Effects of the restriction of access to feed on growth performance and the development of the proximal part of the gastrointestinal tract in young broilers

Abstract

Three experiments were conducted to evaluate the capacity of young broilers to adapt gastrointestinal tract (GIT) development to feed restriction (FR) programs, based on the physical restriction to the access to feed, and the effects on feeding behavior and growth performance. In experiment 1, we studied the effects of the age of the birds and the length of the fasting period on growth performance, anticipatory feeding behavior, and the development of the proximal part of the GIT. In experiment 2, we evaluated the effects of the length of the FR period in the response of broilers with or without a previous training to fasting in terms of growth performance (essay 1), GIT development (essay 2), and the retention of the digesta in the proximal part of the GIT during the fasting period (essay 3). In experiment 3, we studied the effects of the length of the FR period on the development of the GIT and growth performance in broilers varying in growth potential. In experiment 1, broilers were fed ad libitum (AL) from 1 to 6 d of age and then, fasted for 0, 4, 6, or 8 h per day from 7 to 19 d of age. At 10, 13, 16, and 19 d of age, immediately before the start of the FR period, the development of the crop and gizzard were evaluated. The experimental design was completely randomized and the linear (L) and quadratic (Q) effects of the length of the fasting period on feed intake (FI), body weight (BW) gain, feed conversion ratio (FCR), and GIT traits were determined. In addition, the effects of broiler age and the interactions with FR on GIT development were studied. From 7 to 19 d of age, FI (L, Q; P < 0.05) and BW gain (L; P < 0.01) decreased as the length of the FR period increased, with most of the differences observed from 7 to 13 d of age and with 6 or more hours of fasting. However, FCR was not affected by FR length. The relative weight of the crop (% BW) and its fresh content increased (L; P < 0.001) and the moisture of the digesta (%) decreased (L; P < 0.001) as the fasting period increased. The dried content (g) of the crop increased with FR, with most of the differences observed with 6 or more hours of fasting (L, Q; P < 0.001). At 19 d of age, the Lactobacillus spp. count in the crop increased (L; P < 0.05) with increases in the length of the FR period. Fasting did not affect any gizzard trait at any age. Physical restriction of the access to feed for 6 h or more reduced BW gain but did not affect FCR in broilers from 7 to 19 d of age. Feed restriction for 4 to 8 h stimulated FI in anticipation to the fasting conditions and increased crop development in broilers. In experiment 2, three essays were conducted to evaluate the effects of FR on growth performance, GIT development, and the retention of the digesta in the proximal part of the GIT in broilers from 6 to 14 d of age. In essay 1, broilers were fed AL from 1 to 5 d of age and then, assigned at random to 4 treatments that consisted in applying a FR period of 0, 4, 6, or 8 h/d from 6 to 14 d of age. Fasting impaired FCR (L; P < 0.001) and reduced FI and BW gain (L, Q; P < 0.001). Most of the differences observed occurred in birds fasted for 6 h or more. In essay 2, 7 treatments were tested: a positive control group with birds fed AL throughout the experiment, and 6 extra groups organized as a 3 × 2 factorial with birds fasted for 4, 6, or 8 h under trained (TR, restricted access to feed daily from 6 to 14 d of age) or non-trained (NTR, restricted access to feed only on d 14) conditions. On d 14, after the end of the fasting period, all birds had free access to feed for 60 min and then, the crop and gizzard were examined. In relative terms (% BW), the empty crop was heavier (P < 0.01) in the TR than in the NTR birds. The fresh digesta content of the crop increased with the length of the FR period (L; P < 0.001) with the greatest differences between the TR and the NTR groups being observed in birds under 4 h of fasting (P < 0.05 for the interaction). Gizzard traits studied were not affected by treatment. In essay 3, the experimental design and all management conditions were similar to those indicated for essay 2. On d 14, immediately after the end of the fasting period, all birds were fed for 60 min and then the feed was removed. After feed removal, the empty weight, digesta content (% BW), and pH of the crop and gizzard were determined at one hour interval for the next 4 h. The crop content increased with training (P < 0.05) and with the length of the fasting period (P < 0.001). However, independent of treatment, the effects of FR on crop content decreased with time and in fact, crop contents of all birds were negligible 3 h after feed removal (P < 0.001 for the interaction with time). Gizzard content was greater (P < 0.05) in TR than in NTR birds for the first 2 h after feed removal but not thereafter (P < 0.05 for the interaction with time). Crop (P < 0.001) and gizzard (P < 0.05) pH decreased with time after feed removal. In summary, fasting periods of 6 h/d or more impaired broiler performance from 6 to 14 d of age. Crop content, measured 1 h after the end of the fasting period, increased with training and with the length of the FR. In addition, fasting increased the storage capacity of the crop and the retention time of the digesta in the proximal part of the GIT, effects that were more pronounced in trained birds and that increased with the length of the fasting period. In experiment 3, the effects of the length of the FR on growth performance and on the development of the GIT were studied in broilers with high (Cobb 500) or medium (Hubbard JA757) growth potential, hatched over a hatching window (HW) of 24 h (Cobb-24 and Hubbard-24, respectively). A third group of broilers from the same batch of Cobb 500 birds was used but the HW was of 48 h (Cobb-48). Six treatments were organized as a 3 × 2 factorial with the 3 types of broilers and 2 fasting period lengths (4 vs. 8 h/d) applied from 7 to 21 d of age. All birds received a mash diet designed for slow-growth rate chickens. At 21 d of age, immediately after the end of their respective FR period, all birds had free access to feed for 60 min and the GIT was examined. No interactions between FR and broiler type were detected for any trait. From 7 to 21 d of age, the Cobb-24 birds grew faster than the Cobb-48 birds and both faster than the Hubbard-24 birds (P < 0.001). The FCR weas similar for the Cobb-24 and the Cobb-48 birds and better for Cobb than for Hubbard birds (P < 0.001). Broilers fasted for 4 h/d grew faster (P < 0.001) and had better FCR (P = 0.054) than broilers fasted for 8 h/d. The development of the organs of the GIT was not affected by broiler type, except for gizzard pH that was lower for the Cobb than for the Hubbard birds (P < 0.05). An increase in the FR period from 4 to 8 h/d increased the weight and the contents of the crop (P < 0.001) and the number of Lactobacillus spp. colonies (P < 0.01). In summary, prolonged HW reduced the BW of the Cobb broilers but did not affect GIT development at 21 d of age. An increase in the fasting period from 4 to 8 h/d reduced growth performance and increased the digesta content and Lactobacillus spp. counts of the crop of all birds, independently of their growth potential. In summary, the physical restriction of the access to feed for 6 h/d or more decreased FI in broilers for the first days after the implementation of the fasting strategy. The negative effects on growth performance observed during the period of adaptation to the FR strategy decreased with age and were not affected by broiler potential for growth. Fasting conditions changed broiler feeding behavior quickly with an increase in FI immediately before and after the absence of feed availability. In fact, birds anticipated their feeding behavior prior to the fasting period as soon as 3 d after the start of the FR strategy. The increase in the length of the FR period (from 0 to 8 h/d) stimulated the storage capacity of the crop and the retention time of the digesta in the proximal part of the GIT, an effect that was more intense in broilers with a previous training to fasting. Also, the number of colonies of Lactobacillus spp. in the crop increased with the length of the FR period. Fasting, however, had limited effects on gizzard development. In conclusion, fasting broilers for moderate periods of time (i.e., 6 h of darkness per day after 7 d of age), as proposed by the European Union, had limited influence on growth performance in young broilers, an effect that is expected to decrease or even disappear with age. The data confirm that independent of their growth potential, broilers are able to adapt rapidly their feeding behavior to the fasting conditions by stimulating the storage function of the crop before and after the FR period to compensate for the reduction in feed availability.