mammals, but in addition have those that detect ultra-violet light. Most objects appear very different under UV light and we know that certain compounds, such as aflatoxin, have a characteristic fluorescence. The cones in the birds’ eyes also contain lipids that give them the ability to detect subtle dif- ferences in shades of colour that to us are indistinguishable. There will certainly be a colour change in any diet when we change the ingredient composition. Given a choice, very young birds prefer feed that is green, although into the sec- ond week and beyond this preference changes to red. It is in- triguing to consider whether continuity of feed colour across a diet change, by using feed colourants, could help with diet transition?


Feed ingredients and nutrient change Feeds are changed to adjust the nutrient profile of diets in keeping with the birds’ evolving requirements. Most often, these changes are quite subtle and it seems unlikely that changing major ingredients by, say, ±10% is going to induce short-term feed refusal in young broilers. The components of certain ingredients, such as glucosinolates and sinapine in canola and rapeseed products, depress feed intake over time, although there is no indication that this effect occurs sponta- neously when these ingredients are first introduced with a diet change. There is little room for ingredient selection or re- fusal with crumbled and pellet diets. It is the nutrients in a diet that are the major factor controlling long-term feed in- take. Broilers still eat according to their energy needs and an imbalance in amino acids (AAs) can dramatically depress feed intake. Classic studies with broilers many years ago showed that infusing an imbalanced mixture of AAs into the carotid


artery caused an immediate reduction in feed intake. This is likely an evolutionary defense mechanism preventing the fu- tility of ingesting an imbalanced array of amino acids. More recent studies have shown that feeding an imbalanced ratio of amino acids causes a change in feed intake only 5-6 hours after ingesting the feed, and again the effect was most nota- ble in broiler rather than layer bird strains. Hopefully, our diet changes do not have this formulation error. There is usually an increase in dietary energy in successive di- ets fed to broilers. Since broilers still eat according to their en- ergy needs, this change will be associated with a downward trend in feed intake. Again, it is unlikely that birds immediate- ly recognize a change in dietary energy concentration, and so this is of limited importance in any immediate feed refusal. But over a 24 hour period, this increase in energy between the grower and the starter diet will contribute to an ‘apparent’ decline in feed intake 1-2 days following transition and adds to the paranoia about broilers not being able to adjust to changes (possibly in texture) in the early diet. Table 2 shows the effect of diet change from starter (@ 3000kcal/kg) to grower (@ 3100kcal/g) at 17 days of age. On day 17 broilers consume 87g of starter. The next day, the expectation according to the management guide is that 93g/d will be consumed, but because of the introduction of the higher energy grower diet, intake is only 90g, assuming that broilers adjust their energy intake within 24 hours and consume 279kcal/d. This seems quite a small change, but for a flock of 50,000 broilers the reduction of 150kg in intake ‘be- low standard’ the day following transition is invariably blamed on the feed change per se, and often on the inevitable texture change that comes with it. Broiler


▶ POULTRY WORLD | No. 1, 2021


Detailed obser- vations show that for individ- ual birds ‘feed refusal’ is ob- served mainly in the first 20 min- utes but that within 24 hours of the diet change, there is compensatory feeding that normalises, or even exceeds, expected feed intake for that day.


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