Concept and application of ideal protein for pigs

Abstract

Knowledge about the amino acid requirements and the response of pigs to the amino acid supply is essential in feed formulation. A deficient AA supply results in a reduction in performance while an oversupply is costly and leads to excessive nitrogen excretion with a potentially negative environmental impact. Amino acid requirements are determined to a large extent by the protein deposition in the body and, for lactating sows, by the protein exported in the milk. The concept of ideal protein was developed more than 50 years ago and refers to a protein with an amino acid profile that exactly meets the animal's requirement so that all amino acids are equally limiting for performance. Because Lys typically is the first-limiting amino acid, the ideal amino acid profile is often expressed relative to Lys. Although the ideal protein profile is often assumed to be constant for a given production stage, (small) changes in the ideal protein profile can occur within a production stage. This can be caused by changes in the relative contribution of the different components of amino acid requirements during the productive life on the animal (e.g. changes in the relative contribution of growth and maintenance). Amino acids requirements can be determined experimentally using dose-response studies. The design of the study, the chosen response criterion, and the statistical model affect the requirement estimate. Although considerable experimental work has been carried out to determine the requirements for Lys, Met, Thr, and Trp in growing pigs (and to a lesser extent in sows), little is known about the requirements for the other essential amino acids. Experimental dose-response studies generally focus on the requirement and less on the overall response (i.e. what are the consequences of an amino acid deficiency?). This latter aspect is, to some extent, accounted for in modelling approaches that quantify the response of the animal to the amino acid supply in a dynamic way. The paper describes the origin of ideal protein and illustrates how fundamental concepts of amino acid nutrition have been integrated in practical modeling approaches for the nutrition of growing pigs and sows.

Keywords: Amino acids; Gestation; Growth; Ideal protein; Lactation; Modeling; Pigs.

Figure 1

Growth response of growing pigs to the SID Val:Lys content in the diet. Each point (●) indicates the response of an individual animal [37]. The solid red line indicates the adjusted linear-plateau model to the data and solid blue line the adjusted curvilinear plateau model. Dashed lines indicate the respective estimated SID Val:Lys requirements estimated by the two models.

Figure 2

Utilization of SID Lys between 30 and 115 kg as predicted by the InraPorc model. Data are expressed on a g/kg diet basis. At 65 kg of body weight, the grower is replaced by a finisher diet with a lower SID Lys content.

Figure 3

SID Lys and SID Thr:Lys requirements for growing pigs estimated by the InraPorc and NRC models. Both models were parameterized to represent similar conditions and growth was simulated between 20 and 140 kg of body weight for a pig consuming 2.24 kg/d and with an average daily gain of 785 g/d. The feed intake (gamma function of maintenance) and protein deposition (Gompertz function) curves used in InraPorc were parameterized in the NRC model as third-order polynomials with similar shapes as the functions used in InraPorc.

Figure 4

Utilization of SID Lys in a first parity sow as predicted by the InraPorc model. The sow is offered a gestation diet at a restricted level up to the last week of gestation. The gestation diet is then replaced by a lactation diet (with a higher SID Lys content), which is offered ad libitum during lactation. The gestation diet is offered again at a restricted level during the interval between weaning and conception.

Figure 5

SID Lys requirements for sows estimated by the InraPorc and NRC models. Both models were parameterized to represent similar conditions. A second parity sow weighing 165 kg at service was fed 2.21 kg/d until 90 d of gestation and 2.61 kg/d thereafter until farrowing (13.5 pigs/litter, 1.4 kg birth weight). The body weight after farrowing was 210 kg and the sow consumed on average 5 kg/d during lactation (11.5 piglets weaned, 230 g/d weight gain during 21 d of lactation).