Effects of feed allowance and indispensable amino acid reduction on feed intake, growth performance and carcass characteristics of growing pigs

The hypothesis that pigs placed on diets with reduced indispensable amino acid (AA) content attempts to offset the reduction in the nutrient density with increased feed intake was tested. In the experiment, feeds with a high or a low AA content were administrated to pigs fed ad-libitum or restrictively according to a 2 × 2 factorial design. Ninety-six barrows were housed in 8 pens (12 pigs/pen) equipped with automatic feeders. Within pen, and from 47 body weight (BW) onwards, 6 pigs were fed ad libitum. The others pigs were allowed to consume, as a maximum, the feed amounts indicated by the breeding company feeding plane to optimize the feed efficiency. In early (86–118 kg BW) and late (118–145 kg BW) finishing, the pigs of 4 pens received feeds with high indispensable AA contents (8.1 and 7.5 g lysine/kg in the two periods, respectively). The other pigs received feeds with reduced indispensable AA contents (lysine, methionine, threonine and tryptophan) by 9 and 18% in early and late finishing, respectively. Body lipid and protein (Pr) retentions were estimated from BW and back-fat depth measures recorded at the beginning and the end of each period. Nitrogen excretion was estimated as actual intake minus estimated N-retention (Pr/6.25). Pigs were slaughtered at 144 kg BW. Restricted feeding decreased feed intake (-7%), daily gain (-5%), carcass weight (-2.6%) and back-fat depth (-8.0%) but increased gain:feed ratio (+2%). The AA restriction increased feed intake (+5.9%), carcass weight (+4.9%) and intramuscular fat (+17.6%), and reduced carcass weight variation (-36%), with no effects on the feed efficiency and the estimated Pr (142 g/d). N excreted was reduced by feed (-9%) and dietary AA (-15%) restrictions. Irrespectively of the feeding level, the pigs responded to a reduction of the dietary essential AA content by increasing their feed intake.

本研究旨在验证一项假说：当日粮中必需氨基酸（indispensable amino acid, AA）含量降低时，猪可通过提高采食量以抵消营养浓度的下降。本实验采用2×2析因设计，将高低两种必需氨基酸含量的日粮分别以自由采食或限制饲喂的方式饲喂给试验猪。
试验共使用96头阉公猪，饲养于8个栏舍（每栏12头），栏舍内配备自动饲喂器。自猪只体重（body weight, BW）达47 kg起，每栏内6头猪采用自由采食模式，剩余6头猪的最大饲喂量参照育种公司的饲喂标准设定，以优化饲料利用效率。
在育肥前期（体重86~118 kg BW）与育肥后期（体重118~145 kg BW），4个栏舍的猪只饲喂高必需氨基酸含量日粮（两个阶段的赖氨酸含量分别为8.1 g/kg和7.5 g/kg）；其余栏舍的猪只饲喂降低了必需氨基酸含量的日粮，其中育肥前期、后期的赖氨酸、蛋氨酸、苏氨酸和色氨酸含量分别降低9%和18%。
通过各试验阶段起始与结束时记录的体重与背膘厚度数据，估算猪体的脂肪与蛋白质（Pr）沉积量。氮排泄量则通过实际氮摄入量减去估算的氮沉积量（Pr/6.25）计算得到。所有试验猪均在体重达144 kg时进行屠宰。
试验结果显示：限制饲喂可降低采食量（-7%）、日增重（-5%）、胴体重（-2.6%）与背膘厚度（-8.0%），但可提升饲料转化率（gain:feed ratio）2%。必需氨基酸限制饲喂可提高采食量（+5.9%）、胴体重（+4.9%）与肌内脂肪含量（+17.6%），并降低胴体重变异系数（-36%），但对饲料利用效率与估算的蛋白质沉积量（142 g/d）无显著影响。
饲喂限制与日粮必需氨基酸限制均可减少氮排泄量，分别降低9%与15%。无论饲喂水平如何，猪只均会通过提高采食量来应对日粮必需氨基酸含量的降低。