Effects of high neutral detergent fiber diets on the N-balance of pigs fed Thr-limited diets

An experiment was conducted to test the hypothesis that increased levels of fiber in diets fed to pigs would reduce N-balance, but be ameliorated with increased Thr. A total of 96 gilts (initial BW: 28.98 ± 2.0 kg) were housed in individual metabolism crates allowing for total and separate collection of feces and urine. Pigs were allotted to 12 diets [0.90% standardized ileal digestible (SID) Lys] with 8 replicate pigs per diet using a randomized complete block design. Diets were prepared in a 3 × 4 factorial arrangement with 3 levels of fiber [(6, 11, or 15% neutral detergent fiber (NDF)] added as wheat middlings and distiller’s dried grains with solubles and 4 SID Thr:Lys concentrations (0.45, 0.50, 0.55, or 0.60). Pigs were fed experimental diets for 14d, including a 7d adaptation period and subsequent 5d urine and fecal collection period. Apparent total tract digestibility (ATTD), retention, and biological value (BV) of N were greatest (P < 0.05) for pigs fed diets containing 6% NDF. Retention and BV of N were greater (P < 0.05) in pigs fed diets with SID Thr:Lys ratios of 0.50, 0.55, and 0.60 than in pigs fed diets with SID Thr:Lys ratios of 0.45. Results indicate that the optimal SID Thr:Lys may increase with increased dietary fiber. Nitrogen balance in pigs fed diets containing 11% or more NDF may be increased with SID Thr:Lys ratios greater than 0.60.

Table 1. Nitrogen balance of pigs fed diets of varying Thr and fiber concentrations

	6% NDF					11% NDF					15% NDF					P-value
	0.45 SID Thr:Lys	0.50 SID Thr:Lys	0.55 SID Thr:Lys	0.60 SID Thr:Lys		0.45 SID Thr:Lys	0.50 SID Thr:Lys	0.55 SID Thr:Lys	0.60 SID Thr:Lys		0.45 SID Thr:Lys	0.50 SID Thr:Lys	0.55 SID Thr:Lys	0.60 SID Thr:Lys	Pooled SEM	Fiber level	Thr level	Fiber level × Thr level
Fecal N output, g/5 d	149	140	123	154		214	205	217	203		248	274	250	253	11	<0.0001	0.742	0.341
Urinary N output, g/5 d	135	118	105	92		175	114	112	108		154	127	114	100	36	0.344	<0.001	0.848
ATTD of N, %	82.7	84.3	85.4	82.1		73.8	77.1	75.9	77.3		72.3	72.7	74.0	71.4	1.5	<0.0001	0.046	0.237
N retention, %	67.0	71.8	73.0	71.5		51.7	64.5	63.6	65.1		55.2	58.6	62.0	60.1	4.1	<0.0001	<0.0001	0.200
BV, %	81.1	84.8	85.5	87.1		70.4	83.5	83.8	84.3		76.3	80.6	83.8	84.0	5.3	0.036	<0.0001	0.442