Feed is the fundamental basis for farmed fish, and feed costs typically account for 50-60% of the total production cost. Therefore, addressing the feed issue is one of the most critical aspects in aquaculture. Currently, the best feed conversion ratio (FCR) is around 1.2, while some poor ones exceed 2. In reality, the digestibility of fish on feed is influenced by multiple factors such as feed formulation, processing technology, feeding methods, and water environmental conditions—each of which significantly impacts the FCR and overall feed cost. Based on my own experience in aquaculture, I will discuss some of the main issues affecting the feed coefficient.
**First, the Rationality of the Feed Formula**
The nutrient content of the feed has a significant impact on the feed coefficient. Protein and fat are the two main nutritional indicators. The higher their levels, the lower the FCR, and the better the digestion and absorption of fish. However, even with similar nutrient levels, differences in the types and structures of active ingredients can lead to varying results. For example, when comparing barley, oats, and wheat, the protein digestibility follows the order: barley > oats > wheat. This is mainly due to the lower quality of wheat protein, particularly in terms of lysine and threonine content. The biological value of protein calculated from essential amino acid indices was 55 for wheat, 70 for oats, and 73 for barley. Excessive nutrients in the feed can also cause digestive problems. For instance, high protein levels may not translate into proportional weight gain, leading to wasted protein and increased pollution from metabolites. Therefore, it’s crucial to maintain an optimal balance of nutrients. Research shows that there should be a proper ratio between protein and non-protein in fish feed, and other nutrients can only be effectively utilized when protein is sufficient. Lack of certain nutrients can also hinder the use of others, while complementary nutrients can enhance overall feed utilization.
Fish nutrition has its unique characteristics. While they can efficiently utilize proteins and fats, they struggle with carbohydrates and cellulose. Feeding habits vary among species, so feed formulas must be tailored accordingly. Only when the nutrients match the fish's needs can digestion and absorption improve, lowering the feed coefficient. This means that the feed coefficient largely depends on how well the formula aligns with the fish's requirements.
**Second, Quality of Feed Ingredients and Processing**
The nutrient composition of feed ingredients varies greatly depending on the variety, origin, grade, foreign matter content, storage conditions, and time. These factors directly affect the nutritional quality of compound feed. High-quality raw materials ensure that the final feed meets the expected nutrient levels. However, prolonged storage reduces the solubility and digestibility of proteins, especially at higher temperatures. After two years of storage at 24°C, protein digestibility can drop by 8%. Other nutrients, like vitamin E, are also affected, with fat decomposing due to lipase activity, leading to rancidity. Using such substandard materials severely compromises feed quality.
Processing techniques are equally important, especially for aquafeeds. Factors like particle size, steam pressure, and temperature during processing influence feed quality. Studies show that particle sizes between 10-20 mesh have a digestibility rate of 11%, while those above 50 mesh reach 73%. Finer particles offer better gelatinization, adhesion, and stability in water, making them easier for fish to digest. Standards for aquatic feed require freshwater fish feed to pass through a 20-mesh sieve, with no more than 30% passing through a 40-mesh sieve. Shrimp feed must pass through a 40-mesh sieve, with no more than 20% passing through a 60-mesh sieve. Both excessively fine or coarse particles increase the feed coefficient.
Steam pressure and conditioning time determine the degree of maturation. Insufficient heating leads to poor gelatinization, while over-processing can destroy nutrients, particularly lysine. For aquatic feeds, the ideal moisture content for conditioning is about 12%, with a material temperature of 70°C. This not only improves production efficiency but also ensures high-quality feed.
**Third, Feeding Technology**
Feeding techniques play a major role in determining the FCR. Key factors include feeding frequency and rate. The number of feedings depends on how quickly the fish digest their food, while the feeding rate should match the fish's maximum satiety. Incorrect timing or amounts can increase the FCR. For example, overfeeding causes the gut to be full, reducing contact between the intestinal wall and feed, thus lowering digestion and increasing FCR. Similarly, inconsistent feeding schedules can make fish hungry, reducing feed intake and utilization. Understanding fish behavior and practicing rational feeding can maximize feed efficiency and reduce the FCR.
**Fourth, Water Environmental Factors**
Among environmental factors, water temperature and water quality have the greatest impact on the FCR. Fish are ectothermic, meaning their metabolic activity changes with water temperature. At lower temperatures, their metabolism slows down, reducing food intake and digestion efficiency. For example, carp fed at 20-27°C show much better feed utilization compared to when the temperature is 14-15°C. Low water temperatures generally lead to higher FCRs.
Water quality, particularly dissolved oxygen levels, also plays a critical role. When dissolved oxygen is above 4 mg/L, fish eat more and digest better. Research shows that carp can double their feed efficiency when dissolved oxygen is between 3-6 mg/L. It's clear that maintaining good water quality is essential for optimizing the FCR and ensuring efficient aquaculture practices.
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