| Three experiments were designed in this study to investigate the effect of feeding level on reproductive performance over a four-parity period using a high-producing sow genotype, protein metabolism and hormone secretion of pregnant and non-pregnant sow. Whole-body protein turnover was measured with the [15N]glycine single-dose end-product method.Trial 1 21 Landrance X Meishan crossbred gilts at 97kg of mean body weight were mated and allocated to four different feeding level (high, NRC, middle, low) randomly. During gestation, sows were raised in automatically weighed cages in the first and the second parity, and in partially slotted concrete-floored pens in the third and the fourth parity, ' and fed with a same diet(DE13.38MJ/kg,CP13.5%), weighed 7:00am every day, as well as providing approximately 2, 1.2, 1, 0.6M(the requirement of maintenance) during early gestation, increasing 20% at the base of prophase during mid pregnancy, and increasing 25% at the base of prophase during late pregnancy. Metabolism experimentation was conducted at 1, 30, 85, 105 d of gestation. The results were as follows:1. The live weight gain of sows is significantly affected by feeding level (PO.05), and live weight gain significantly affects sow reproductive performance (PO.05). The results showed that maternal body gain and total live weight gain in pregnancy is the main factors affecting sow reproductive performance (PO.05), and the early live weight gain has the negative effect on litter size (PO.05), and the late live weight gain significantly affects birth weights (PO.05).2. Feeding level has significantly no effects on sow reproductive performance from the first to the fourth parity(P>0.05). At the high feeding level, the litter size of sows significantly was improved at the second parity, but significantly decreased at the fourth parity(PO.05), and increased individual piglets birth weights at all parities(PO.05). Low feed intake is beneficial to improve litter size, butlower the number of live pigs born, piglets birth weight and survival rate at weaning, delaying the onset of postweaning estrus.3. Feeding level apparently affects energy digestibility(EDR) and metabolic rate (EMR) of pregnant sows. High feed intake apparently increases EDR and EMR (P<0.05), on the contrast, low feed intake level decreases ED and EMR (PO.05). There are little effect with stages and parities (P>0.05). The apparent digestible energy(ADE) requirement for pregnant sow is 15.16 MJ/day at least.. 4. Feeding level has little influence on nitrogen (N) digestibility in pregnant sows (P>0.05). The results showed that the rate of N retention increased linearly with N intake(P<0.05). The fecal nitrogen and urine nitrogen excretion of gilts significantly increase under high feed intake level (P<0.05), there is a declining trend in nitrogen biological value and nitrogen retention rate(P<0.05). However, there are opposite results under NRC, middle and low feed intake level (PO.05). the rate of nitrogen retention increases in NRC and middle feed intake groups , but nitrogen retention declines in late pregnancy in high and low groups. The N retention, N biological value and retention rate of pregnant sows decreases in the second parity.5. According to best litter size, from parity 1 to 3, the optimal total pregnant gain is 39kg, N intake 34.8g/d, N retention 18.6g/d, ADE intake 21.5MJ/d. Gain and nutrients model are: in early pregnancy, gain 6kg, N intake 26.4g/d, N retention 12.87 g/d, ADE intake 16.3MJ/d; in the middle pregnant stage, gain 17kg, N intake 31.7g/d, N retention 13.6g/d, ADE intake 19.6MJ/d, in the late stage, gain 16kg, N intake 46.20g/d, N retention 28.98g/d, ADE intake 28.60MJ/d. According to live birth piglets number and weaned piglets number, from parity 1 to 3, total pregnant gain is 55kg and 9kg in early stage, 24kg in middle stage, and 22kg in late stage, respectively, N intake: 44.2g/d, 34g/d, 36.5g/d and 62g/d, N retention: 22.12 g/d, 14.22 g/d, 15.15 g/d and 34.98g/d, ADE intake: 27.07 MJ/d, 20.9 MJ/d, 22.30 MJ/d, 38.00MJ/d.
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