Lippens Marijke
Merelbeke, Belgium
Quantitative feed restrictions are used to induce an early growth retardation (followed by compensatory growth), which makes it possible to control the high incidences of leg problems, metabolic diseases and increased fat deposition, common for our modern meat-type chickens. The objective of the present trial was to look at the influence of the grower protein content on the compensatory growth capacity and carcass composition of feed restricted broilers of two different Ross lines. The influence of lowering the CP-content of the starter was examined. The influence of feed protein content (starter or grower) is quite different between the two lines. Lowering the protein content of the starter diet of early-restricted birds can give good results on performance and carcass composition for both lines. However, especially Ross 508 chickens seemed to have their benefit of receiving a high protein grower after early feed restrictions. Reduced mortality was difficult to indicate because of the small number of birds in trial.
Introduction
As modern meat-type chickens show a high incidence of leg problems, metabolic diseases and increased fat deposition, a slower growth rate is pursued to avoid these negative selection responses. Early quantitative feed restriction seems to be a possibility to control these unwanted losses without significant reduction in final body weights (Plavnik and Hurwitz, 1991; Lippens et al., 2000). However, little is known about protein requirement before and during the period of compensatory growth. Model-based calculations of Plavnik and Hurwitz (1989) show an increased requirement for most of the essential AA. Other authors could not show any positive effect of increased AA contents during 'catch-up' growth (Jones and Farrell, 1992 and Leeson and Zubair, 1997).
The objective of the present trial was to look at the influence of the grower protein content on the compensatory growth capacity and carcass composition of feed restricted broilers of two different Ross lines. Also, the influence of lowering the CP-content of the starter was examined.
Materials and methods
A 2-factorial experiment (6 x 2) was set up with 1630 Ross 308 arid 1630 Ross 508 unsexed broiler chickens to investigate the effect of feed protein content on the capacity to catch up in growth of feed restricted birds. Starter diets were given from day 0 until day 10. Grower diets were given from day 11 until 42 days of age.
The following treatments were applied:
1- ad libitum with normal protein starter and grower diet (22% CP, 12.42 MJ AMEn and 20 % CP, 12.85 MJ AMEn, respectively) (ad lib NP);
2 - 80% of ad lib NP from day 4 during 4 days followed by a normal protein grower (20 % CP, 12.85 MJ AMEn) (80 % NP-NP);
3 - 80 % of ad lib NP from day 4 during 4 days followed by a high protein grower (22% CP, 12.85 MJ AMEn) (80 % NP-HP);
4 - ad libitum with low protein starter and normal grower diet (20% CP, 12. 42 MJ AMEn and 20% CP, 12.85 MJ AMEn, respectively) (ad lib LP);
5 - 80 % of ad lib LP from day 4 during 4 days followed by a normal protein grower (20 % CP, 12. 85 MJ AMEn) (80 % LP-NP);
6 - 80 % of ad lib LP from day 4 during 4 days followed by a high protein grower (22 % CP, 12. 85 MJ AMEn) (80 % LP-HP).
The crude protein content of the high protein grower was formulated with respect for the ideal amino acid balance. The two ad libitum treatments consisted of 3 replicates of 100 birds per pen. The other treatments had 2 replicates of 100 birds per pen and 2 replicates of 33 birds per pen. The birds were kept on floor pens at a stocking density of 16 birds per m2.
A lighting schedule with increasing photoperiod (d 4-13: 6 h light, d 14-20: 10 h light, d 21-27: 14 h light, d 28-34: 18 h light; from d 35: 23 h light) was used. Main air temperature started at 30°C during the first three days to gradually decrease to 17°C at the end of the trial.
Results and discussion
Table 1 shows the performance data at the end of the trial. Although body weight was not significantly changed with treatment, still it seems that Ross 508 chickens had a higher capacity to compensate for growth retardation in comparison with 308 birds. Within the normal protein starter groups it probably had to do with the lower relative growth retardation of the 508 line at the age of 8 days (12. 5 % against 8. 5 %), rather than differences in growth rate. However, within the low protein starter groups, the opposite counted (10 % against 14 %) which made the catch up growth of the 508 line even more pronounced. Giving a high protein grower can further stimulate compensatory growth. Especially Ross 508 chickens fed a low protein starter seemed to have a higher need for protein during 'catch-up' growth. Lowering the protein content of the starter brought about a trend of deterioration of the FC, which could be prevented by a quantitative feed restriction. A higher protein content of the grower could further ameliorate FC significantly. The expected reduced mortality after early feed restriction was limited to a positive trend in the 308 line. Indeed, as the data were from a small number of birds, the effect of restriction may have been obscured.
The effect of treatment on carcass composition was quite different between lines (Table 2). For Ross 308 birds, especially feed restrictions, not grower protein content, had a positive influence being a significant effect on total fat content (g/kg total animal inclusive feathers) and a slight trend on abdominal fat content and breast meat percentage. In contrast, significant changes of carcass composition should be expected if restricted birds of the 508 line are fed with a high protein grower. There were positive effects on total protein content, yield and breast meat percentage.
Lowering the protein content of the starter can give good results. However, attention should be paid to less favourable circumstances (f. e. low chick quality) in which case the combination of a low protein starter and feed restrictions can result in minor performances.
References
Jones, G.P.D. and Farrell, DJ. (1992). Early-life feed restriction of broiler chickens I, Methods of application, amino acid supplementation and the age at which restrictions should commence, British Poultry Science 33: 579-587.
Leeson, S. and Zubair, A.K. (1997). Nutrition of the broiler chicken around the period of compensatory growth, Poultry Science 76: 992-999.
Lippens, M, Room, G., De Groote, G. and Decuypere, E. (2000). Early and temporary quantitative food restriction of broiler chickens. Effects on performance characteristics, mortality and meat quality. British Poultry Science 41: 343-354.
Plavnik, I. and Hurwitz, S. (1989). Effect of dietary protein, energy, and feed pelleting on the response of chicks to early feed restriction. Poultry Science 68: 1118-1125.
Plavnik, I. and Hurwitz, S. (1991). Response of broiler chickens and turkey poults to food restriction of varied sevirity during early life. British Poultry Science 32: 343-35.
From Proceedings of the "13th
