| Six experiments were conducted to study the optimal standardized ileal digestible (SID) valine (Val) to lysine (Lys) ratios for growing and finishing pigs as well as its regulatory effects on feed intake. The first 4 experimments were conducted to determine the SID Val to Lys ratio required for 25 to 50 kg (Exp. 1),50 to 70 kg (Exp.2),70 to 90 kg (Exp.3) and 90 to 120 kg (Exp.4) pigs fed low crude protein diets supplemented with crystalline amino acid. The first 3 experiments utilized 150 pigs (Duroc x Landrace x Large White), while Exp.4 utilized 90 finishing pigs. Pigs in all 4 experiments were randomly allocated to 1 of 5 diets with 6 pens per treatment (3 pens of barrows and 3 pens of gilts) and 5 pigs per pen for the first 3 experiments and 3 pigs per pen for Exp.4. Diets for all experiments were formulated to contain SID Val to Lys ratios of 0.55,0.60,0.65,0.70 or 0.75. In Exp.1 (25 to 50 kg), average daily gain (ADG) increased (linear and quadratic, P< 0.05) with increasing dietary SID Val:Lys ratio. The SID Val:Lys ratio to maximize ADG was 0.62 using a linear broken-line model and 0.71 using a quadratic model. In Exp. 2 (50 to 70 kg), ADG increased (linear and quadratic, P< 0.05) as the SID Val:Lys ratio increased. Serum urea nitrogen (SUN) decreased (linear and quadratic, P< 0.05) with an increased SID Val:Lys ratio. The SID Val:Lys ratios to maximize ADG as well as to minimize SUN levels were 0.67 and 0.65 using a linear broken-line model and 0.72 and 0.71 using a quadratic model, respectively. In Exp.3 (70 to 90 kg), ADG increased (linear and quadratic, P< 0.05) and SUN decreased (linear and quadratic, P< 0.05) as dietary SID Val:Lys ratio increased. The SID Val:Lys ratios to maximize ADG as well as to minimize SUN levels were 0.67 and 0.67 using a linear broken-line model and 0.72 and 0.74 using a quadratic model, respectively. In Exp.4 (90 to 120 kg), Gain:Feed (G:F) was improved (linear and quadratic, P< 0.05) as dietary SID Val:Lys ratio increased. The SID Val:Lys ratio to maximize G:F was 0.68 using a linear broken-line model and 0.72 using a quadratic model. In Exp.5,18 piglets, with an initial body weight (BW) of 18.9 ± 2.3 kg, were divided into 3 treatments with 6 replicates per treatment to study the effects of SID Val:Lys ratios on feed intake and the expression of regulatory factors of pigs. Diets were formulated to contain SID Val to Lys ratios of 0.45,0.65 and 0.85. The experiment lasted for 14 d. Average daily feed intake (ADFI) increased (linear and quadratic, P< 0.05) by increasing dietary SID Val to Lys ratio. Compared with the control treatment, the expression of gastral cholecystokinin (P< 0.05) was lower in pigs fed diet of SID Val:Lys ratio of 0.85. The expression of hypothalamic pro-opiomelanocortin was lower (P< 0.05) in pigs fed diet of SID Val:Lys ratio of 0.65 than the control treatment. In Exp.6,6 pigs, with an initial BW of 15.7 ± 0.8 kg, were divided into 3 groups to study the effect of SID Val:Lys ratios on the postprandial plasma amino acid concentrations of pigs. The SID Val:Lys ratios were set the same as in Exp.5. Pigs were cathetered in the precaval vein. Results showed that postprandial plasma Val concentration was higher in pigs fed diets of SID Val;Lys ratios of 0.65 and 0.85 than the control treatment at 1,2,3 or 4 h after meal. In conclusion, the dietary SID Val:Lys ratios required for 25 to 50 kg,50 to 70 kg,70 to 90 kg and 90 to 120 kg pigs were estimated to be 0.62,0.66,0.67 and 0.68 using a linear broken-line model and 0.71,072,0.73 and 0.72 using a quadratic model. Diets supplemented with crystalline Val can increase the feed intake of piglets. The regulatory mechanisms may be that dietary crystalline Val increased the plasma Val content and decreased the cholecystokinin and pro-opiomelanocortin mRNA expression. |
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