Although a linear relationship is expected when the relative growth rate dW/dt is plotted against lnW (Emmans, 2022) for chickens raised under good conditions, the plots in this study were better described by a polynomial of order 2 (quadratic). This would suggest that the growth conditions were sub-optimal, which could be attributed to stress induced by high temperature, humidity, poor nutrition and poor ventilation. Nonetheless, the Gompetz function gave the best fit, implying the robustness of this function even for data generated by less than optimal conditions.
Growth trajectories in chickens can be altered by manipulation of the environment, for instance, the feed, provided to a given genotype. Higher initial specific growth rates (L), maturation rates and lower age at inflection were observed in groups with heavier weights, similar to results from a selection experiment in meat-type chickens carried out for many generations (Mignon-Grasteau et al., 2000). Depending on the cost of the feed, it could therefore be possible to identify optimal slaughter ages for chickens of a given genotype subject to a specific environment (type of feed), which would influence the initial specific growth rate (L), rate parameter or maturing index (k), age at inflection, weight at inflection, mature weight (A) and average daily gain (ADG). This may allow flexible management depending on the feed resources available to a producer.
The higher mature body weight and lower maturation index observed for the BAO-MOLM and OV-MOLM genotype-diet combinations are similar to observations made in Holstein-Friesians (Berry et al., 2005; Koenen and Groen, 1996), beef cows (Brown et al., 1976; Lopez de Torre et al., 1992) and sheep (Lewis et al., 2002) where a negative correlation between asymptotic mature live weight and maturation rate was reported. The degree of maturity was proportional to the rate parameter, indicating that combinations of genotype-diet with higher rate parameter may be more biologically efficient for certain markets.
It appears the lower NDF concentration in the COM diet may have contributed to the faster growth and lower age at inflection observed under the COM diets compared to the MOLM-based diets (Simasiku, 2023). A higher NDF concentration in the MOLM-based diet may have reduced diet digestibility and hence available nutrients for growth. Dietary fibre might have decreased protein and energy digestibility which in turn led to poor growth performance in chickens (Ahmed et al., 2015). Dietary fibre is important in regulating intestinal morphology, development of digestive organs, absorption of nutrients, growth and intestinal microbiota (Tejeda and Kim, 2021). The ADG and MDG were consistent with this explanation. Furthermore, the chickens on the COM had a better feed conversion ratio (FCR) than those on the MOLM-containing diet (3.51 ± 0.18 and 4.76 ± 0.22, respectively), implying a higher efficiency of nutrient utilization in the former (Simasiku, 2023). These results contrast those of other studies where Moringa oleifera leaf meal or aqueous extracts reduced feed intake (Ashong and Brown, 2011; Alabi et al., 2016) and gave better FCR than controls (David et al., 2012; Ebenebe et al., 2012). The differences may be attributed to genotypes used and the supplementation form.
The heavier weights of BAO than OV chickens at inflection point and at maturity may reflect heterosis effects. Kgwatalala et al (2015) also reported heavier weights in Tswana x Black Australorp than Tswana chickens at 20 weeks of age. It would appear to be advantageous to cross Ovambo to Black Australorp due to the heavier weights attained and this is further enhanced by provision of a commercial diet. The optimal slaughter weights, however, will be dependent on the cost of feed which was not determined in this study.
The data used for the growth curve estimation was obtained by weighing birds in groups (weeks 1–4) or individually (weeks 5–20) on a weekly basis, without keeping track of the individual identification within each pen. The different growth curves were easily fitted and parameter estimates obtained, which suggests that the same strategy could be used to estimate growth curve parameters in a commercial set up, by random sampling of a sizeable number of birds on a weekly basis to monitor flock performance.