Effects of Dietary Threonine Level on Growth Performance, Carcass Quality and Immune Function of Yellow-Feathered Broilers from 43 to 63 Days of Age

LIN Xiajing; XI Pengbin; GOU Zhongyong; CHENG Zhonggang; RUAN Dong; ZHENG Chuntian; JIANG Shouqun

doi:10.12418/CJAN2023.147

2023 , Vol. 35 >Issue 3: 1552 - 1560

DOI: https://doi.org/10.12418/CJAN2023.147

RESEARCH PAPER

Effects of Dietary Threonine Level on Growth Performance, Carcass Quality and Immune Function of Yellow-Feathered Broilers from 43 to 63 Days of Age

LIN Xiajing ,
XI Pengbin ,
GOU Zhongyong ,
CHENG Zhonggang ,
RUAN Dong ,
ZHENG Chuntian ,
JIANG Shouqun ^,^*

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Guangdong Provincial Key Laboratory of Animal Breeding and Nutrition, Key Laboratory of Animal Nutrition and Feed Science in South China, Ministry of Agriculture and Rural Affairs, State Key Laboratory of Livestock and Poultry Breeding, Institute of Animal Science, Guangdong Academy of Agricultural Sciences, Guangzhou 510640, China

*professor, E-mail: jsqun3100@sohu.com

Received date: 2022-09-22

Online published: 2023-03-16

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Abstract

This experiment was conducted to investigate the effects of dietary threonine level on growth performance, carcass quality, fat metabolism and immune function of yellow-feathered broilers from 43 to 63 days, so as to determine the optimal threonine requirement. One thousand and six hundred 43-day-old Lingnan yellow-feathered broilers were selected and randomly divided into 5 groups with 8 replicates in each group and 40 chickens in each replicate. The chickens in the five groups were fed the diets containing 0.50% (basal diet), 0.57%, 0.64%, 0.71% and 0.78% threonine, respectively. The experiment lasted for 21 days. The results showed as follows: 1) dietary threonine level could significantly affect the ratio of feed to gain of yellow-feathered broilers from 43 to 63 days of age (P<0.05), and the ratio of feed to gain in 0.57%, 0.64%, 0.71% and 0.78% threonine groups was significantly lower than that in 0.50% threonine group (P<0.05), but had no significant effects on the average daily gain and average daily feed intake (P>0.05). There was a significant quadratic correlation between dietary threonine level and the ratio of feed to gain (P<0.05). According to the regression equation of the ratio of feed to gain, the optimal dietary threonine level for yellow-feathered broilers was 0.68%, the daily threonine requirement was 0.61 g, and the ratios of threonine to lysine, methionine and tryptophan were 0.76, 1.94 and 4.00, respectively. 2) The eviscerated yield percentage in 0.64%, 0.71% and 0.78% threonine groups was significantly higher than that in 0.50% and 0.57% threonine groups (P<0.05). The meat color brightness (L^*) value of breast muscle in 0.78% threonine group was significantly lower than that in 0.50% and 0.64% threonine groups (P<0.05), and the meat color redness (a^*) value of breast muscle in 0.71% and 0.78% threonine groups was significantly higher than that in 0.50% and 0.64% threonine groups (P<0.05). 3) The serum triglyceride content in 0.78% threonine group was significantly lower than that in 0.50% and 0.64% threonine groups (P<0.05), the serum low density lipoprotein cholesterol content was significantly lower than that in 0.50% threonine group (P<0.05), and the serum insulin content was significantly higher than that in 0.50% threonine group (P<0.05); the serum insulin like growth factor-1 content in 0.64% and 0.71% threonine groups was significantly higher than that in 0.50% threonine group (P<0.05). 4) The T lymphocyte proliferation rate in peripheral blood in 0.64%, 0.71% and 0.78% threonine groups was significantly higher than that in 0.50% and 0.57% threonine groups (P<0.05). According to the regression equation of T lymphocyte proliferation rate, the optimal dietary threonine level for yellow-feathered broilers was 0.75%, the daily threonine requirement was 0.67 g, and the ratios of threonine to lysine, methionine and tryptophan were 0.83, 2.14 and 4.41, respectively. In conclusion, under the conditions of this experiment (the crude protein and lysine levels in the basal diet were 17% and 0.50%, respectively), dietary supplementation with appropriate threonine can improve the growth performance, carcass quality and meat quality of yellow-feathered broilers from 43 to 63 days of age, and significantly affect the fat metabolism. Based on the ratio of feed to gain, the optimal threonine level and daily threonine requirement for yellow-feathered broilers from 43 to 63 days of age are 0.68% and 0.61 g, and the ratios of threonine to lysine, methionine and tryptophan are 0.76, 1.94 and 4.00, respectively.

Key words： yellow-feathered broilers; threonine; growth performance; carcass quality; immune function; nutrient requirement

Cite this article

LIN Xiajing , XI Pengbin , GOU Zhongyong , CHENG Zhonggang , RUAN Dong , ZHENG Chuntian , JIANG Shouqun . Effects of Dietary Threonine Level on Growth Performance, Carcass Quality and Immune Function of Yellow-Feathered Broilers from 43 to 63 Days of Age[J]. Chinese Journal of Animal Nutrition, 2023 , 35(3) : 1552 -1560 . DOI: 10.12418/CJAN2023.147

苏氨酸是家禽体内最后发现的必需氨基酸,是肉鸡继赖氨酸、蛋氨酸之后的第三限制性氨基酸,它是影响肉鸡生产性能的重要限制因素^[1]。研究表明,苏氨酸可以促进机体蛋白质合成^[2]和肠道组织发育^[3],提高生长性能^[4]或者产蛋性能^[5],改善机体免疫功能^[6]和胴体品质^[7]。黄羽肉鸡是我国本土肉鸡,由地方优质品种杂交培育而成,更加符合我国消费者的饮食习惯。黄羽肉鸡产业的发展在整个畜禽行业中都具有举足轻重的地位,但目前对黄羽肉鸡苏氨酸的研究甚少,本研究室已通过生长性能、免疫功能等关键指标研究得出1~21日龄^[8]和22~42日龄黄羽肉鸡苏氨酸适宜需要量,而黄羽肉鸡大鸡阶段的苏氨酸需要量还需要进一步研究。因此,本试验以快大型岭南黄羽肉鸡为研究对象,研究饲粮苏氨酸水平对43~63日龄黄羽肉鸡生长性能、胴体品质和免疫功能的影响,以确定43~63日龄黄羽肉鸡的苏氨酸适宜需要量,为科学配制黄羽肉鸡饲粮提供参考。

1 材料与方法

1.1 试验动物及分组处理

试验选用1 600只43日龄健康、发育良好的快大型岭南黄羽肉公鸡,根据体重一致原则将试验鸡分为5个组,每组8个重复,每个重复40只鸡。

1.2 试验饲粮

试验采用玉米-花生粕型基础饲粮(豆粕的苏氨酸含量较高,故使用花生粕替代常规饲粮中的豆粕),其营养水平参考《黄羽肉鸡营养需要量》(NY/T 3645—2020)^[9]和《鸡饲养标准》(NY/T 33—2004)^[10],并参照《中国饲料成分及营养价值表(2017年第28版)》设计饲粮配方。5个组饲粮中分别添加0、0.07、0.14、0.21和0.28 g/kg(实测值分别为0.50%、0.57%、0.64%、0.71%和0.78%)苏氨酸。除苏氨酸外,各组饲粮其他营养水平均一致,基础饲粮组成及营养水平见表1。

表1 基础饲粮组成及营养水平(饲喂基础)

Table 1 Composition and nutrient levels of the basal diet (as-fed basis)%

项目 Items	含量 Content
原料 Ingredients
玉米 Corn	71.67
花生粕 Peanut meal	21.10
项目 Items	含量 Content
鱼粉 Fish meal	1.00
大豆油 Soybean oil	2.00
石粉 Limestone	1.16
磷酸氢钙 CaHPO₄	1.03
氯化钠 NaCl	0.25
预混料 Premix¹⁾	1.00
L-赖氨酸盐酸盐 L-Lys·HCl	0.42
DL-蛋氨酸 DL-Met	0.12
L-色氨酸 L-Trp	0.01
L-异亮氨酸 L-Ile	0.12
L-缬氨酸 L-Val	0.12
合计 Total	100.00
营养水平 Nutrient levels²⁾
代谢能 ME/(MJ/kg)	12.76
粗蛋白质 CP	17.00
钙 Ca	0.80
有效磷 AP	0.35
赖氨酸 Lys	0.90
蛋氨酸 Met	0.35
色氨酸 Trp	0.17
异亮氨酸 Ile	0.64
缬氨酸 Val	0.75
苏氨酸 Thr	0.50

1)预混料为每千克饲粮提供 The premix provided the following per kilogram of the diet:VA 12 500 IU,VD₃ 3 000 IU,VE 27.5 IU,VK 5 mg,VB₁ 2.5 mg,VB₂ 7.5 mg,VB₆ 5 mg,VB₁₂ 0.025 mg,烟酸 niacin 50 mg,D-泛酸钙 D-calcium pantothenate 15 mg,叶酸 folic acid 1.25 mg,生物素 biotin 0.05 mg,胆碱 choline 500 mg,Fe 60 mg,Cu 4.8 mg,Mn 72 mg,Zn 60 mg,I 0.42 mg,Se 0.18 mg。

2)粗蛋白质、钙、有效磷和苏氨酸为实测值,其他为计算值。CP, Ca, AP and Thr were measured values, while the others were calculated values.

1.3 饲养管理

试验开展前对栏舍进行全面消毒。试验鸡采用地面平养,使用干燥谷壳作为垫料,各组饲养管理和环境条件均一致。饲粮以颗粒料形式,试验鸡自由采食和饮水,保持鸡舍的通风,全期自然光照,常规程序免疫。试验期21 d。

1.4 测定指标

1.4.1 生长性能

在试验开始和结束(63日龄),试验鸡空腹12 h后进行称重,并统计耗料量,计算平均日增重、平均日采食量和料重比。每日观察记录鸡只的健康状况、采食与排泄物情况,及时对死亡和异常试验鸡进行处理,统计死淘率。

1.4.2 胴体品质和免疫器官指数

试验结束当天,每个重复选接近平均体重的试验鸡2只,参照全国家禽育种委员会公布的《家禽生产性能指标名称和计算方法》进行分割,称屠体重、全净膛重、胸肌重、腿肌重和腹脂重,并计算屠宰率、全净膛率、胸肌率、腿肌率和腹脂率(以屠体重为基础计算全净膛率,以全净膛重为基础计算胸肌率、腿肌率和腹脂率);同时分离肝脏、法氏囊、脾脏和胸腺,称重记录,计算各免疫器官与屠体重的相对值,得到免疫器官指数。

1.4.3 肉品质

63日龄时,每个重复选接近平均体重的试验鸡2只,将2侧无皮无骨的胸肌完整剥离,去除胸肌周围的脂肪组织等,测定胸肌pH、肉色和滴水损失。肉色:屠宰后45 min用色差仪(CR-410型,美能达公司,日本)测定其亮度(L^*)、红度(a^*)和黄度(b^*)值,每个胸肌样品选择3个位置。pH:屠宰后45 min用pH计(HI8424型,北京哈纳科仪科技有限公司)对每个胸肌样品选择3个位置检测。系水力:从每个胸肌样品上取一块面积5 cm²、厚1cm左右的圆形肉样,称重并记录,将肉样置于上下各18层滤纸之间,然后放于钢环膨胀压缩平台中,加压35 kg,持续恒定加压5 min,之后迅速撤去压力,称取加压后的重量,分别于45 min和24 h(4 ℃下储存)2个时间点测定系水力。

系水力(%)=100×[(肉样加压前重量-肉样加压后重量)/肉样加压前重量]。

1.4.4 血清生化指标

63日龄时,每个重复选取接近平均体重的试验鸡2只,使用不含抗凝剂的采血管翅静脉采血2 mL,3 000 r/min离心15 min,分离血清。试验鸡血清中甘油三酯(TG)、低密度脂蛋白胆固醇(LDL-C)和尿酸含量均使用多功能酶标仪(Spectra Max M-5,Molecular Devices公司,美国)测定,相关试剂盒由南京建成生物工程研究所提供;血清中胰岛素(INS)、三碘甲状腺原氨酸(T₃)、甲状腺素(T₄)、胰岛素样生长因子-1(IGF-1)、白细胞介素-6(IL-6)和免疫球蛋白G(IgG)含量采用酶联免疫吸附试验(ELISA)法,使用多功能酶标仪(Spectra Max M-5,Molecular Devices公司,美国)测定,相关试剂盒购买于北京厚泽鸿业科技有限公司。

1.4.5 血液细胞免疫指标

63日龄时,每个重复选取接近平均体重的试验鸡2只,使用含有抗凝剂的采血管翅静脉采血3 mL,常规分离淋巴细胞,按照试剂盒说明书加入CD4⁺和CD8⁺单克隆抗体(Southern Biotechnology Associates,美国),采用流式细胞仪(BD Accuri C6 Plus,Becton,Dickinson and Company公司,美国)测定,通过计算得到CD4⁺/CD8⁺值。T淋巴细胞增殖率采用四甲基偶氮苯唑盐(MTT)法,使用多功能酶标仪(Spectra Max M-5,Molecular Devices公司,美国)测定。

1.5 数据统计分析

试验结果数据以平均值和均值标准误(SEM)形式表示。所有数据采用SAS 8.0软件进行单因素方差分析,采用Duncan氏法进行多重比较检验差异显著性,统计显著水平为P<0.05。对差异显著的关键评价指标采用二次曲线模型(y=Ax²+Bx+C,y代表对应指标测定值,x代表饲粮苏氨酸水平,A和B分别为方程二次项和一次项的系数,C为方程的常数项)进行回归分析,其中只有料重比和外周血T淋巴细胞增殖率指标呈显著的二次线性相关(P<0.05)。

2 结果与分析

2.1 饲粮苏氨酸水平对43~63日龄黄羽肉鸡生长性能的影响

由表2可见,当基础饲粮粗蛋白质和赖氨酸水平分别为17%和0.50%时,饲粮苏氨酸水平可以显著影响43~63日龄黄羽肉鸡料重比(P<0.05),其中0.57%、0.64%、0.71%和0.78%苏氨酸组料重比显著低于0.50%苏氨酸组(P<0.05);不过,饲粮苏氨酸水平对平均日增重和平均日采食量均无显著影响(P>0.05)。由表3可见,饲粮苏氨酸水平与黄羽肉鸡料重比呈显著的二次相关(P<0.05),根据料重比回归方程计算得出,43~63日龄黄羽肉鸡饲粮苏氨酸最适水平为0.68%,苏氨酸每日需要量为0.61 g,苏氨酸与赖氨酸、蛋氨酸和色氨酸的比值分别为0.76、1.94和4.00。

表2 饲粮苏氨酸水平对43~63日龄黄羽肉鸡生长性能的影响

Table 2 Effects of dietary threonine level on growth performance of yellow-feathered broilers from 43 to 63 days of age

项目 Items	饲粮苏氨酸水平 Dietary threonine levels/%					均值标准误 SEM	P值 P-value
项目 Items	0.50	0.57	0.64	0.71	0.78	均值标准误 SEM	P值 P-value
初始体重 Initial body weight/g	952.31	954.81	952.63	956.69	954.25	2.51	0.736
终末体重 Final body weight/g	1 620.01	1 646.52	1 640.13	1 645.23	1 626.76	13.81	0.582
平均日增重 ADG/(g/d)	31.79	32.94	32.74	32.79	32.03	0.67	0.678
平均日采食量 ADFI/(g/d)	91.64	90.63	90.14	89.70	88.30	0.90	0.144
料重比 F/G	2.89^a	2.76^b	2.76^b	2.75^b	2.77^b	0.04	0.048

	同行数据肩标无字母或相同字母表示差异不显著(P>0.05),不同字母表示差异显著(P<0.05)。表4~表8同。
	In the same row, values with no letter or the same letter superscripts mean not significantly different (P>0.05), while with different letter superscripts mean significantly different (P<0.05). The same as Table 4 to Table 8.

表3 饲粮苏氨酸水平与黄羽肉鸡料重比回归分析

Table 3 Regression analysis of dietary threonine level and feed to gain ratio of yellow-feathered broilers

项目 Item	P值 P-value	决定系数 R²	回归方程 Regression equation	饲粮苏氨酸最适水平 Optimal dietary threonine level/%	苏氨酸每日需要量 Daily requirement of threonine/g
料重比 F/G	0.025	0.893	y=4.227x²-5.768x+4.705	0.68	0.61

2.2 饲粮苏氨酸水平对黄羽肉鸡胴体品质的影响

由表4可见,当基础饲粮粗蛋白质和赖氨酸水平分别为17%和0.50%时,饲粮苏氨酸水平显著影响黄羽肉鸡全净膛率(P<0.05),其中0.64%、0.71%和0.78%苏氨酸组全净膛率显著高于0.50%和0.57%苏氨酸组(P<0.05)。

表4 饲粮苏氨酸水平对黄羽肉鸡胴体品质的影响

Table 4 Effects of dietary threonine level on carcass quality of yellow-feathered broilers%

项目 Items	饲粮苏氨酸水平 Dietary threonine levels/%					均值标准误 SEM	P值 P-value
项目 Items	0.50	0.57	0.64	0.71	0.78	均值标准误 SEM	P值 P-value
屠宰率 Dressing percentage	90.84	90.63	90.93	90.75	90.88	0.23	0.898
全净膛率 Eviscerated yield percentage	74.03^c	73.99^c	75.09^b	75.81^ab	76.21^a	0.30	<0.001
胸肌率 Breast muscle rate	16.95	17.79	17.67	17.43	17.80	0.34	0.353
腿肌率 Leg muscle rate	23.11	23.40	23.63	23.30	23.68	0.35	0.320
腹脂率 Abdominal fat rate	4.59	4.12	4.28	3.94	3.93	0.25	0.195

2.3 饲粮苏氨酸对黄羽肉鸡肉品质的影响

由表5可见,当基础饲粮粗蛋白质和赖氨酸水平分别为17%和0.50%时,饲粮苏氨酸水平显著影响黄羽肉鸡胸肌肉色L^*值和a^*值(P<0.05),其中0.78%苏氨酸组胸肌肉色L^*值显著低于0.50%和0.64%苏氨酸组(P<0.05);0.71%和0.78%苏氨酸组胸肌肉色a^*值显著高于0.50%和0.64%苏氨酸组(P<0.05);饲粮苏氨酸水平对胸肌pH、45 min系水力、24 h系水力和肉色b^*值均无显著影响(P>0.05)。

表5 饲粮苏氨酸水平对黄羽肉鸡肉品质的影响

Table 5 Effects of dietary threonine level on meat quality of yellow-feathered broilers

项目 Items		饲粮苏氨酸水平 Dietary threonine levels/%					均值标准误 SEM	P值 P-value
项目 Items		0.50	0.57	0.64	0.71	0.78	均值标准误 SEM	P值 P-value
pH		5.81	5.90	5.79	5.70	5.84	0.06	0.192
45 min系水力 WHC_{45 min}/%		22.26	16.94	21.56	24.67	16.60	2.54	0.117
24 h系水力 WHC_{24 h}/%		26.45	26.05	27.85	28.43	26.78	1.00	0.424
肉色 Meat color	亮度 L^*	60.19^a	59.40^ab	61.20^a	59.54^ab	57.83^b	0.71	0.023
	红度 a^*	10.82^c	11.33^bc	10.98^c	11.86^ab	12.52^a	0.29	<0.001
	黄度 b^*	11.60	12.02	11.62	11.18	11.89	0.61	0.889

2.4 饲粮苏氨酸水平对黄羽肉鸡免疫器官指数的影响

由表6可见,饲粮苏氨酸水平对黄羽肉鸡法氏囊指数、脾脏指数和胸腺指数均无显著影响(P>0.05)。

表6 饲粮苏氨酸水平对黄羽肉鸡免疫器官指数的影响

Table 6 Effects of dietary threonine level on immune organ indices of yellow-feathered broilers%

项目 Items	饲粮苏氨酸水平Dietary threonine levels/%					均值标准误 SEM	P值 P-value
项目 Items	0.50	0.57	0.64	0.71	0.78	均值标准误 SEM	P值 P-value
法氏囊指数 Bursa of Fabricius index	0.10	0.12	0.11	0.10	0.11	0.01	0.800
脾脏指数 Spleen index	0.15	0.16	0.15	0.14	0.14	0.01	0.635
胸腺指数 Thymus index	0.24	0.28	0.26	0.29	0.25	0.02	0.611

2.5 饲粮苏氨酸水平对黄羽肉鸡血清生化指标的影响

由表7可见,当基础饲粮粗蛋白质和赖氨酸水平分别为17%和0.50%时,饲粮苏氨酸水平显著影响黄羽肉鸡血清中TG、LDL-C、INS和IGF-1含量(P<0.05),其中0.78%苏氨酸组血清TG含量显著低于0.50%和0.64%苏氨酸组(P<0.05);0.78%苏氨酸组血清LDL-C含量显著低于0.50%苏氨酸组(P<0.05);0.78%苏氨酸组血清INS含量显著高于0.50%苏氨酸组(P<0.05);0.64%和0.71%苏氨酸组血清IGF-1含量显著高于0.50%苏氨酸组(P<0.05)。

表7 饲粮苏氨酸水平对黄羽肉鸡血清生化指标的影响

Table 7 Effects of dietary threonine level on serum biochemical indices of yellow-feathered broilers

项目 Items	饲粮苏氨酸水平 Dietary threonine levels/%					均值标准误 SEM	P值 P-value
项目 Items	0.50	0.57	0.64	0.71	0.78	均值标准误 SEM	P值 P-value
甘油三酯 TG/(mg/L)	269.89^a	195.61^ab	213.99^a	193.66^ab	176.23^b	11.42	<0.001
低密度脂蛋白胆固醇 LDL-C/(mmol/L)	1.93^a	1.76^ab	1.88^ab	1.88^ab	1.64^b	0.06	0.005
胰岛素 INS/(mIU/mL)	6.36^b	9.36^ab	7.80^ab	9.70^ab	13.41^a	0.95	<0.001
三碘甲状腺原氨酸 T₃/(ng/mL)	1.41	1.19	1.30	1.11	1.30	0.10	0.293
甲状腺素 T₄/(μg/dL)	2.01	1.90	2.09	1.85	1.94	0.07	0.169
胰岛素样生长因子-1 IGF-1/(ng/mL)	18.21^b	22.88^ab	30.22^a	36.09^a	25.07^ab	2.76	<0.001
免疫球蛋白G IgG/(mg/mL)	1.18	1.53	2.46	1.13	0.96	0.52	0.250
白细胞介素-6 IL-6/(pg/mL)	32.84	39.40	29.46	41.46	35.91	3.56	0.129

2.6 饲粮苏氨酸水平对黄羽肉鸡外周血淋巴细胞亚群和T淋巴细胞增殖率的影响

由表8可见,当基础饲粮粗蛋白质和赖氨酸水平分别为17%和0.50%时,0.64%、0.71%和0.78%苏氨酸组黄羽肉鸡外周血T淋巴细胞增殖率显著高于0.50%和0.57%苏氨酸组(P<0.05),饲粮苏氨酸水平对CD4⁺/CD8⁺值无显著影响(P>0.05)。由表9可见,饲粮苏氨酸水平与黄羽肉鸡外周血T淋巴细胞增殖率呈显著的二次相关(P<0.05),根据T淋巴细胞增殖率回归方程计算得出黄羽肉鸡饲粮苏氨酸最适水平为0.75%,苏氨酸每日需要量为0.67 g,苏氨酸与赖氨酸、蛋氨酸和色氨酸的比值分别为0.83、2.14和4.41。

表8 饲粮苏氨酸水平对黄羽肉鸡外周血淋巴细胞亚群和T淋巴细胞增殖率的影响

Table 8 Effects of dietary threonine level on lymphocyte subsets and T lymphocyte proliferation rate in peripheral blood of yellow-feathered broilers

项目 Items	饲粮苏氨酸水平 Dietary threonine levels/%					均值标准误 SEM	P值 P-value
项目 Items	0.50	0.57	0.64	0.71	0.78	均值标准误 SEM	P值 P-value
CD4⁺/CD8⁺	1.67	1.88	1.83	1.83	1.68	0.11	0.593
T淋巴细胞增殖率 T lymphocyte proliferation rate/%	0.093^c	0.122^b	0.153^a	0.172^a	0.163^a	0.010	<0.001

表9 饲粮苏氨酸水平与黄羽肉鸡外周血T淋巴细胞增殖率回归分析

Table 9 Regression analysis of dietary threonine level and T lymphocyte proliferation rate in peripheral blood of yellow-feathered broilers

项目 Item	P值 P-value	决定系数 R²	回归方程 Regression equation	饲粮苏氨酸最适水平 Optimal dietary threonine level/%	苏氨酸每日需要量 Daily requirement of threonine/g
T淋巴细胞增殖率 T lymphocyte proliferation rate	0.021	0.979	y=-1.283x²+1.913x-0.546	0.75	0.67

3 讨论

3.1 饲粮苏氨酸水平对43~63日龄黄羽肉鸡生长性能的影响

苏氨酸具有调节肉鸡采食量和体重的作用^[11],当饲粮中的苏氨酸缺乏或者过量时,动物的生长性能会下降,只有当苏氨酸水平达到最适需要量时,动物的生产性能才可以最大程度地发挥出来。Dozier等^[12]研究饲粮添加6种苏氨酸水平对42~56日龄肉鸡生长性能的试验结果发现,当饲粮苏氨酸水平为0.67%时料重比最佳,0.68%时平均日增重最佳。Kidd等^[13]研究表明,42~56日龄肉鸡根据料重比最小化为评定指标,得出苏氨酸需要量为0.67%。本试验结果显示,当苏氨酸水平低于《黄羽肉鸡营养需要量》行业标准^[9](0.67%)30%以上时,会显著提高黄羽肉鸡的料重比;结合回归分析结果,以料重比为判定指标,黄羽肉鸡饲粮苏氨酸最适水平为0.68%,这个与国标《鸡饲养标准》^[10]一致,稍高于行业标准。本试验血清生化指标中的IGF-1含量结果表明,苏氨酸可能是通过调节体内血清激素的合成而影响生长性能的提高;也有可能是由于饲粮中的苏氨酸水平太低,导致饲粮中氨基酸的不平衡影响蛋白质合成,从而降低了饲料转化率。

3.2 饲粮苏氨酸水平对黄羽肉鸡胴体品质和肉品质的影响

肉鸡肌肉中苏氨酸脱氢酶活性最高,其代谢后的产物除了合成蛋白质,还参与了肌内素的生成^[14]。研究发现,饲粮苏氨酸水平从0.71%提升至0.77%可以显著提高42日龄肉鸡的胸肌率,从0.71%提升至0.84%后还能显著提高腿肌率^[15]。Taghinejad-Roudbaneh等^[16]研究也发现,在大鸡阶段提高苏氨酸水平可以显著提高胸肌率和腿肌率。本试验结果显示,黄羽肉鸡的全净膛率随苏氨酸水平的提高呈线性提高,但胸肌率和腿肌率无显著变化。结果说明,饲粮苏氨酸水平适宜时,可以加快黄羽肉鸡机体蛋白质合成和沉积,改善胴体品质。

肉色是直观判断肉品质好坏的关键指标,其中L^*值越高说明肉质越差,a^*值越高说明肉质越好^[17]。本试验结果表明,饲粮苏氨酸水平越高(0.78%),对肉色改善作用越佳。目前,关于苏氨酸对肉品质的作用鲜有报道。有研究发现,在饲粮中添加苏氨酸可以提高机体血红蛋白含量^[18],而许多研究均发现血红蛋白对肉色的红度和评分具有重要影响作用^[19],这个可能是苏氨酸改善肉色的原因。

3.3 饲粮苏氨酸水平对黄羽肉鸡免疫功能的影响

苏氨酸是机体免疫球蛋白的主要成分,参与了免疫系统的构成^[20]。机体苏氨酸缺乏时,会影响T、B淋巴细胞的产生,进而抑制抗体的形成^[21]。聂伟等^[22]研究表明,给蛋鸡饲喂含有0.54%~0.57%的苏氨酸饲粮后,能够显著提高T淋巴细胞增殖率,而苏氨酸缺乏会减少盲肠T细胞数量^[23]。本试验结果显示,饲料苏氨酸水平对黄羽肉鸡外周血T淋巴细胞增殖率有显著影响,根据回归分析结果,苏氨酸为0.75%时能够获得最佳的T淋巴细胞增殖率。以上结果说明,苏氨酸对于43~63日龄黄羽肉鸡的免疫功能有改善效果,但机体免疫功能改善需要的苏氨酸水平要高于生长性能所需要的(0.68%)。

3.4 饲粮苏氨酸水平对黄羽肉鸡血清糖脂代谢指标的影响

机体脂肪代谢与必需氨基酸存在着密切关系^[24]。研究发现,饲喂苏氨酸缺乏的饲粮会显著降低肝脏的脂质含量,增加体脂含量^[25],而添加适宜苏氨酸可以降低胸肌中的脂肪沉积^[26]。此外,不添加苏氨酸饲粮组的小鼠会显著上调参与β-氧化的基因——过氧化物酶体增殖物激活受体α(PPARα)和酰基辅酶A氧化酶(ACOX)基因的表达水平,说明机体缺乏苏氨酸会增加TG的外排并增加脂肪酸的降解^[27]。Rezaeipour等^[28]研究发现,与不额外添加苏氨酸的饲粮组相比,添加苏氨酸组试验鸡的血液TG和极低密度脂蛋白胆固醇含量显著降低。Westermeier等^[29]和王红梅^[30]研究也得出相似的结论。INS可以促进葡萄糖进入脂肪细胞,并通过调节肝脏和脂肪组织中的脂肪酸合成酶活性来抑制脂肪酸合成^[31]。本试验研究结果显示,高水平的苏氨酸(0.78%)能够显著降低黄羽肉鸡血清TG和LDL-C含量,显著提高血清INS含量,说明苏氨酸能够减少黄羽肉鸡脂肪沉积,这可能是由于苏氨酸对脂肪代谢相关的酶具有调控作用,具体机理还需要进一步深入研究。

4 结论

在本试验条件下(基础饲粮粗蛋白质和赖氨酸水平分别为17%和0.50%),饲粮补充适宜水平的苏氨酸可提高43~63日龄黄羽肉鸡生长性能、胴体品质和肉品质,对机体的脂肪代谢有显著影响。以料重比为判定指标,确定43~63日龄黄羽肉鸡饲粮苏氨酸最适水平为0.68%,苏氨酸每日需要量为0.61 g,苏氨酸与赖氨酸、蛋氨酸和色氨酸的比值分别为0.76、1.94和4.00。

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