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1. A total of 7960 Cobb and Ross broiler males were reared on various photoperiods or continuous illumination in two trials to 35 or 40 d. Tibial breaking strength was measured in both, and tibial ash content determined in the first of the two trials. 2. Tibial breaking strength was significantly affected by photoperiod, body weight, testicular weight, and genotype. Although peak bone strength occurred at about 7 h for Ross and at 12 h for Cobb, with reductions in strength for both shorter and longer photoperiods than these peaks, the removal of body weight effects showed that tibial breaking strength was negatively correlated with photoperiod per se. However, after removal of photoperiodic influences, breaking strength was positively correlated with both body weight and testicular weight. 3. Ross birds had greater tibial breaking strengths than Cobb, irrespective of whether the comparisons were made at the same photoperiod, the same body weight, or the same ash content. 4. Ash content increased according to the logarithm of photoperiod, but was unrelated to body weight. Tibial breaking strength increased with ash content. Ash contents were similar for Ross and Cobb birds. 5. Despite the significant differences in breaking strength and ash content, there were no broken bones or signs of leg abnormality in any of the 300 birds sampled. Tibial breaking strength data may therefore be of limited value in assessing the benefits of photoperiod to welfare in modern broilers.

This is the second paper in a series that reports the allometric relationships between some of the physical parts and body protein weight of commercial broiler strains reared, sexes separate, on different dietary protein levels. In this trial, four commercial broiler strains were sampled at day old and then weekly from each of three dietary protein treatments to determine the weights of the physical parts and the chemical composition of each of 936 birds. Allometric regressions were compared between strains, sexes and dietary protein levels using linear regression with groups. Whereas these regressions were similar over strains and sexes, some interactions were evident between factors, the largest differences occurring when broilers were fed differing dietary protein levels. These differences may be explained on the basis that lipid is deposited to different extents in each of the parts in response to dietary protein. Day-old breast meat and wing weights fell below the regression that best fitted the remaining observations and so were omitted from allometric analyses. The allometric regressions presented are an attempt to provide information that would enable the prediction of the weights of breast meat, thigh, drum and wing at different stages of growth of broilers whose genotype and feed composition are adequately described. Keywords: Breast meat, thigh, drum, wing, carcass yield

This paper reports the allometric relationships between some of the physical parts and body protein weight of three commercial broiler strains reared, sexes separate, on four dietary protein levels to six weeks of age. Birds were sampled at day old and then weekly from each of the treatments to determine the weights of the physical parts and the chemical composition of each of 582 birds. Allometric regressions were compared between strains, sexes and dietary protein levels using linear regression with groups. Whereas these regressions were similar over strains and sexes, some interactions were evident between factors, and differences occurred when broilers were fed differing dietary protein levels. These differences may be explained on the basis that lipid is deposited to different extents in each of the parts in response to dietary protein, although this was not tested. The allometric regressions presented are an attempt to provide information that would enable the prediction of the weights of breast meat, thigh, drum and wing at different stages of growth of broilers whose genotype and feed composition are adequately described.Keywords: Breast meat, thigh, drum, wing, carcass yield, simulation modelling

1. Individually-caged Ross 308 broiler breeders were transferred temporarily at 147 d from 8 to 14-h photoperiods and back to 8 h at 5-d intervals from 5 to 25 d and at 32 d. Control birds were maintained on 8 h or transferred permanently to 14 h. 2. Five long days had minimal effect on age at first egg, between 10 and 25 long days progressively accelerated sexual development, and maximum advance was achieved by 32 long days. However, a model involving the ogive of a normal distribution with a mean of 14 long days and an SD of 5.3 indicated that 28 long days would be sufficient to maximise sexual development. 3. The similarity of this model for broiler breeders with one previously produced for egg-type pullets suggested a common physiological mechanism for photosexual responses to two opposing changes in photoperiod.

Ross 308 broiler breeder females, maintained in litter-floor pens, were reared from 2 d of age on 8-h photoperiods and transferred abruptly to an 11-, 12-, 13-, or 14-h photoperiod at 20 wk of age and at a mean BW of 2.09 kg. Four qualitative dietary treatments were superimposed upon each photoperiodic treatment, but, in the absence of any significant interactions of photoperiod with diet, only photoperiodic means are reported. Age at 50% egg production (ASM) was progressively advanced as photoperiod increased from 11 to 13 h but was similar for 13-h and 14-h birds. Egg production was significantly related to ASM, with peak egg numbers being stimulated by a transfer to 13-h photoperiods. Rate of lay after 48 wk of age declined more rapidly in the 14-h birds than in any of the other groups, suggestive of an earlier onset of adult photorefractoriness. More eggs were laid outside the nest box and more cracked and dirty eggs were produced by birds on 11- and 12-h photoperiods than on 13- or 14-h photoperiods. Mean egg weight was positively correlated with ASM but negatively related to photoperiod; the number of eggs thought to be double-yolked was not significantly affected by photoperiod. Total egg mass output was not significantly affected by photoperiod. Birds exposed to 11-h and 12-h photoperiods were heavier at 60 wk than birds transferred to 13- or 14-h photoperiods. These findings show that a transfer from 8 to 13 h at 20 wk optimizes egg-laying performance in broiler breeder females, especially when they are kept beyond 60 wk of age.

This paper describes the responses of two genotypes of male broilers to constant 8- and 16-h photoperiods, and to an abrupt transfer from 8 to 16 h at 10, 15 or 20 d. Body weight, feed intake, and feed conversion efficiency were not significantly different at any stage during the 35 d study. Mortality and the incidence of Sudden Death Syndrome were similar for all lighting groups at 35 d. When these data were pooled with previously reported data for female broilers, growth and feed conversion efficiency post 21 d and through to depletion for constant 8-h and birds transferred from 8 to 16 h at 20 d were significantly superior to constant 16-h birds. Constant 8-h birds ate about half their feed during the dark period, whilst 16-h birds consumed no more than 10%. Birds which had been started on 8 h and transferred to 16 h at 10, 15 or 20 d reduced their rate of nocturnal feeding when changed to the longer photoperiod, however, they still consumed more feed in the 8-h dark period than birds that had always been given 16 h illumination. Cobb and Ross genotypes responded similarly to all lighting treatments. Keywords : Photoperiod, broiler growth, nocturnal feeding South African Journal of Animal Science Vol. 38 (3) 2008: pp. 159-165