The ingredient and nutrient composition of the protein supplement provided to Bonsmara calves can be seen in Table I.
Table I The ingredient and nutrient compositions of protein supplement provided to Bonsmara calves
Ingredient | g/kg As fed | | Nutrient | Composition |
Maize | 576.0 | | Dry matter (%) | 89.90 |
Cottonseed oilcake | 286.0 | | Metabolisable energy (MJ/kg) | 10.89 |
Salt | 11.0 | | Crude protein (%) | 29.60 |
Urea | 40.0 | | Crude fiber (%) | 0.28 |
Monocalcium phosphate | 28.0 | | Fat (%) | 2.96 |
Molasses powder | 28.0 | | Ash (%) | 3.15 |
Limestone | 22.0 | | Calcium (%) | 1.32 |
Sulphur | 6.0 | | Phosphorous (%) | 1.12 |
Commercial vitamin and Mineral premix | 3.0 | | | |
With regards to the Gompertz parameter estimates (Table II), an interaction between sex and year was only observed for the A parameter (P =0.026). The interaction indicated that bulls in 2015 tended to have heavier mature weights than in 2014, while the mature weights of heifers from the different production years did not differ.
Comparing the main effects separately, animals from 2015 tended to have heavier asymptotic weights (A) than that of the 2014 production year (P <0.10). With respect to the other parameter estimates, no differences were observed between the two production years (P >0.05). For the effect of sex, bulls had heavier mature weights than heifers (878.96 and 564.36 kg, respectively; P <0.001) and were also older than heifers at the point of maximal growth (C) (290.50 and 184.03 days, respectively; P <0.001). No differences were observed between bulls and heifers for the maturation rate (B) parameter (~0.0043; P =0.810). The models determined for the different sexes accounted for more than 92% of the variation of the data, while with regard to production year, 89% of the variation was accounted for by the respective models. The sexual dimorphism between bulls and cows accounts for the differences observed with the Gompertz parameter estimates. Males are heavier than females when at the same age. Males are also later maturing than females as they reach heavier mature weights. The growth curves of bulls and heifers estimated by the Gompertz function are illustrated in Figure I. The Gompertz parameters estimates obtained in this study indicate heavier weights for heifers than for bulls (Figure I) which contradicts the notion that bull calves are heavier than heifer calves at the same age. Actual birth data showed that the average birth weights of bull calves (40.7 kg) were in fact heavier than heifer calves (36.5 kg) (P <0.05). This unexpected trend illustrated by the Gompertz growth curve, may be partially due to the significantly lower C parameter estimate of heifers which indicates that the point of inflection of heifers is reached almost 100 days before that of bulls. The inflection point in various sheep breeds has been shown to occur shortly after the weaning age; thus adjusting the C parameters of both sexes to coincide with shortly after weaning age (e.g. 220 days) should give a better indication of early growth.
It is also suggested that dam age affects the heifer progeny growth and reproductive performance; not necessarily the dam live weight (Beard et al., 2019). Mature beef cows have a tendency to wean heavier calves (Beard et al., 2019), that possibly may expand the number of heifers reaching puberty earlier than that of bull calves. In terms of post-weaning growth, these models are still appropriate to predict body weights of bulls and heifers with more than 92% of the variation being accounted for by the Gompertz models fitted to the respective growth curves of bulls and heifers.
Table II Comparison of the Gompertz growth parameter estimates between bull and heifer calves for the 2014 and 2015 production years (LSM ± S.E)
Main effect | Parameter | R2 |
A | B | C |
Sex | Bull | 878.96±23.06 | 0.0045±0.0007 | 290.50±12.07 | 0.929 |
| Heifer | 564.36±29.90 | 0.0042±0.0009 | 184.03±15.64 | 0.920 |
P-value | | <0.001 | 0.810 | <0.001 | |
Year | 2014 | 689.11±19.30 | 0.0038±0.0006 | 226.31±10.10 | 0.891 |
| 2015 | 754.21±32.46 | 0.0049±0.0010 | 248.23±16.99 | 0.899 |
P-value | | 0.093 | 0.337 | 0.274 | |
To determine intake trends of calves reared on a hay-based diet, two groups of five bull calves and five heifer calves, of similar body weights born from the Kromme Rhee Bonsmara stud cows, were group fed in 0.25 ha camps from weaning until about two years age. The animals were reared on lucerne hay which was provided ad libitum in hay racks and a protein supplement (Table I) which was provided every second day at a rate of 2 kg per animal for heifers and bulls weighing under 350 kg and 380 kg, respectively. When the average weights of the animals exceeded these body weights, the proportion of supplement was increased to 3 kg per animal. Wastage from the hay rack was monitored daily over a two week period and was used to correct intake over the rearing period. The hay racks were positioned on concrete slabs to facilitate the collection of wastage hay that was pulled from the trough and trampled. The wastage was collected before feeding each day, weighed and expressed as a proportion of hay supplied the previous day. The average wastage was calculated from the two week monitoring period and was used to correct the amount of hay consumed by subtracting the average wastage proportion. Feed refusals were determined weekly, with fresh hay being supplied daily. Feed intakes and body weights were recorded on a monthly basis. From the intake study, the average production performance characteristics for bulls and heifers over the study period were determined and presented in Table III. The end weights of bulls tended to be heavier than that of heifers (P =0.109), while the growth rates of bulls (0.837 kg/day) were significantly higher than that of the heifers (0.633 kg/day).
Overall, no differences in average daily feed intake (~9.88 kg/day) or feed conversion ratio (FCR) (~13.72 kg feed / kg gain) were observed in this study. Under feedlot conditions, De Bruyn (1991) observed growth rates of 1.094 kg/day and FCR of 7.09 kg feed/kg gain for Bonsmara steers. Esterhuizen et al., (2008) observed growth rates of 1.52 kg/day for Bonsmara steers in a feedlot, while steers on pasture receiving supplement grew at a rate of 0.98 kg/day. Under feedlot conditions (high energy and protein), Bonsmara steers present FCRs of less 5-6 kg feed/kg (Strydom et al., 2001), while in the current prolonged study the cattle were reared on simulated pasture rearing conditions on a forage based diet.
Table III Production performance means of group fed Bonsmara bull and heifer calves reared for 369 days under simulated backgrounding conditions (LSM ± S.E.)
Trait | Bulls | Heifers | P-value |
Start age (days) | 222±45 | 236±43 | 0.789 |
Starting weight (kg) | 274±56.3 | 271±24.0 | 0.941 |
End age (days) | 599±40 | 605±31 | 0.916 |
End weight (kg) | 583±39.5 | 504±8.1 | 0.109 |
Average daily gain (kg/day) | 0.837±0.046 | 0.633±0.043 | 0.045 |
Average daily intake (kg/day) | 10.12±0.530 | 9.64±0.339 | 0.398 |
Feed conversion ratio (kg/feed/ kg weight gain) | 12.13±1.30 | 15.30±1.58 | 0.160 |
Average feed intake was regressed against average body weight of the calves for both sex groups in order to determine how intake increases in growing cattle. Linear regressions were then developed that can be used to predict the average daily intake as well as cumulative feed intake (CFI). The regression coefficients of bulls and heifers could not be compared statistically due to there being only two repetitions per sex group.
Intake of bulls can be predicted by:
Daily feed intake (kg) = 0.1653 + 0.0235 W
(R2=0.783)
CFI (kg) = 7.973W -1895.8
(R2= 0.680)
Intake of heifers can be predicted by:
Daily feed intake (kg) = 0.0284W – 1.4199
(R2=0.763)
CFI (kg) = 7.889W -1802.0
(R2= 0.673). Where W represents the average live weight of the cattle (kg).
These equations can be utilised to predict the level of growing calves consuming a forage based diet. Under these conditions, calves are reared on grass/ forage-based systems with the provision of a protein supplement (contributing ~37% of daily intake) on alternate days to meet growth requirements of the animals. Under feedlot conditions, Van der Westhuizen et al., (2009) estimated intake of Bonsmara bulls using a polynomial function to show that intake increases from 8.4 kg to 11.0 kg in the first 10 weeks of feedlot finishing, after which intake tends to level out. In the current study, the animals were not reared on concentrate diets and so intake of the growing animals kept increasing in order to meet the requirements of the animal.
Hence intake increased linearly through the study period. At each monthly weighing, average daily intake was expressed as a percentage of average body weight and the average intake as a percentage of body weight was calculated over the rearing period. The intake expressed as a percentage of body weight was calculated every month and analysed between the groups. No differences were observed between the sexes (P =0.453) with an average percentage intake of 2.43% being observed in this study. This is lower than average percentage intake calculated from the models determined by Van der Westhuizen et al., (2009) which showed that bull calves consumed ~3% of body weight. The bulkiness of the forage consumed, physically limit the intakes of the animals and therefore calves consuming a forage based diet are expected to present lower intakes than feedlot calves in relation to body weight