Comparison of Ruminal Degradability, Indigestible Neutral Detergent Fiber, and Total-Tract Digestibility of Three Main Crop Straws with Alfalfa Hay and Corn Silage

Abstract

Three main crop straws including corn straw (Zea mays, CS), rice straw (Oryza sativa, RS), and wheat straw (Triticum aestivum, WS), and two forages including alfalfa hay (Medicago sativa, AH) and corn silage (Zea mays, CSil) were analyzed in order to compare their ruminal degradability, indigestible neutral detergent fiber (iNDF), intestinal digestibility (ID), and their total-tract digestibility (TTD) of crude protein (CP), neutral detergent fiber (NDF), and acid detergent fiber (ADF) using both an in situ nylon bag technique and a mobile nylon bag technique. The forage samples were incubated in the rumen for 6, 12, 16, 24, 36, 48, 72, and 288 h, respectively, to determine their ruminal degradability. Prior to intestinal incubation, forage samples were incubated in the rumen for 12 h and 24 h to determine the ruminal degradable content of CP, NDF, and ADF, respectively, and for 288 h to determine their iNDF₂₈₈ content. Residues from the ruminal undegradable fractions (12 h for CP, 24 h for NDF and ADF) were subsequently inserted into the duodenum through a cannula to determine their intestinal digestible content. Here, the TTD of CP, NDF, and ADF were determined as the ruminal degradable content + intestinal digestible content. The results showed that AH had the highest iNDF_2.4 (calculated as acid detergent lignin content × 2.4) and iNDF₂₈₈ values (379.42 and 473.40 g/kg of NDF), while CS and CSil had the lowest iNDF_2.4 values (177.44 and 179.43 g/kg of NDF). The ruminal degradability of CP, NDF, and ADF for CS, RS, and WS were lower than those of AH and Csil during the first 48 h of incubation. The potential degradation fraction of CP, NDF, and ADF for CSil was the highest; CS, RS, and WS were intermediate; and AH was the lowest (p < 0.05). CS, RS, and WS had a lower intestinal digestibility with respect to their rumen undegradable content of NDF (p < 0.05), and lower TTD of CP, NDF, and ADF (p < 0.05) compared to AH and CSil. General regression equations with satisfactory accuracy (R² ≥ 0.828) were derived to predict iNDF₂₈₈ and TTD based on their chemical compositions and the ruminal degradation kinetics of different forages. Incorporating this information into rations could improve our ability to optimize main crop straws utilization and milk production.

Keywords: indigestible neutral detergent fiber; main crop straws; ruminal degradability; total-tract digestibility.

Figure 1

The real-time degradability of various forages during 288 h in situ incubation. (A) CP, (B) NDF, and (C) ADF. Data are expressed as means ± standard deviation. p-values of Forage Type, Time, and Forage Type × Time mean the effect of forage types, the effect of incubation times, and the interaction effect of forage types and incubation times. Data were fitted to a one-phase association exponential model with three parameters: y = a + b × [1 − exp (−c × t)], where y is the ruminal degradation of CP, NDF, and ADF at time t, a is the soluble or rapidly degradable fraction (g/kg); b is the potentially degradable fraction (g/kg); c is the constant rate of degradation of b (%/h); t is the time of incubation (h). Regression, p < 0.05; R² for CS, RS, WS, AH, and CSil = 0.93 to 0.99, as shown in the graph. CS, corn straw; RS, rice straw; WS, wheat straw; AH, alfalfa hay; Csil, corn silage; DM, dry matter; CP, crude protein; NDF, neutral detergent fiber; ADF, acid detergent fiber. Each forage was incubated in 6 replicates (two for each time point per cow) in the rumen.

Figure 2

Ruminal degradable content, intestinal digestible content, and total-tract undigested content of CP, and NDF, and ADF in different forages. (A) CP, (B) NDF, and (C) ADF. RD, ruminal degradable content was determined after ruminal incubation for 12 h (CP) and 24 h (NDF and ADF); ID, intestinal digestible content; TTUD, total-tract undigested content. CS, corn straw; RS, rice straw; WS, wheat straw; AH, alfalfa hay; Csil, corn silage; DM, dry matter; CP, crude protein; NDF, neutral detergent fiber; ADF, acid detergent fiber. Each forage was incubated in 6 replicates (two for each time point per cow) in the rumen and 12 replicates for intestinal incubation.