In an ADM-sponsored 84-day feeding trial involving juvenile triploid rainbow trout in France, fish fed ADM’s NURSal starter feed reached an average weight of 25.0 grams – 17% heavier than fish fed a competing micro-extruded feed and 33% heavier than those fed a crumble feed. NURSal also produced a more uniform population and favorable gut microbiome indicators, while feed conversion ratios remained statistically comparable across all three diets.
Trial Design and Feeding Conditions
Early-stage nutrition can influence growth, feed efficiency and hatchery operations throughout the salmonid production cycle. A recent trial highlighted in Aquaculture Magazine examined how three commercially available starter feeds performed in juvenile rainbow trout.
The 90-day study was conducted in France and compared ADM’s NURSal cold-extruded starter feed with a competing micro-extruded feed and a conventional crumble feed. The trial involved 12 tanks, each holding 850 triploid rainbow trout in 500-liter tanks, at a water temperature of 15°C. Each of the three dietary treatments was replicated in four tanks.
Fish were fed four times daily, with feeding rates adjusted every two weeks according to biomass sampling and observed appetite. The reported growth comparisons were based on measurements taken after 84 days.
Growth Performance: NURSal Fish Reached 25 Grams
The largest difference among treatments was in average final body weight.
| Performance measure | NURSal | Micro-extruded feed | Crumble feed |
|---|---|---|---|
| Average body weight after 84 days | 25.0 g | 21.4 g | 18.7 g |
| Weight advantage for NURSal | — | 17% | 33% |
| Overall feed conversion ratio (FCR) | 0.66 | 0.62 | 0.65 |
| Coefficient of variation in body weight | 17% | 23% | 25% |
Source: ADM-sponsored trial results published by Aquaculture Magazine. The article reports no statistically significant differences in overall FCR among the three treatments.
Fish receiving NURSal were significantly heavier at the end of the reported growth period. According to the article, they also reached the 25-gram target approximately seven days earlier than fish receiving the alternative feeds.
The growth advantage was not accompanied by a statistically significant improvement in feed conversion. The reported FCR values were 0.66 for NURSal, 0.62 for the micro-extruded feed and 0.65 for the crumble feed. This distinction matters for producers evaluating feed costs: the trial supports a growth advantage under the tested conditions, but it does not demonstrate that NURSal uses less feed per unit of weight gain.
More Uniform Fish Could Simplify Hatchery Operations
The study also found meaningful differences in population uniformity. The coefficient of variation (CV) in body weight was 17% for NURSal, compared with 23% for the micro-extruded feed and 25% for the crumble feed.
A lower CV indicates that individual fish weights were more tightly clustered around the population average. In practical terms, a more uniform group may require less intensive grading and experience less size-related competition. It may also make transfers between production stages and scheduling of juvenile deliveries more predictable.
For hatcheries, these operational benefits could be valuable even when feed conversion ratios are similar.
What the Study Found About Gut Microbiome Characteristics
Researchers also evaluated the fish gut microbiome, analyzing samples from 20 fish per dietary treatment using flow cytometry fingerprinting conducted by Kytos BV.
The results showed differences in microbial community composition and activity among the diets. Both NURSal and the crumble-feed treatment had significantly higher alpha-diversity scores than the micro-extruded-feed treatment (adjusted p less than 0.001). Alpha diversity reflects the variety of microbial cell types detected and how evenly they are represented.
NURSal also showed a significant reduction in opportunistic bacterial populations compared with the crumble-feed treatment, based on the study’s Growth Risk Index (adjusted p less than 0.05).
These findings suggest that starter-feed selection may influence the developing gut microbial community. However, microbiome indicators alone do not establish improved disease resistance, survival or long-term health, and the results should not be interpreted as proof of those outcomes.
Potential Implications for RAS Facilities
The article highlights the possible economic implications of reaching target weights sooner, particularly for recirculating aquaculture systems. RAS facilities require substantial investment in tanks, filtration, pumps and water-treatment equipment, making production throughput an important operational consideration.
ADM estimates that reaching 25 grams approximately seven days sooner could translate into roughly 31 production days saved over a year. This is a projected annualized benefit, rather than a result directly measured over a full year in the trial.
If a similar growth advantage were consistently achieved under commercial conditions, it could help producers move fish through nursery systems more quickly and make better use of existing tank capacity. Actual results would depend on farm-specific conditions, feed costs, stocking practices and the ability to maintain the observed performance at commercial scale.
What Trout Producers Should Take Away
The trial reinforces the importance of evaluating starter feeds beyond their purchase price. Growth rate, feed conversion, population uniformity, health indicators and time spent in nursery systems can all affect production economics.
At the same time, the results come from a specific trial involving triploid rainbow trout under controlled conditions. The study was sponsored by ADM, the manufacturer of NURSal, and the findings should be considered alongside independent research, farm-level performance data and the total cost of each feeding program.
For trout producers, the central question is whether differences observed in controlled trials translate into measurable improvements in their own hatchery operations.