Background:Two studies were conducted to evaluate the effects of dietary natural betaine on sow reproductive performance during summer (Exp. 1) and non-summer months (Exp. 2). Treatments were designed as a 2 × 2 factorial arrangement with factors including dietary betaine (0 or 0.2％) and period of supplementation (lactation or post-weaning until 35 days post-insemination). In Exp. 1, 322 and 327 sows and in Exp. 2, 300 and 327 sows representing young (parity 1 and 2) and mature (parity 3 to 6) sows, respectively, were used. Results:In Exp. 1, supplementation of betaine during lactation increased sow body weight losses (?11.95 vs.?14.63 kg;P=0.024), reduced feed intake (4.12 vs. 4.28 kg/d;P=0.052), and tended to reduce percentage of no-value pigs (P=0.071). Betaine fed post-weaning reduced weaning-to-estrus interval (5.75 vs. 6.68 days;P=0.054) and farrowing rate (86.74％vs. 91.36％;P=0.060), regardless of parity group. Post-hoc analysis with sows clustered into 3 parity groups (1, 2 and 3, and 4+) indicated that betaine fed in lactation to parity 4+sows (P=0.026) and betaine fed post-weaning to parity 1 sows increased the number of pigs born in the subsequent cycle (P≤0.05). In Exp. 2, betaine fed during lactation tended to reduce the weaning-to-estrus interval (6.64 vs. 7.50 days;P=0.077) and farrowing rate (88.23％vs. 83.54％;P=0.089), regardless of parity group. Feeding betaine post-weaning reduced number of pigs born (13.00 vs. 13.64;P=0.04) and pigs born alive (12.30 vs. 12.82;P=0.075), regardless of parity group. Conclusions:Using 0.2％betaine during the non-summer months did not benefit sow performance. During the summer, betaine supplementation in lactation increased subsequent litter size in parity 4+sows. Betaine fed during the post-weaning period reduced the wean-to-estrus interval and farrowing rate, increased total number of pigs born for parity 1 sows and reduced total number of pigs born to parity 4+sows. Further research is needed to determine if the detrimental effects on feed intake and farrowing rate may be correlated and depend on dietary betaine level.