%) in RU8, but normal far- rowing rates in RU15 (83%). In conclusion, this study showed that 15 d of postweaning altrenogest treatment of primiparous sows may allow follicle turnover in sows that had large follicles at weaning and that this was associated with an improved reproductive performance. It also showed that shorter treatment with altrenogest (4 or 8 d) is beneficial for sows with small follicles at weaning, but is not recommendable for sows with large follicles at weaning. Key words: altrenogest, follicular development, litter size, pregnancy rate, sow ©2011 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2011. 89:397–403 doi:10.2527/jas.2010-3246 INTRODUCTION A negative energy balance during first lactation af- fects subsequent follicular development (Quesnel et al., 2000), ovulation rate (Hazeleger et al., 2005), oocyte development (Zak et al., 1997b), and embryo develop- ment (Algriany et al., 2004), and ultimately leads to embryo mortality (Almeida et al., 2000). As a result, primiparous sows commonly have increased intervals from weaning to estrus, reduced farrowing rates, and reduced litter sizes (Morrow et al., 1989). Administering an orally active progestagen from weaning onward improves reproductive performance postweaning. Several studies have found greater ovula- tion rates (Koutsotheodoros et al., 1998), better syn- chronization of estrus (Martinat-Botté et al., 1994; Martinat-Botté, 1995), and improved embryo survival (Patterson et al., 2008) for primiparous sows treated with altrenogest postweaning. These positive effects are probably related to an improvement in follicular development. In a previous study, altrenogest-treated, primiparous sows were found to have larger follicles at the start of the follicular phase than nontreated con- The effect of different postweaning altrenogest treatments of primiparous sows on follicular development, pregnancy rates, and litter sizes 1 J. J. J. van Leeuwen,* S. I. Williams,† M. R. T. M. Martens,‡ J. Jourquin,§ M. A. Driancourt,# B. Kemp,* and N. M. Soede* 2 *Adaptation Physiology Group, Department of Animal Sciences, Wageningen University, PO Box 338, 6700AH, Wageningen, the Netherlands; †Facultad de Ciencias Veterinarias, Universidad Nacional de La Plata, PO Box 296, La Plata, Argentina; ‡Intervet/Schering Plough Animal Health, PO Box 50, 5830 AB Boxmeer, the Netherlands; §Janssen Animal Health, Turnhoutseweg 30, B-2340 Beerse, Belgium; and #Intervet/Schering Plough Animal Health, R&D Pharmaceuticals, BP 67131, 49071 Beaucouze, France 1 The authors thank Intervet/Schering Plough Animal Health (Boxmeer, the Netherlands) and Janssen Animal Health (Beerse, Belgium) for their financial support, Isowean S. A. (Monte Buey, Cordoba, Argentina) and its employees for their participation in this study, Pablo Villafane (private practice, Lincoln, Buenos Aires, Argentina) for finding the swine facilities, student Anniek Peek for her help, and Intervet SPAH Brasi (São Paulo, Brazil) for supplying the Regumate. 2 Corresponding author: Nicoline.Soede@wur.nl Received June 17, 2010. Accepted October 8, 2010. 397 Published December 4, 2014