- Grass carp production is the largest finfish aquaculture industry in China, with an increasing volume of commercial feed production. This justifies the need to improve our understanding of the dietary nutrient requirements of this species in order to produce them more efficiently.
- Available data on the amino acid requirements of grass carp show high variability and do not cover the whole production cycle, especially for the market sizes produced in South China.
- Data on the nutrient digestibility of ingredients is essential to formulate diets on a digestible nutrient basis and to better evaluate the nutritional value of ingredients. Although the number of studies concerning the amino acid digestibility of ingredients by grass carp is limited, the available data cover the major ingredients used by the grass carp feed producers.
- More studies investigating amino acids requirements and digestibility of different ingredients for different life stages of grass carp would further contribute to the rapid development of this industry.
Grass carp (Ctenopharyngodon idellus Valenciennes, 1844) is a native Chinese freshwater fish, with 97 % of the global production (Figure 1) originating from China in 2014 (FAO, 2016). Grass carp represents the largest aquaculture industry in China - their production in 2016 was 5,898,799 tonnes. Grass carp currently accounts for about 11 % of the whole aquaculture production in China, with an average annual growth rate of 10 %.
More than 86 % of grass carp produced in China come from the regions highlighted in Figure 2 (China Fishery Statistical Yearbook, 2017). The top three regions are Hubei, Guangdong and Hunan, whose contributions accounted for 16.4 %, 13.9 % and 12.1 % respectively of the total grass carp production in 2016. The most commonly adopted farming techniques for grass carp include polyculture in ponds and pens, and mono or polyculture in cages installed in lakes and reservoirs. Polyculture is commonly practiced with other carp species and bluntnose bream. Intensification of grass carp farming is supported by increasing availability of commercial feed.
In 2016, the estimated feed output was around 1.90 million tonnes to produce 1.65 MT of grass carp in South China (Guangdong, Guangxi, Hainan, Fujian, Guizhou and Yunan). There are four main stages of grass carp in the South China market and the popularity of feed type (pelleted, extruded) is different for each stage. Although extruded feed is perceived to be more expensive by the market, it allows for better feed management, resulting in a better feed conversion ratio (FCR), and ultimately higher cost savings. South China has a particularly widespread use of extruded feed. The share of extruded feed is growing, with a high proportion (about 70 %) being recorded for the ‘Tonghuan’ stage (around 1 kg) (Table 1). Further growth of the grass carp industry requires a more precise understanding of their nutrient requirements under practical conditions, especially of the amino acids which determine the quality of dietary protein. In addition, knowledge of the nutrient digestibility of raw materials is essential to meet the nutrient requirements of fish more precisely, as well as to better evaluate the nutritional value of raw materials. In this paper, we provide a short overview of the available data on the amino acid requirements of grass carp and the nutrient digestibility of commonly used raw materials.
Amino acid requirements of grass carp – a review of available data
Nutrient requirement data provide a basis for the formulation of practical diets and ensure the feed quality of the target species. Amino acid requirement data play a vital role in this regard. Several studies have been published on the amino acid requirements of grass carp, especially in the last few years (Table 2). These studies have largely used a dose-response method with purified (casein and gelatin) or semi-purified (fish meal, casein, gelatin and peanut meal) diet formulations.
Methionine and Cysteine
Methionine is typically the first limiting amino acid in grass carp feed formulated using the common plant protein sources (e.g., soybean meal, cottonseed meal, rapeseed meal). Studies have shown dietary total sulfur amino acid (TSAA) requirements to be 1.21 % for fry grass carp (3.15g, Wang, 2006) and 1.20 for adult grass carp (259g, Tang et al., 2012). As cysteine can be synthesized metabolically from methionine, its appearance in the diets can replace a portion of the methionine in the TSAA requirements (He et al., 2016). Several studies in fish have shown the replacement value of cysteine for methionine in the TSAA requirement to be at least 40 % (NRC, 2011). No study exploring the optimal ratio of methionine and cysteine in grass carp has been published. However, practical feed ingredients used in grass carp feed formulations are typically high in cysteine and therefore, it is important to emphasize adequate levels of dietary methionine. Generally, in grass carp commercial diets with 0.50 % dietary cysteine, the methionine level should be around 0.70 % to meet the TSAA requirements (~1.2 %).
Lysine is often the second limiting amino acid in the commercial grass carp feed. Diets deficient in lysine were reported to reduce growth and feed efficiency in grass carp (Wang et al., 2005). Huang et al. (2003) determined the optimal dietary lysine requirement of juvenile grass carp (4.39g initial weight) to be 1.61 % (as-fed basis), based on the weight gain using a quadratic model. Later, Wang et al. (2005), using second-order polynomial regression, showed that the lysine requirement for the optimal growth and feed efficiency of juvenile grass carp (3.15g initial weight) is 2.24 % of the dry diet on a total basis and 2.07 % of the dry diet on a digestible basis. It is to be noted that Huang et al. 2003 also recorded much lower fish growth than the later study (specific growth rate of 1.05 versus 1.62), which partly explains the differences in the lysine requirements between the two studies. In a much more recent study, Li et al. (2014), using a quadratic model, determined the lysine requirement to be only 1.09 % diet for optimizing weight gain of adult grass carp (460g initial weight) fed with semi-purified diets. Differences in the values between studies, even for the same life stages, indicate the need for more studies to improve our understanding of the lysine requirements of grass carp.
After methionine and lysine, threonine is often the next limiting amino acid in the practical diet formulations of many fish species including carp (Tibaldi and Tulli, 1999). In addition to muscle protein synthesis, threonine also plays a major role in the intestinal growth and immune function of animals. Gao et al. (2014) reported that grass carp juveniles fed threonine deficient diets (0.73 %, 1.03 %) exhibited intestinal villus exfoliation, twist, severe fusion, and reduced anterior intestinal villus height and serosa thickness. The study also found that the optimal dietary threonine requirement of juvenile grass carp (4.02g initial weight) was around 1.37 % diet. In addition, two other studies reported threonine requirement values: 1.42 % of dry feed for juvenile grass carp (Wen et al., 2009; 8.35g initial weight) and 1.16 % diet for adult grass carp (Hong et al., 2015; 442g initial weight).
Besides these (methionine, lysine and threonine), the estimated requirements for other essential amino acids (EAA) published in various studies is summarized in Table 2.
In 2002, the Ministry of Agriculture of the People’s Republic of China published standard dietary nutrient requirements for different growing stages of grass carp to be followed by the aquaculture industry. While the requirements of certain amino acids (e.g., Trp, Val, Phe) listed in Table 3 are slightly lower than those published in Table 2, values for the first two limiting amino acids (TSAA and Lys) are given as ‘≥’ (equal to or higher than the given value). Clearly, guidelines are cautiously set to produce practical feed with at least (and possibly higher than) the given values for these amino acids to minimize the risk of performance drop under commercial farming conditions.
Nutrient digestibility of different raw materials in grass carp
Formulating diets on a digestible nutrient basis not only increases the consistency of the targeted animal performance, but also reduces feed wastage and nutrient pollution of the aquatic environment. Available data on the apparent digestibility coefficient (ADC, %) of dry matter (DM), crude protein (CP), crude lipid (CL) and amino acids for grass carp in various raw materials are presented in Table 4.
Law investigated the digestibility of different ingredients (fishmeal, soybean meal, maize, copra cake, rice bran, napier and carpet grass meals) in pelleted feed for juvenile grass carp (15.3g) back in 1986. The author found that copra cake and rice bran were poorly digested, while fish meal and soybean meal were well digested (> 90 % protein digestibility) by grass carp. Since the study is quite old, the processing conditions applied and the quality of raw materials produced then may not represent those available today, and therefore ADC values produced in this study are only indicative. The study also documented that grass carp, unlike most other fish species, can digest grass meals. In the early 2000s, the researchers (Luo et al., 2001; Lin et al., 2001; Ye et al., 2003) of Southeast Agricultural University in China conducted a series of trials to determine the ADCs of 17 different protein ingredients (native and imported fish meals, crab meal, casing meal, meat meal, soybean meal, expanded soybean, sesame meal, rapeseed meal, yellow rapeseed meal, black rapeseed meal, double-low rapeseed meal, corn germ meal, distiller’s grains meal, cottonseed meal, corn gluten meal, brewers dried yeast) and 9 different energy ingredients (wheat middling, wheat bran, rice bran, corn distillers, standard starch, wheat grain, rice, barley grain, corn grain) in grass carp (initial body weight 180g). Test diets comprising 70 % of reference diet and 30 % of a test ingredient (on an air-dry weight basis) were used in their studies. Chromic oxide (Cr2O3, 0.5 %) was used as an inert marker. The authors collected feces from the outlet by placing a feces collecting net (Figure 3). Feces trapped in the net were removed every two hours and used to estimate nutrient digestibility. Nutrient digestibility values obtained from these studies are presented in Table 4.
Subsequently, Jiang et al. (2006) reported DM, CP, CL and amino acid digestibilities of eight different raw materials (peanut meal, soybean meal, double-low rapeseed meal, rice bran, rapeseed meal, cottonseed meal, wheat middling and fishmeal) in grass carp (initial body weight 65g). In this study, the DM ADCs of fishmeal (59.07 %) and soybean meal (57.99 %) were much lower than those reported by Lin etal. (2001) for the same raw materials (83.06 % for fishmeal and 75.44 % for soybean meal). However, Jiang et al. (2006) recorded high CP digestibility for fishmeal and soybean meal, with smaller differences between the two studies (76.9 vs. 87.5 % for fish meal and 75.7 % vs. 87.5 % for soybean meal). Among several other factors that may have contributed to the differences observed between the two studies, water temperatures were lower in the study by Jiang et al. (2006) than in the study by Lin et al. (2001) (21.5±1.0ºC vs. 25±1.0ºC). Lower than optimal water temperatures can result in lower digestibility (Jiang et al. 2006). It is also worth mentioning that although both the studies used Cr2O3 as the inert maker, the inclusion rate was lower (0.1 % versus 0.5 %) in the study by Jiang et al. (2006) than that used by Lin et al. (2001). Chromic oxide inclusion levels are typically recommended at 0.5-1 % in digestibility studies to minimize analytical errors. Therefore, the possible influence of this factor cannot be ignored when explaining the low digestibility of raw materials obtained by Jiang et al. (2006).
Jiang et al. (2006) also compared the effect of two different methods of feces collection, siphoning vs. stripping, on the ADCs of DM and CP for several raw materials (Figure 4). In the siphoning method, feces were collected within 4 hours of defecation. The study showed that for the majority of ingredients, differences between the two methods are small (0.4-3.1 % lower for stripping versus siphoning) for CP and DM ADC. However, for certain ingredients (e.g., double-low rapeseed meal, meat and bone meal), the stripping method produced lower digestibility values than the siphoning method (7-8 % lower for DM digestibility of both the raw materials, 7 % lower for CP digestibility of double-low rapeseed meal). The digestive tract of grass carp is very long and coiled (Figure 5) (Mokhtar et al., 2015), making it difficult to delineate the distal segment to be used for feces stripping. Furthermore, the fish would be under stress if stripping was practiced repeatedly in a short time interval. Given the small differences recorded in the digestibility values for the majority of the raw materials, the author recommended the values produced by the siphoning method for practical application. Table 4 contains the ADCs of raw materials reported in this study, produced only from the siphoning method.
In 2012, a digestibility experiment using rice bran, corn meal and cassava meal was carried out by Thu in grass carp (about 250g) stocked in 500-L cylindroconical tanks. The study showed that grass carp digested more DM, CP, ash and lipid from rice bran and corn meal than from cassava meal. Rice bran appeared to have a high digestibility of DM (80.3 %), protein (87.9 %) and ash (97.4 %) for grass carp. However, this finding is in contradiction with the conclusion of Law (1986), who suggested that grass carp digested rice bran poorly. The better rice bran quality as well as the larger (older) fish used in the recent study (250g vs. 15.3g) probably explain the differences between the two studies. Thu (2012) recorded high DM and CP digestibility values for cassava (80.3 % for DM and 87.9 % for CP), while very low ash digestibly was observed in cassava (14.39 %). The anti-nutritional factors present in cassava meal likely reduced the ash (mineral) digestibility, as they are known to interfere with digestion by binding to digestive enzymes or directly to some mineral elements (Francis et al., 2001).
In some cases, even for the same raw materials, amino acid digestibility differed between studies (Figure 6). For example, the methionine digestibility of imported fish meal reported by Ye et al. (2003) (87.3 %) was much higher than that by Jiang et al. (2006) (41.45 %), while the methionine digestibility of double-low rapeseed meal reported in Ye et al. (2003) (47.2 %) was much lower than that in Jiang et al. (2006) (65 %) (Fig.6). While the differences can be related to raw material quality, the influence of other factors such as those related to fish size (180g grass carp in Ye et al. (2003) vs. 65g grass carp in Jiang et al. (2006)) and inert marker (Cr2O3) content (0.5 % in Ye et al. (2003) and 0.1 % in Jiang et al. (2006)) among others cannot be discounted.
Improvements have been made in the digestibility calculations by several authors over the years (You et al., 1993; Sugiura et al., 1998; Forster, 1999; Bureau et al., 1999). Recently, in 2015, the Ministry of Agriculture of the People’s Republic of China published “Procedure of apparent digestibility coefficient determination in fish and shellfish” (NY/T 2713-2015), where the equation proposed by Bureau et al. (1999) was recommended for use in digestibility calculations. While Jiang et al. (2006) and Thu (2012) have used improved equations proposed by You et al. (1993) and Sugiura et al. (1998) respectively, others have used the old equation from Cho and Slinger (1979). This may have resulted in some errors in the ADC values reported and partly explain the differences observed in the ADCs between studies for the same raw material and nutrient.
In summary, grass carp production is the largest finfish aquaculture industry in China. Published data summarized in this paper on the EAA requirements and raw material digestibility of grass carp will allow feed producers to be more accurate when formulating diets of grass carp. More studies are clearly needed to further strengthen the database, covering the whole life stage of grass carp under practical production conditions.
ADC – apparent digestibility coefficient
CL – crude lipid
CP – crude protein
DM – dry matter
EAA – essential amino acids
FCR – feed conversion ratio
TSAA – total sulfur amino acid