Effect of feeding a by-product feed-based silage on nutrients intake, apparent digestibility, and nitrogen balance in sheep

The SMS was collected fresh from a local mushroom (Pleurotus eryngii) farm. The original mushroom substrate consisted of 70 % sawdust, 15 % corn cobs, and 15 % rice bran. The by-product feed-based silage was manufactured as described in Kim et al. [1] at the experimental farm of Konkuk University located in Chung-Ju city in Chung-Buk province. The SMS (45 %) was mixed with RPB (21 %), rye straw (11 %), rice bran (10.8 %), corn taffy residue (10 %), protected fat (1.0 %), bentonite (0.6 %), and microbial additive (0.6 %), and ensiled in two folds of polyvinyl bags that were placed in a 40-kg capacity plastic bag for 30 d. The microbial inoculants used in this experiment were isolated and identified previously in our laboratory [2, 3], and included Enterobacter ludwigii KU201-3, Bacillus cereus KU206-3, Bacillus subtilis KU3, Bacillus subtilis KU201-7, Saccharomyces cerevisiae, and Lactobacillus plantarum. The mixture was inoculated with the strains (each added at 0.17 % [v/w] to ensure suppying 1 × 10⁹ cfu/g fresh biomass). Bacillus sp. and Enterobacter sp. were cultured in plate count broth (5 g casein, 2.5 g yeast extract, and 1 g/L dextrose) at 36 °C for 24 h, Saccharomyces sp. was cultured in yeast malt broth (0711, Difco Laboratories Inc., Detroit, MI, USA) at 30 °C for 48 h, and Lactobacillus sp. was cultured in MRS broth (0881, Difco Laboratories Inc.) at 36 °C for 24 h.

All animal care protocols were approved by the Konkuk University Institutional Animal Care and Use Committee. Six crossbred ram lambs (a mean body weight of 55 + 2 kg) were randomly allotted to two dietary treatments: a control diet (formulated concentrate mix + rye straw) and a treatment diet (formulated concentrate mix + BF-based silage). The chemical composition of concentrate mix, rye straw, and BF-based silage is shown in Table 1. The BF-based silage contained about 4-times higher CP and lower neutral detergent fiber (NDF) than rye straw. The formulated concentrate mix was composed of 23 % corn grain, 17.5 % wheat bran, 14 % coconut meal, 10 % wheat grain, 7 % palm meal, 6% rapeseed meal, 5.8 % molasses, 5 % distillers grain, 5% corn gluten feed, 1.5 % wheat flour, 1.4 % limestone, 1.3 % tapioca, 1 % bentonite, 0.5 % dehydrate salt, 0.45 % red clay, 0.25 % calcium phosphate, 0.2 % calcium sulfate, and 0.1 % vitamin-mineral premix. Ingredients and chemical composition of diets fed to sheep are shown in Table 2. Control or treated diet was fed in equal portions in the amount of 850 g dry matter (DM, approximately 1.5% of body weight) at 07:00 and 18:00 h. The amount of diet DM corresponded to the requirements of growing ram lambs [7].

During the experiment, sheep always had free access to fresh water. Diets were randomly assigned to sheep at the start of the trial provided that sheep would not receive the same diet in two consecutive trials. The experiment consisted of 5 d transition, 10 d preliminary, and 7 d collection periods. The number of observations per treatment was 6. During the experiment, the animals were kept in individual metabolism crate (1.6 m × 0.5 m) that permitted separate collection of feces and urine. Daily fecal output during the collection period was dried at 60 °C. Feces were thoroughly mixed at the end of the collection period to obtain a composited sample, which was subsequently ground through a 2 mm screen prior to storage. Daily urine output was collected in plastic bottles containing 15 ml of 13.5 N H₂SO₄. After weighing, 2 % of the urine volume was refrigerated and bulked for the collection period.

On the last day of the experiment (d 7), 2 hours after the morning feeding, rumen fluid (250 ml) was collected with a stomach tube using an electric vacuum pump. The fluid was strained through four layers of cheesecloth prior to the determination of pH (HI9321, Hanna Instrument, Portugal). Each 5 ml of fluid was transferred to a tube containing two drops of concentrated H₂SO₄ and another 5 ml to a tube containing 1 ml of 25 % (w/v) of metaphosphoric acid plus 5 ml isocaproic acid for the determination of NH₃-N and volatile fatty acids (VFA), respectively.

Representative samples of the test feeds that supplied to the sheep were collected and stored at −20 °C. Immediately before the analysis, all the samples were dried and ground to pass through a 1-mm filter using a sample mill (Cemotec, Tecator, Sweden). The DM fraction was quantified by drying the samples at 60°C for 48 h until constant weight. The CP, ether extract (EE), and crude ash contents were determined by the AOAC method [8]. Neutral detergent fiber (NDF, heat-stable α-amylase), acid detergent fiber (ADF) were determined using the method of Van Soest et al. [9]. Ruminal NH₃-N was determined according to Chaney and Marbach [10]. Ruminal VFA was determined according to Erwin et al. [11] using a Trace GC Ultra gas chromatograph (Trace GC ultra, Thermo, Italy).

Data were analyzed as a completely randomized design and subjected to one-way analysis of variance (ANOVA) using the general linear model procedure. Means separation was performed using t-test (Statistix7, 2000) [12] and significance was declared at p ≤ 0.05. Data are presented as least squares means and standard errors of the means.

Ruminal parameters of sheep fed the different diets is shown in Table 3. The ruminal pH of both treatments was 6.6. Ruminal pH above 6.2 means adequate rumen fermentation [13]. Additionally, feeding BF-based silage did not affect ruminal acetate, propionate, and butyrate concentrations, and acetate: propionate ratio in sheep, but increased ruminal NH₃-N (p < 0.05). The NH₃-N is used as a nitrogen source by ruminal microbes which can degrade both nonstructural and structural carbohydrates [14]. It was found that BF-based silage had a higher non-protein nitrogen portion than that of rye straw [1]. In the present study, feeding BF-based silage increased dietary CP intake by 44 %, resulting in 71 % higher ruminal NH₃-N concentration. Adequate ruminal NH₃-N concentrations for ruminal microbial synthesis were in 5 ~ 23 mg/dl [15, 16]. The ruminal NH₃-N concentration for the treated group were in the upper normal range. The increased ruminal NH₃-N could increase ruminal VFA concentration [17], but feeding BF-based silage in the present study did not affect ruminal VFA concentrations.

The total feed intake are shown in Table 4. Compared to feeding rye straw, feeding BF-based silage increased CP, EE, and crude ash intake (p < 0.05), but decreased fiber (NDF, ADF and CF), OM, and nitrogen free extracts (NFE) intake (p < 0.05). Apparent nutrient digestibility and digestible nutrient intake are shown in Table 5. Digestible CP and EE intakes increased and digestible DM, OM, fiber (NDF, ADF, and CF), and NFE intakes decreased by replacing rye straw with BF-based silage (p < 0.05). Compared to control treatment, sheep fed BF-based silage were observed to have 1.8, 2.9 and 8.8 percentage units lower apparent digestibility of DM, NDF, and crude ash, respectively (p < 0.05). It appeared that about 2-fold increase in EE intake induced increased EE digestibility. The decreased DM digestibility by feeding BF-based silage was attributed to the high ash content in BF-based silage. The decreased fiber (NDF, ADF, and crude fiber) digestibility was attributed to the high lignin content in the sawdust-based mushroom substrate included in the BF-based silage [1]. It seems natural that fiber in sawdust and corn cobs constituting major source in the SMS and BF-based silage would be less digested than that in rye straw. Calculated total digestible nutrients (TDN) of both diets did not alter, indicating that BF-based silage was equivalent to rye straw in energy value for sheep. In detail, TDN content of BF-based silage was estimated to be 44.1 % when TDN content of commercial concentrate mix was assumed to be normally 79.0 %.

The N balance of sheep fed the different diets is shown in Table 6. Because of the CP content difference between rye straw (4.2 %) and BF-based silage (16.9 %), N intake significantly increased 6.9 g/d by feeding BF-based silage (p < 0.05). Feeding BF-based silage increased N excretion, absorption and retention by 6, 5.1, and 0.9 g/d (p < 0.05), respectively. But N retention, expressed % intake or % absorbed, was not different between rye straw and BF-based silage, indicating that N retention efficiency was similar between rye straw N and BF-based silage N. Normally with dietary N intake increased, N absorption and total N excretion were increased [18] as shown in this study. The high ruminal NH₃-N could be available mostly as a nitrogen source for ruminal microbial growth, but excessive NH₃-N would be absorbed through the rumen wall or flow into the lower digestive tract and further absorbed, resulting in increased N retention for the BF-based silage-fed sheep.