In order to match nutrient requirements with bird age it is necessary to change dietary specifications throughout the production cycle. Although requirements change slowly over time, we can only practically accommodate this transition with rather abrupt changes to diet formulation. Luckily, birds easily adapt.
Our current strains of broilers show outstanding genetic potential in terms of growth rate fuelled by ever increasing feed intake. This genetic potential is necessarily underpinned by evermore sophisticated environmental control and providing diets that match requirements to age and level of performance.
For broilers, depending on market age, we usually impose 2 or 3 diet changes. It is generally recognised that with higher sustained genetic potential, we now have bird strains that are perhaps more sensitive to any given diet change. An alternative view is that any adverse response may have always occurred but has only more recently been measured with the introduction of more sophisticated monitoring systems.
In broilers, there is often very transient feed refusal or feed wastage when the pelleted grower diet is first introduced. There is surprisingly little information available on how broilers respond to these abrupt diet changes that are necessary as part of modern lifecycle feeding regimes. As with many situations in life, the bird likes consistency in its environment, including in its feed, and takes time to adjust to any new situation.
There are likely 2 general factors that impact transient change to feed intake in response to diet change, namely:
With a change in feed formulation comes a change in the nutrient composition, as well as a change in ingredient composition. Additionally, and perhaps of more importance to the young broiler, these changes are also associated with a major change in feed texture. For broilers, any response to feed change is very transitory, as will be discussed later. Research suggests that this transition is exceptionally short-lived and is much less dramatic than suggested by our casual management observations.
It is difficult to accept that any nutrient change in the feed could have an immediate and short-lived impact on feed intake. While dietary nutrient composition is definitely involved in long-term feed intake regulation (>24h), it is more likely that changes in the physical characteristics of the feed are the main reason behind an alteration in the very short-term feed intake. Such physical characteristics are feed texture, feed colour, feed taste and feed density/hardness. Table 1 indicates the possible relevance of these feed characteristics.
The physical nature of the feed, namely, the combination of texture and hardness, are the overwhelming factors impacting feed intake at the time of diet change, as clearly seen in the transition from crumbs to pellets in young broilers. Changes in ingredient composition and nutrient profile are intimately linked, and together can represent an important additional novel change during this transition. Feed colour and taste are of lesser importance but this maybe because we don’t yet fully understand their significance to the bird.
It seems unlikely that change in feed colour or taste could have a major impact on transitory feed intake. The bird has more taste buds than originally documented, so it’s a myth that the bird has no sense of taste. Broilers seem to have more taste buds than layer strains. They respond measurably to ‘bitter’ tastes but much less to a ‘sweet’ taste, hence the myth about stimulating early feed intake of chicks with sugar, etc.
There are numerous reports of compounds that birds find unpalatable, which was the basis for developing varieties of sorghum that wild birds are reluctant to eat. The active compound in bird-resistant sorghum is closely related to cinnamaldehyde, a compound we have shown to have a negative linear effect on feed intake in broilers.
Similarly, we have shown that broilers reduce their intake when feed is artificially flavoured with ‘mushroom’, which is possibly an evolutionary defence mechanism against the ingestion of mycotoxins. There are no reports of any flavours that stimulate feed intake which mitigates against the development of any unique flavours to aid in the diet transition process.
It is also a myth that birds are colour-blind and so feed colour may be of more significance than is generally assumed. Birds have red, blue and green cones in their eyes, as do most 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 differences 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 second week and beyond this preference changes to red. It is intriguing to consider whether continuity of feed colour across a diet change, by using feed colourants, could help with diet transition?
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 spontaneously when these ingredients are first introduced with a diet change. There is little room for ingredient selection or refusal with crumbled and pellet diets. It is the nutrients in a diet that are the major factor controlling long-term feed intake.
Broilers still eat according to their energy needs and an imbalance in amino acids can dramatically depress feed intake. Classic studies with broilers many years ago showed that infusing an imbalanced mixture of amino acids into the carotid artery caused an immediate reduction in feed intake. This is likely an evolutionary defense mechanism preventing the futility 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 notable in broiler rather than layer bird strains. Hopefully, our diet do not have this formulation error.
There is usually an increase in dietary energy in successive diets fed to broilers. Since broilers still eat according to their energy needs, this change will be associated with a downward trend in feed intake. Again, it is unlikely that birds immediately recognise 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 (@ 3,000 kcal/kg) to grower (@ 3,100 kcal/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 279 kcal/d. This seems quite a small change, but for a flock of 50,000 broilers the reduction of 150kg in intake ‘below 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 management guides cannot accommodate this transient energy change in predictions of daily feed intake, since timing (days of age) of the diet change is so variable.
While colour, taste and nutrient/ingredient composition may contribute to a transient change in feed intake in response to novel feeds, it is clear that the main factor impacting ‘feed refusal’, is feed texture. For broilers this is most noticeable in the change from starter crumbs to grower pellets.
We feed pellets because it allows for rapid intake within competitive feeding systems and reduces the net energy for maintenance of feeding activity and so improves the F:G. Delaying the introduction of pellets because of the perception that young birds “will not eat pellets”, ensures reduced feed efficiency. You cannot expect the same feed efficiency of birds fed pellets (vs small crumbs) introduced at 15d vs as late as 24d, as sometimes occurs. Research suggests that when given a choice, young broilers prefer large particles and generally avoid the smallest particles (Table 3).
Birds do not like change in any aspect of their environment and take time to adjust to novel situations. This is particularly important to the bird when it comes to feed, since a novel offering may be hazardous and so they are initially wary and take time to investigate and eventually accept the new feed. However, this adjustment time may be much shorter than is suggested by anecdotal observations on the farm.
The broilers initial reluctance to eat pellets is associated with certain behavioural changes. Birds actually approach the feeder more often at this time, but this is often with a ‘closed beak’ which supports the observation of birds ‘playing’ with the feed or even scattering feed onto the litter. In the first 20 minutes there are also more instances of birds picking up the pellets but not swallowing them, so again they may be dropped onto the litter. In one study, dropping pellets onto the litter increased 10-fold in the first 20 minutes after initially offering pellets, although actual wastage was just a few grammes/bird. Pellet hardness and colour have been shown to have little impact on this transient ‘feed refusal’ behaviour.
These feed phobia behaviours therefore form the basis for the supposition that broilers “won’t eat pellets” at an early age, and thus the necessity to feed crumbs much later in the production cycle. However, such feed phobia are going to occur regardless of age and delaying the introduction of pellets to, say, 21-24 days is, in fact, going to have a bigger impact on reduced growth than accepting any transient step-back at an early age.
Research findings do not support the contention that changing to pellets sets the bird back by 2-3 days. Detailed observations show that for individual birds ‘feed refusal’ is observed mainly in the first 20 minutes but that within 24 hours of the diet change, there is compensatory feeding that normalises, or even exceeds, expected feed intake for that day. We rarely measure this overall intake pattern but rather respond to that first 20 minutes of clearly seen feed phobia.
Consequently, transient feed refusal is an inevitable and inherent behaviour but one that we can perhaps temper. Since birds are reacting to a novel situation, an obvious approach is to make sure that birds have access to some pellets prior to the changeover. Adding 5% pellets to a crumbed feed 5-7 days ahead of the changeover is one approach, while a more common solution is to feed 50:50 (crumbs: pellets) as the first delivery of feed at the transition time.
An even more radical approach would be to offer the first pelleted feed as a ‘poor quality’ pellet. But even with an abrupt change from crumbs to pellets, the economic loss in terms of wasted feed or reduced growth rate is probably much less than suggested by casual observation of initial bird behaviour. We should accept these transition issues and get it over and done with as early as possible, to capitalise on the amazing ability of the broiler to eat pelleted feed and so promote growth and feed efficiency.