The experimental procedures involving animals for the current study were carried out in accordance with the Chinese Guidelines for Animal Welfare and Experimental Protocol, and were approved by the Animal Care and Use Committee of Wuhan Polytechnic University. Twenty-four crossbred healthy female piglets (Duroc × Landrace × Yorkshire) reared by sows were weaned at 7 ± 2 days of age and were housed in a temperature-controlled nursery barn (28–30 °C). After a 5-day period of adaptation, piglets (12 ± 2 days of age, average body weight of 3.44 ± 0.28 kg; d 0 of the trial) were randomly allotted to one of three treatment groups: control, β-CG, and β-CG + NAC groups. Each treatment had eight piglets. Piglets were fed a liquid diet that contained milk replacer (containing hydrolysed wheat protein rather than soybean protein) and casein as protein sources (Table (Table1).1). The liquid diet was formulated according to the ratio of diet/water (1:4)22. During the 5-day period of adaptation, all piglets were fed a liquid diet containing 10% casein. At day 0 of the trial, piglets in the control group were continuously fed the same diet, whereas those in the β-CG and β-CG + NAC groups were fed a liquid diet containing 9.5% casein and 0.5% β-CG for 2 days. At day 2 of the trial, pigs in the β-CG + NAC group were orally administrated with 50 mg (kg BW)−1 NAC, while pigs in the other two groups were orally administrated with the same volume of sterile saline for 3 days. In order to exclude the possible effects of β-CG-induced food intake reduction on the intestinal indices of piglets, the control and β-CG + NAC piglets were fed the same amounts of liquid diet as the β-CG piglets. The dosage of β-CG was chosen according to the result of our preliminary study in which the piglets fed a liquid diet containing 1% or 2% β-CG exhibited severe diarrhea and a depression of feed intake, while those fed a liquid diet containing 0.5% β-CG showed a slight reduction in feed intake and a moderate increase in diarrhea incidence. The dosage of NAC was chosen in the present study in accordance with our previous work22. The current experiment lasted for 5 days.

Ingredient composition and nutrient levels of milk replacer powder diets (as-fed basis).

aMilk replacer contained hydrolyzed wheat protein rather than soybean protein.

bPremix provided the following vitamins and trace minerals per kilogram of milk replacer powder diet: Fe, 100 mg; Cu, 20 mg; Mn, 40 mg; Zn, 100 mg; I, 0.4 mg; Se, 0.3 mg; retinol acetate, 3.6 mg; cholecalciferol, 0.05 mg; DL-α-tocopheryl acetate, 30 mg; Menadione, 3 mg; thiamin, 2 mg; riboflavin, 6 mg; pyridoxine, 7 mg; Cyanocobalamin, 0.04 mg; biotin, 0.1 mg; Folic acid, 2 mg; niacin, 40 mg; D-calcium pantothenate, 20 mg.

cAnalyzed value.

Body weights of piglets were recorded every day and the incidence of diarrhea was observed 3 times per day. Fecal scores were determined according to stool conditions: 0, firm and shaped; 1, pasty; 2, semiliquid; and 3, liquid54. The occurrence of diarrhea was defined as the maintenance of a fecal score at 2 or 3 for 2 consecutive days. Then, the incidence of diarrhea was calculated according to the formula as reported by Liu et al. (2008): incidence of diarrhea = total number of pigs with diarrhea/(total number of pigs × experimental days) × 100%, where “total number of pigs with diarrhea” was defined as the sum of the number of pigs with diarrhoea observed each day55.

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