Feed is the fundamental material for farmed fish, and feed costs typically account for 50-60% of the total production cost in aquaculture. Therefore, solving the feed issue is one of the most critical challenges in fish farming. Currently, a good feed conversion ratio (FCR) is around 1.2, while poor ones can exceed 2. In reality, the digestibility of fish on feed is influenced by multiple factors such as feed formulation, processing techniques, feeding methods, and water environmental conditions—these are the main elements affecting FCR and overall feed cost. Drawing from my own experience in fish farming, I'd like to share some key factors that influence the feed coefficient.
**First, the rationality of the feed formula**
The nutritional content of the feed itself plays a major role in determining the feed coefficient. Protein and fat levels are two of the most important indicators, and higher levels usually lead to a lower FCR and better digestion and absorption by the fish. However, even with similar nutrient levels, differences in the presence and structure of active ingredients can significantly affect performance. For example, among barley, oats, and wheat, the protein digestibility follows the order: barley > oats > wheat. This is mainly due to the lower lysine and threonine content in wheat, which reduces its biological value. Calculated from essential amino acid indices, the protein biological value is 55 for wheat, 70 for oats, and 73 for barley.
Excess levels of certain nutrients can also cause digestive issues. For instance, too much protein may not translate into proportional weight gain, leading to wasted protein and increased metabolic waste that pollutes the water. Therefore, the nutrient content in feed should be optimized. Studies have shown that there needs to be an appropriate balance between protein and non-protein sources. Only when protein levels are sufficient can other nutrients be effectively utilized. Similarly, a deficiency in certain nutrients can hinder the utilization of others. Complementary ingredients in the feed can enhance overall feed efficiency.
Fish nutrition has unique requirements. While they can efficiently use proteins and fats, they struggle with carbohydrates and cannot easily digest cellulose. Their feeding habits vary, so it's essential to tailor feed formulas to the specific species. When the nutrients in the feed match the fish’s needs, digestion and absorption improve, leading to a lower feed coefficient. This means that the feed coefficient is largely determined by how well the feed formula aligns with the fish’s nutritional demands.
**Second, quality of feed ingredients and processing**
The nutritional composition of feed ingredients varies greatly depending on their type, origin, grade, impurity levels, storage conditions, and duration. This directly impacts the nutritional value and quality of the compound feed. High-quality raw materials ensure that the final feed meets the required nutrient levels. However, prolonged storage can reduce the nutritional value of ingredients. For example, although crude protein levels remain stable during storage, protein solubility and digestibility gradually decrease. After two years of storage at 24°C, protein digestibility may drop by 8%. Other nutrients, especially vitamin E, may also degrade over time. Fat breakdown by lipase can lead to rancidity, further reducing feed quality.
Processing technology is also crucial in aquafeed production. Factors like particle size, steam pressure, and temperature play a significant role in feed quality. Research shows that feed particles in the 10–20 mesh range have a digestibility rate of 11%, while those in the 30–50 mesh range reach 51%, and above 50 mesh reach 73%. Smaller particles provide a larger surface area, improving quenching and tempering effects, resulting in better gelatinization and adhesion. This helps prevent particle loss in water and improves digestibility.
For freshwater fish feed, the standard requires that 100% 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 can increase the feed coefficient. Steam pressure and conditioning time also determine the degree of ripening. If the moisture is too low or the temperature insufficient, the feed may not gelatinize properly, reducing digestibility. Over-processing can damage nutrients, particularly lysine, which leads to reduced feed availability and a higher FCR.
For aquatic feeds, the ideal moisture level for quenching and tempering is about 12%, with a material temperature of around 70°C. This not only boosts production efficiency but also ensures high-quality feed.
**Third, feeding technique**
Feeding techniques significantly impact the feed coefficient, especially the frequency and rate of feeding. The number of feedings depends on how long food stays in the fish’s digestive tract, while the feeding rate is based on the fish’s maximum satiety. Incorrect timing or rates can raise the FCR. For example, feeding too frequently or in large amounts can cause the fish’s gut to become full, reducing contact between the intestinal wall and the feed, thus lowering digestion efficiency and increasing the FCR. Irregular feeding schedules can also make fish hungry, reducing feed utilization.
Understanding the feeding habits of the fish and implementing proper feeding practices can maximize feed efficiency and help reduce the FCR.
**Fourth, water environment factors**
Among the environmental factors, water temperature and water quality are the most influential on the feed coefficient. Fish are poikilothermic, meaning their metabolism changes with water temperature. At lower temperatures, their metabolic activity decreases, leading to less food intake and longer digestion times, which lowers feed utilization. For example, carp show higher feed efficiency at 20–27°C compared to 14–15°C, where the FCR is more than double.
Dissolved oxygen levels in the water also play a critical role. When dissolved oxygen is above 4 mg/L, fish can eat more and digest better. Studies show that carp can double their feed efficiency when dissolved oxygen ranges from 3–6 mg/L. It’s clear that maintaining adequate dissolved oxygen levels is vital for achieving a lower feed coefficient.
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