Recommendations for lighting programs during the rearing period for broilers generally include a dark period. In Canada, the Codes of Practice for Chickens, Turkeys and Breeders (https://www.nfacc.ca/pdfs/codes/poultry_code_EN.pdf) require that a minimum of 4 hours of continuous dark is included in a photoperiod program from day 5 of placement to no sooner than 7 days prior to catching. However, this remains a controversial subject, and the inclusion of darkness is not included in all programs world-wide.
The published data supports the positive impacts of allowing a dark period for sleep on numerous aspects of a broiler production program. For example, growth rate and feed conversion are improved, health is improved, diurnal rhythms function as they should, behavioural expression, including the expression of positive behaviours, is broader. These responses result in significant improvements in productivity and bird welfare.
The traditional thought pattern involved providing broilers continuous (24 hours of light (24L)) or near-continuous (23 hours of light and 1 hour of dark (23L:1D)) light to allow them near-constant visual access to feed and watering systems, and the ability to move throughout the barn facility as they wish. With that thought pattern, a common belief was that 23L:1D or 24L:0D would result in heavier body weights at the time of marketing compared to those broilers reared with the utilization of a dark period. However, numerous research projects have clearly indicated that this is not the actual response. In our work (Schwean-Lardner et al., 2012a), we found that as early as 32 days of age, broilers were heavier at market age when allowed 4 hours of consecutive darkness compared to 23L:1D, and by 49 d of age, heaviest weights were found when broilers were reared under 17L:7D. At this age, allowing 10 hr of darkness for broilers actually resulted in numerically heaviest weights compared to birds reared under 23L. In addition, feed efficiency improved with the addition of darkness, so not only can producers improve body weights with a simple addition of darkness in a photoperiod program, but can also reduce feed purchases through improved feed conversion ratios. The addition of darkness in a photoperiod program also improves the health of broilers, reducing mortality and morbidity. The differences generally are noted with respect to both skeletal and metabolic related diseases (Schwean-Lardner et al., 2013). Other health improvements are noted in reduced footpad dermatitis lesions, and significant improvements in bird mobility (Schwean-Lardner et al., 2013). The addition of darkness also changes bird behaviour. Birds given a minimum of 4 hr of darkness spend more time performing positive behaviours such as stretching, playing and preening, move in their environment more and spend more time at the feeder than do birds on continuous or near-continuous photoperiods (Schwean-Lardner et al., 2012b).
How does this Impact Feed Efficiency and Gut Transit?
Much of the earlier data from this lab did not determine what the mechanisms are that can impact feed efficiency when longer periods of darkness are provided for broilers. However, recent work by Shynkaruk et al. (2019) may have an answer. Their work found that if broilers know when darkness is coming, feeding patterns are adjusted to include an anticipatory feeding prior to lights off. By doing this, they maintain feed in the gastro-intestinal tract for the majority of the dark period.
With respect to the feeding behaviour, birds increase the number of times they go to the feeder during the photophase in a linear manner. In other words, they have more meals, and more drinking bouts, during the photoperiod every hour that the lights are on (Table 1).
The feeding activity increases approx. 30-120 min before lights go off (Schwean-Lardner et al., 2014). This additional feed intake results in birds with heavier crops and gizzards (Shynkaruk et al., 2019; Table 2), which may improve their function, resulting in improved digestibility of feedstuffs.
Regardless of the time of lights off, the crops and gizzards of birds on each of these lighting programs emptied at approximately the same time, just prior to lights returning on for the following day (Figure 1 and 2). This suggests that retention time of feedstuffs is longer in the digestive tract, which may also improve feed efficiency for these birds. Classen et al. (2016) reported that enzymes found in feed, water, saliva or other microorganisms can initiate digestion in the crop, and that this may be more effective if the feedstuff remains there for a longer period of time. A longer retention time in the gizzard, as well as a more muscular gizzard, can also increase digestibility of feedstuffs (Table 2). The remaining gastrointestinal tract showed similar trends in emptying, with each section reaching near-empty at approximately the same time before lights come on regardless of the lighting program tested.
In conclusion, the latter study (Shynkaruk et al., 2019) has provided evidence that birds anticipate darkness (when a set dark onset is used) and eat accordingly to maintain feedstuffs throughout the gastrointestinal tract until just prior to lights on. The gut is not completely empty at any time. This increased time, along with indications of increased functioning of the crop and gizzard, may at least in part explain the improved feed efficiency when diurnal photoperiods are used for broilers. The increased feed efficiency, along with reduced mortality and heavier body weights at market age, should result in increased profits for producers.
Table 1. Effect of darkness on the average number of feeding and drinking bouts in broiler chickens (average of males and females at wk 2 and 4) (Shynkaruk et al., 2019).
Table 2. Effect of darkness on empty crop and gizzard weight and content expressed as a percentage of body weight (Shynkaruk et al., 2019)
Figure 1. Effect of dark exposure and time of day on crop content expressed as a percentage of body weight. The dark periods corresponding to the lighting treatments are as follows: 2000 to 600 (10D), 2300 to 600 (7D), 200 to 600 (4D), and 500 to 600 (1D).
Figure 2. Effect of dark exposure and time of day on gizzard content expressed as a percentage of body weight. The dark periods corresponding to the lighting treatments are as follows: 2000 to 600 (10D), 2300 to 600 (7D), 200 to 600 (4D), and 500 to 600 (1D).
Presented at the 2021 Animal Nutrition Conference of Canada. For information on the next edition, click here.