The effects of supplemental dietary threonine (Thr) on laying performance, expression of intestinal mucin 2 (MUC2) and secretory IgA (sIgA), and intestinal microbiota of laying hens fed a low CP diet were investigated. A total of 240 Lohmann Brown laying hens, 28 wk of age, was allocated to 3 dietary treatments, each of which included 5 replicates of 16 hens. Hens were fed a control diet (16% CP), a low CP diet (14% CP), or a low CP diet supplemented with 0.3% L-Thr for 12 weeks. Chemical analyses of the diets for Thr are 0.49, 0.45, and 0.69%, respectively. Lowering dietary CP impaired egg production and egg mass of laying hens. Dietary Thr supplementation to the low CP diet increased (P < 0.05) egg production and egg mass. In addition, ileal sIgA contents and MUC2 and sIgA mRNA expression were increased (P < 0.05) by dietary Thr addition. Dietary CP reduction reduced (P < 0.05) intestinal bacterial diversity, whereas dietary Thr supplementation to the low CP diet recovered the bacteria diversity and significantly increased the abundance of potential beneficial bacteria. In conclusion, dietary Thr supplementation to a low CP diet could affect intestinal health and hence productivity via regulating intestinal mucin and sIgA expression, and microbial population of laying hens.

In addition to being an essential amino acid in protein synthesis, threonine is an integral component of gut function. To verify the effects of l-threonine on gut function, Babcock Brown layers (n = 960; 40 wk of age) were allocated to 5 dietary treatment groups, each of which included 6 replicates of 32 hens. Each group received the same basal diet formulated with corn, peanut meal, and crystalline amino acids. l-Threonine was added to the basal diet at 0 (control), 0.1, 0.2, 0.3, and 0.4% for 8 wk to achieve 0.47 (NRC), 0.57, 0.67, 0.77, and 0.87% threonine, respectively. Expressions of jejunal and ileal mucin 2 mRNA were increased linearly by increasing l-threonine (P < 0.01). At 0.4% l-threonine, the concentrations of IgA antibody in the mucosa of the ileum increased linearly (P < 0.01). No differences attributable to treatment were found among groups in the activity of digestive enzymes in the jejunum or ileum. It was concluded that dietary threonine requirements as reported in current NRC recommendations are insufficient for modern commercial laying hens raised in summer climates. The results suggest that threonine might function as a nutrient immunomodulator in maintaining intestinal barrier function.

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The effects of chitooligosaccharide (COS) supplementation in laying hen diets on egg production, egg quality, blood biochemistry, antioxidant capacity and immunity during the late laying period were investigated in this 10-week trial. A total of 3000 Fengda No.1 laying hens, 52 weeks of age, were randomly allocated to three treatment groups, each of which included five replicates of 200 hens. Treatments consisted of the basal diet only or the basal diet supplemented with COS at 75 or 125 mg/kg. Hens fed with both COS supplemented diets had improved hen-day egg production, egg mass and feed conversion ratio than control birds. The higher level of COS supplementation exerted positive effects on eggshell strength and eggshell thickness, but significantly decreased yolk colour. Addition of 125 mg/kg COS was also found to significantly increase serum albumin content and significantly decrease serum cholesterol level. Dietary supplementation with COS significantly decreased serum triglyceride level compared with birds fed the control diet. Moreover, COS significantly increased total antioxidative capacity and significantly decreased malondialdehyde level in serum, but had no significance on the activities of antioxidant enzymes. The only change in blood immune response compounds observed in this study was a significantly increased concentration of C3 when laying hens were fed COS supplemented diets. In conclusion, COS could improve laying performance and egg quality in hens during the late laying period. Inclusion of COS in the diet might be beneficial to hens' health by lowering serum lipids, enhancing antioxidant activity and slightly enhancing immune ability. HIGHLIGHTS COS improved laying performance and egg quality in hens during the late laying period. Inclusion of COS in the diet might be beneficial to hens' health by lowering serum lipids, enhancing the antioxidant activity and slightly enhancing immune ability. The optimum concentration of COS is 75 mg/kg in the basal diet in order to achieve an increased egg production, egg quality and health condition.

Many countries and cities around the world are located in humid subtropical areas, such as the south of China. Summers in these areas are very hot and humid, with daily temperature averages above 30°C and 75% RH. It has been reported that high temperatures reduce feed intake, egg production, egg weight, and egg quality [1][2][3]. The detrimental effects of high temper- SUMMARYBabcock Brown layers (n = 960), 40 wk of age, were allocated to 1 of 5 dietary treatments groups, each of which included 6 replicates of 32 hens. Each group received the same basal diet formulated with corn, peanut meal, and crystalline amino acids for 8 wk. l-Threonine was added to the basal diet at 0 (control), 0.1, 0.2, 0.3, and 0.4%, to achieve 0.47 (NRC, 1994), 0.57, 0.67, 0.77, and 0.87% threonine, respectively. Although supplementing the diet with lthreonine did not affect ADFI, FCR, egg weight, or egg quality (P > 0.05), the egg production response to supplemental l-threonine was quadratic, and it was maximized at 0.2% supplemental l-threonine. No differences were observed for uric acid, lactate dehydrogenase, alkaline phosphatase, Ca, and P concentrations among the treatments. Serum total protein concentration increased quadratically to supplemental threonine, and the response was maximized at 0.2 and 0.3% supplemental l-threonine. Serum free threonine increased quadratically as supplemental threonine increased, and the response was maximized between 0.2 and 0.3% supplemental lthreonine. The addition of l-threonine at 0.3% of the diet resulted in linearly increasing levels of IgG and total Ig (P < 0.05) as compared with those of the control group. In conclusion, current NRC (1994) recommendations for dietary threonine are insufficient for modern commercial laying hens strains reared under subtropical summer climates. We suggest that 0.2% threonine resulted in optimal egg production, whereas 0.3% l-threonine may have had a positive effect on the humoral immune response of laying hens under conditions of high temperature and humidity.

1. Tryptophan (Trp), besides its role as an essential amino acid in protein synthesis, may also have other important effects on laying hens under summer conditions. 2. Babcock Brown layers (n = 768), 40 weeks of age, were allocated to 4 treatment groups, each of which included 6 replicates of 32 hens. Each group received the same basal diet, formulated with maize and soybean meal, for 8 weeks. Hens were fed on the basal diet with 0·0, 0·2, 0·4, and 0·8 g/kg L-Trp to achieve dietary concentrations of 1·7, 1·9 g/kg, 2·1 g/kg or 2·5 g/kg of Trp, respectively. 3. Supplementing L-Trp had no affect on laying performance. Adding 0·2 or 0·4 g/kg L-Trp improved egg shell strength compared with those fed on the control diet. Serum albumin concentration increased at 0·4 g/kg compared with those receiving 0·0 or 0·8 g/kg Trp. The addition of Trp at 0·4 g/kg increased serum IgM concentration quadratically. Serum superoxide dismutase (T-SOD) increased linearly and quadratically at 0·4 g/kg. 4. In conclusion, we suggest that 0·2 to 0·4 g/kg Trp may have beneficial effects on laying hens under conditions of high temperature and humidity.

Threonine (Thr) may be a limiting amino acid for laying hens fed diets with lowered protein level. An experiment was conducted to examine laying performance, and the intestinal immune function of laying hens provided diets varying in digestible Thr levels. Lohmann Brown laying hens (n = 480), 28 weeks of age, were allocated to six dietary treatments, each of which included five replicates of 16 hens. Dietary crude protein (CP) 16.18% diet was offered as the positive control diet. L-Thr was added to the negative diet (14.16% CP) by 0, 1.0, 2.0, 3.0 and 4.0 g/kg, corresponding 0.44%, 0.43%, 0.49%, 0.57%, 0.66% and 0.74% digestible Thr. At 40 weeks, a reduction in CP level decreased laying performance (p < 0.05). In the low CP, increasing dietary Thr increased (p < 0.05) egg production and egg mass and rose to a plateau between 0.57% and 0.66%. The hens fed 0.66% Thr showed the lowest value (p < 0.05) of feed conversion ratio (FCR). Serum level of uric acid showed the lowest values (p < 0.05) at 0.57-0.66%. In addition, serum-free Thr maximized (p < 0.05) between 0.66% and 0.74%. Digestive trypsin activity decreased (p < 0.05) when hens fed the low-CP diet compared with hens fed CP (16.18%) and hens fed 0.57-0.66%. Expressions of ileal MUC2 mRNA maximized (p < 0.05) at 0.66% Thr. Occludin mRNA increased with increasing Thr level (p < 0.05). sIgA mRNA reached to the maximum level (p < 0.05) at 0.66% and 0.74% Thr. INF-γ mRNA reached to the lowest level (p < 0.05) at 0.65%. Expressions of ileal IL-2, IL-6, IL-1β mRNA decreased with increasing Thr level (p < 0.05). In conclusion, Thr supplementation resulting in optimal laying performance and stimulated the mucosal immune system, suggesting that it is a limiting amino acid in the low-crude-protein diet of laying hens during the peak production period.

A total of 240 Cobb 500 broiler chicks, 18 days old, were allocated to 10 treatments groups, each of which included 4 replicates. Experimental treatments consisted of a 5×2 factorial arrangement with 5 levels of L-threonine (Thr) supplementation and 2 levels of stocking density (11.90 birds mG 2 as the normal stocking density or 16.66 birds mG 2 as the high stocking density). L-Thr was added to a basal diet at 0.0 (control group), 0.25, 0.50, 0.75 and 1.00 g kgG 1 diet. Dietary Thr values were 0.69, 0.71 (deficient), 0.74 (adequate; 100% NRC), 0.76 and 0. 79% (excess) of the current NRC recommendations. At 42 days of age, dietary Thr levels had no significant effect on performance, immunity, plasma total protein and glucose (GLU). However, plasma cholesterol (CHO) levels decreased significantly (p<0.05) with L-Thr supplementation and the lowest value occurred at 0.74% total dietary Thr. In addition, plasma triiodothyronine (T3) and thyroxine (T4) levels were higher (p<0.05) at 0.76% total dietary Thr compared with the control group. The normal stocking density resulted in better performance (p<0.05) compared with the high stocking density. However, stocking density did not affect plasma total protein, total Ig, IgG, IgM, total lipids, GLU, CHO, T3 and T4 levels. Interactions between Thr level and stocking density were observed for plasma levels of total lipids and cholesterol. In conclusion, total dietary Thr level at 0.69% (93% of NRC recommendations) is sufficient to improve broiler performance or immunity under high stocking density. However, 0.74% total dietary Thr level has a positive effect on decreasing plasma total lipids and cholesterol levels during growing-finishing phase.