Use of highfiber, lowcost ingredients, such as coproducts from grain processing industries, in swine diets is increasing. Pigs fed diets containing high levels of fiber have increased intestinal mass due to increased amount of microbial fermentation in the hindgut. Therefore, they also have increased endogenous loss of amino acids in the form of mucins, the proteins that line the intestinal tract. The abrasiveness of fiber stimulates the secretion of mucins as well. These factors may cause the threonine requirement to be increased in high fiber diets, because threonine is present in large amounts in mucins. Therefore, an experiment was conducted to determine the effects of dietary fiber on the optimum threonine:lysine ratio (Thr:Lys) in 25 to 50 kg growing gilts.
Materials and methods
A total of 192 gilts with average initial body weight of 26.29 kg were fed one of 12 diets. The lowfiber basal diet contained approximately 0.40% SID Thr and 0.90% SID Lys. The highfiber basal diet was formulated by adding 15% soybean hulls to the lowfiber basal diet at the expense of corn starch. Ten additional diets were formulated by adding crystalline LThr to each basal diet to create diets containing SID Thr:Lys ratios of 0.45, 0.54, 0.63, 0.72, 0.81, and 0.90. Pigs were weighed at the beginning of the study and on the 28^{th} day, and final body weight, average daily gain (ADG), average daily feed intake (ADFI), and gain:feed ratio (G:F) were calculated.
Brokenline and quadratic analysis were used for each of the growth performance measures. In brokenline analysis, determinations for requirements are defined as the intersection point between the slope and plateau portion of the model. Quadratic analysis fits a quadratic line to the data, and estimates the requirement as the level determined by the apex of the curve. Broken line analysis tends to yield conservative estimates, while quadratic analysis may overestimate requirements. Therefore, a combination of the two types of analyses was used. The concentration of Thr at which the quadratic regression line and the plateau value from the broken line analysis intersected was used to estimate the ideal SID Thr:Lys ratio for those growth performance parameters which differed both linearly and quadratically. The estimated ideals within each fiber level were averaged, and the average was determined to be the ideal SID Thr:Lys ratio.
Growth performance
For pigs fed the lowfiber diets, ADG and G:F increased both linearly (P < 0.01) and quadratically (P < 0.05) as the concentration of Thr increased in the diets (Table 1). Final body weight increased linearly (P < 0.01) as Thr concentration increased. ADFI was not affected by the Thr concentration in the diets.
For pigs fed the highfiber diets, ADG and G:F increased both linearly (P < 0.001) and quadratically (P < 0.05) as the concentration of Thr increased in the diets. Final BW and ADFI increased linearly (P < 0.05) as Thr concentration increased.
Thr:Lys requirement estimates
For pigs fed the lowfiber diets, brokenline analyses estimated the optimum SID Thr:Lys ratio to be 0.60 for ADG (Figure 1) and 0.59 for G:F (Figure 2). Quadratic analyses estimated the optimum SID Thr:Lys ratio to be 0.76 for ADG (Figure 1) and 0.73 for G:F (Figure 2). Combining the brokenline and quadratic analyses yielded an estimated optimum SID Thr:Lys ratio of 0.66 for ADG (Figure 1) and 0.63 for G:F (Figure 2).
For pigs fed the highfiber diets, brokenline analyses estimated the optimum SID Thr:Lys ratio to be 0.66 for ADG (Figure 3) and 0.55 for G:F (Figure 4). Quadratic analyses estimated the optimum SID Thr:Lys ratio to be 0.80 for ADG (Figure 3) and 0.75 for G:F (Figure 4). Combining the brokenline and quadratic analyses yielded an estimated optimum SID Thr:Lys ratio of 0.71 for ADG (Figure 3) and 0.63 for G:F (Figure 4).
Key points
 For 25 to 50 kg growing gilts fed highfiber diets, ADG will be optimized with a Thr:Lys ratio is 0.71.
 For 25 to 50 kg growing gilts fed lowfiber diets, the ideal Thr:Lys ratio to optimize ADG is 0.66.
 The ideal Thr:Lys ratio to optimize G:F in 25 to 50 kg growing gilts is 0.63, regardless of fiber content of the diets.
Table 1. Growth performance of pigs fed experimental diets

Thr:Lys 

Contrasts (Pvalue) 


0.45 
0.54 
0.63 
0.72 
0.81 
0.9 
SEM 
Linear 
Quadratic 
Low Fiber 


Initial BW, kg 
26.16 
26.68 
26.68 
25.73 
26.73 
26.39 
0.8 
0.963 
0.969 
ADG, g 
696 
769 
797 
830 
836 
803 
28.9 
<0.01 
<0.05 
ADFI, g 
1,785 
1,799 
1,777 
1,812 
1,862 
1,830 
102 
0.376 
0.917 
G:F 
382 
423 
450 
455 
445 
435 
20.9 
0.001 
<0.001 
Final BW, kg 
45.66 
49.43 
49.56 
49.5 
50.13 
50.36 
1.15 
<0.01 
0.132 
High Fiber 

Initial BW, kg 
26.47 
26.85 
26.72 
26.57 
26.28 
26.72 
1.31 
0.924 
0.964 
ADG, g 
763 
882 
878 
900 
933 
915 
35.48 
<0.001 
<0.05 
ADFI, g 
1,828 
1,872 
1,835 
1,864 
1,945 
1,989 
64.9 
<0.05 
0.409 
G:F 
421 
461 
465 
472 
470 
464 
12.91 
<0.01 
<0.01 
Final BW, kg 
48.14 
51.86 
51.67 
52.08 
52.7 
52.64 
1.61 
<0.05 
0.145 
Figure 1. Fitted linear breakpoint and quadratic plots of ADG as a function of standardized ileal digestible Thr to Lys ratio with observed treatment means in pigs fed low fiber diets.
Figure 2. Fitted linear breakpoint and quadratic plots of G:F as a function of standardized ileal digestible Thr to Lys ratio with observed treatment means in pigs fed low fiber diets.
Figure 3. Fitted linear breakpoint and quadratic plots of ADG as a function of standardized ileal digestible Thr to Lys ratio with observed treatment means in pigs fed high fiber diets.
Figure 4. Fitted linear breakpoint and quadratic plots of G:F as a function of standardized ileal digestible Thr to Lys ratio with observed treatment means in pigs fed high fiber diets.