Glycine Dynamics in Low Crude Protein Broiler Diets

Growing emphasis on environmental regulation requires global animal production to adopt strategies like feeding low CP diets to minimize nitrogen excretion. However, in some of the animal feeding studies, lowering dietary CP beyond a certain level showed undesirable effects on growth performance and carcass quality of broilers. A number of explanatory approaches is being debated as the possible reasons for the consequences of lowering dietary CP on broiler performance. The difference in the optimal ratio of essential AA between experimental diets (Kobayashi et al., 2013), specific non-essential AA (Corzo et al., 2004) and utilization of free AA compared to peptide bound AA (Namroud et al., 2008) are among the approaches mostly discussed. In this context, concentration of dietary glycine equivalent (Glyequi) and the levels of metabolic precursors of glycine (Gly) have gained significant attention over the years in optimizing dietary needs to maximize animal performance.

This short paper reviews the advances in knowledge concerning low CP diets predominantly focusing on the published studies from the last two decades concerning factors influencing the response to specific non-essential AA’s Gly and Ser in broiler diets.

The potential of Gly to improve growth has been known for decades and the metabolic interconversion of Gly and serine (Ser) is not limited for poultry. Therefore, Gly and Ser are usually assessed together to determine the physiological value of a diet. Most studies use the sum of the concentrations of both Gly and Ser to capture the analogous effect of these AA. This does not account for the fact that dietary Ser only has the same effect as Gly on an equimolar basis. Consequently, Glyequi was proposed as a reference unit, which is calculated as the sum of the concentration of Gly and the molar equivalent of the Ser concentration (Glyequi = Gly + 0.7143 x Ser). Waguespack et al. (2009) conducted three feeding studies to determine the Gly+Ser requirement of broilers in a low CP corn-soy based diet. A single slope break point analysis of the pooled feed conversion ratio (FCR) from the three trials estimated the Gly+Ser requirement to be 21 g/kg (Figure 1).

Multiple researchers have suggested different Gly+Ser requirements over the last two decades. However, comparing the dietary Glyequi values recommended by these dose response studies (Corzo et al., 2004, Powell et al., 2011) is difficult as the authors defined the Glyequi requirements as the dietary concentrations that led to a certain percentage of maximum response. Seigert et al. (2015) conducted a meta-analysis of a performance database from 9626 broilers covering 10 peer-reviewed studies and 11 experiments for varying dietary levels of Gly from 1-21 days (d) of age. The study revealed that the response to dietary Glyequi is highly variable for average daily gain (ADG) and gain to feed ratio (G:F) with 95% of the maximum response achieved at Glyequi concentrations ranging from 11.9 to 19.3 g/kg and from 11.4 to over 23.6 g/kg for ADG and G:F, respectively.

Few of the earlier studies demonstrated that broilers fail to achieve maximum performance when dietary CP is lowered by more than three numerical points even though all known nutrient requirements were met (Hussein et al., 2001, Bregendahl et al., 2002). However, Yuan et al. (2012) evaluated the effect of supplementing Gly to low CP diets in Cobb male broilers and concluded that diets low in CP (<190 g/kg) resulted in decreased body weight (BW) when the Gly + Ser level fell below 17.1 g/kg and increased FCR at levels lower than 18.7 g/kg. The addition of Gly to these diets to a minimum level of 20 g/kg significantly improved performance and was similar in effect to that of diets with 220 g/kg CP. The reduction in performance with low CP diets reported in the above-cited studies may well be due to the underestimation of Gly levels in the diet. Hilliar et al. (2017) reported similar improvement in response with Gly supplementation in diets with CP levels 185 g/kg and 165 g/kg during grower and finisher stages, respectively. Gly supplemented low CP diets showed 7 points improvement in feed efficiency and 254 g improvement in body weight gain (BWG) compared to non-supplemented low CP diets. A dose response trial with low CP diets (177 g/kg and 165 g/kg for grower and finisher respectively), covering five levels of digestible Gly+Ser ranging from 12.4 to 15.7 g/kg and 11.4 to 14.9 g/kg in grower and finisher diets, was done at Wageningen university. The researchers did not find any noticeable effect on production performance neither with three numerical points of CP reduction nor with added Gly compared to the diets with CP level of 209 g/kg and 199 g/kg in grower and finisher stages (van Harn et al., 2018). The authors concluded that a digestible Gly+Ser levels of 12.4 g/kg and 11.4 g/kg in grower and finisher phase, respectively, is sufficient in low CP diets. In contrast, OspinaRojas et al. (2013) reported a linear increase of BWG and G:F of broilers from 21-35 d with increasing dietary total Gly+Ser concentration of 14.7 g/kg to 17.7 g/kg in low CP diets. Although birds endogenously synthesize Gly, 40% of the total requirement of this AA must come from the diet (Graber and Baker, 1973). This might be even more relevant in the context of today’s fast-growing broilers as the synthesis of Gly may fall short to meet specific requirements for protein accretion, high endogenous losses, and needs related to other metabolic processes.

Differences in the optimal dietary concentrations of Glyequi in broiler diets observed in the reported studies may well be explained by the different dietary levels of endogenous Gly precursors of which Thr and choline are quantitatively most important. When diets are merely satisfying essential AA requirements, there is a need to better understand the relationship between Glyequi with Thr. Study by Seigert et al. (2015) showed that certain levels of G:F and ADG can be achieved with distinct combinations of Glyequi and Thr. An increase in dietary Thr levels reduced the Glyequi concentration required to achieve certain response levels. Corzo et al. (2009) in their animal feeding study showed that given the diets are formulated to contain lower CP values, overcoming marginal levels of dietary Gly may be accomplished by allowing moderate excesses of dietary Thr. In this study, dietary interactions were observed for BWG, carcass and breast meat weight. All of these parameters showed improvements with increasing dietary Thr in combination with low dietary Gly + Ser levels (Table 1). However, this reported Gly sparing effect of Thr was not in agreement with the observation from van Harn et al. (2018) wherein supplementing 0.7 g/kg additional Thr to low CP diet supplemented with Gly did not show any beneficial effect on the growth performance of male broilers. Since the growing chick requires a dietary source of Gly at all stages of development, it is of interest to consider contributors to the bird’s endogenous pool of glycine while formulating diet.

A high potential conversion of choline to Gly is reported in mammals (Melendez-Hevia et al., 2009). However, despite the obvious role of Gly in poultry nutrition, very limited information is available on the interactive effect of these nutrients on the animal requirements. Siegert et al. (2015) looked at the magnitude of mutual replacement effects of Glyequi, Thr, and choline using a quadratic regression model. At a fixed choline concentration of 1.05 g/kg DM, the Thr requirement at 95% of maximum G:F ranged from 8.2 to 9.3 g/kg DM when the Glyequi concentration varied between 19.5 and 22.9 g/kg DM (Figure 2a). Likewise, at a fixed Glyequi concentration of 19.5 g/kg DM, the Thr requirement at 95% of maximum G:F ranged from 8.8 to 9.5 g/kg DM when the choline concentration varied between 1.03 and 1.72 g/kg DM (Figure 2b).

In conclusion, the evidence outlined in this paper contributes to further optimization of the dietary Glyequi concentration as well as the other dietary nutrients influencing the response to Glyequi. This enables reducing the dietary CP content without adverse effects on broiler growth performance and concomitantly minimizing the impact of animal production on the environment.