Pacific white leg shrimp, Litopenaeus vannamei, is a popular species among aquaculture farmers due to its high economic value, rapid growth rate, and tolerance to wide ranges in salinity and temperature (Bray et al., 1994; Frias-Espericueta et al., 1999).
Concerns about fishmeal
The high dependence of Pacific white shrimp feed on fishmeal – an important protein source due to its palatability and quality – is becoming increasingly problematic due to concerns about sustainability and price. The quantity of fishmeal used by the aquaculture feed sector has increased considerably, driving up its market price over the past decade. Significant progress has been made to reduce levels of fishmeal in commercial diets for farmed aquatic animals.
Plant protein hurdle
Replacing fishmeal with alternatives such as vegetable proteins can negatively affect production performance. The use of less digestible plant raw material increases the presence of undigested nitrogenous compounds in the intestine, which encourages the formation of ammonia and biogenic amines by intestinal microbiota. These compounds are toxic and consequently can lead to imbalance in the intestine, resulting in inflammatory processes and accelerated turnover of the intestinal tissue, leading to poorer performance (Cabral et al., 2013).
Improving performance with PFAs
Plant raw materials are less digestible and can cause negative effects directly on the gastrointestinal tract. Due to their ability to improve feed efficiency at comparatively low cost, phytogenic feed additives (PFAs) are an important addition to aquafeeds. Derived from plants, PFAs are functional feed additives used to improve animal performance. Plant essential oils have been shown to exert multiple positive effects, such as appetite stimulation, a direct reduction of gut bacteria, stimulation of gastric juices, enhancement of the immune system, as well as anti-inflammatory and anti-oxidant properties (Lambert et al., 2001; Nerio et al., 2010; Peterson et al., 2015; Saravanan et al., 2012; Yeh et al., 2009).
A trial was conducted to evaluate the nutrient sparing effect of a commercially available PFA (Digestarom® P.E.P. MGE) in Pacific white shrimp feed. The trial evaluated the effect of the PFA on growth performance, feed utilization and gut intestine ultrastructure. The relationship between intestinal microorganisms and intestinal structure is complex, but it is known to have an impact on nutrient absorption (Apajalahti et al., 2004).
Five diets were formulated for the experiment (Table 1). Three levels of fishmeal were used; the control diet contained the highest level at 25.0% (FM25). The fishmeal level was then reduced to 22.0% (FM22) and 19.0% (FM19) by substitution with soybean meal and peanut meal. Digestarom® P.E.P. MGE was supplemented to the diets with reduced fishmeal content at a dosage of 200 g/ton of feed.
Table 1. Formulation and proximate composition of the experimental diets (dry matter).
Growth performance and feed utilization
The results of the trial indicate important improvements in measured parameters when using Digestarom® P.E.P. MGE. Weight gain, specific growth rate, feed conversion rate and protein efficiency ratio were all improved (P<0.05) for the shrimp fed low fishmeal diets supplemented with the PFA when compared to those that did not receive the PFA.
The replacement of fishmeal by plant proteins had a negative influence on feed conversion ratio (FCR), which worsened as the level of fishmeal in the diets got lower. However, when the low fishmeal diets were supplemented with Digestarom® P.E.P. MGE there was an improvement in FCR values compared with non-supplemented diets. The growth performance and feed utilization of shrimp fed the experimental diets with lower fishmeal levels was numerically worse than other treatments (Table 2). Of the diets with reduced fishmeal, those supplemented with Digestarom® P.E.P. MGE numerically improved shrimp specific growth rate and FCR (Table 2).
Table 2. Growth performance parameters of juvenile Pacific white shrimp fed different experimental diets.
PER- Protein Efficiency Ratio; HSI – Hepatosomatic Index. Data represent mean ± S.D. of five replicates. No significant differences were found on these parameters (P>0.05).
Structure of the gastrointestinal tract
It is well known that the digestive tract is divided into three distinct regions, according to their relative importance in all crustaceans. While the foregut and hindgut have a chitinous lining and do not play an important role in digestive processes (Brunet et al., 1994), the improvement of microvilli in the mid-gut is important for nutrient absorption. Analysis by transmission electron microscope indicated that shrimp fed the FM25 diet had a better mid-gut structure and a higher number of microvilli than those fed the other diets.
However, in shrimp fed low fishmeal diets, those fed the diet supplemented with Digestarom® P.E.P. MGE had a better gut structure and a higher number of microvilli in the mid-gut compared to those fed the non-supplemented diets.
The author would like to acknowledge Xiao-ling Huang, Ming-hong Xia and Qi-cun Zhou from the Laboratory of Fish Nutrition in Ningbo, and BIOMIN colleagues Yan Zhang, Gonçalo A. Santos, and Pedro Encarnaçao for their input to this research. Full details of the trial can be found on the BIOMIN knowledge center on www.biomin.net.
- Fishmeal is very expensive as aprotein source for aquafeeds.
- Plant based alternatives, while cheaper, have lower digestibility so performance is compromised.
- Digestarom® P.E.P. MGE supplementation improves the performance of white leg shrimp in low fishmeal diets.