African Journals Online
South African Journal of Animal Science

Volume 31, Issue 2, 2001
Abstracts

Selected mineral and heavy metal concentrations in blood and tissues of vultures in different regions of South Africa
Van Wyk, E.Van der Bank, F.H.Verdoorn, G.H.Hofmann, D. 57-63

Abstract: Concentrations of eight essential elements (Ca, Co, Cr, Cu, Fe, Mn, Ni and Zn) and three toxic metals (Al, Pb and Sr) were measured in various tissue samples from African whitebacked (Pseudogyps africanus), Cape griffon (Gyps coprotheres) and Lappetfaced (Torgos tracheliotos) vultures in different regions of South Africa. Blood samples from live African whitebacked vultures were taken from nestlings from two breeding populations, adults from a wildlife reserve and immature birds held in captivity. With the exception of Sr and Mn, concentrations of the majority of metals differed between two or more of the sampling localities and between captive and wild individuals. Birds from Moholoholo had the highest overall blood metal concentrations, while concentrations in birds from Dronfield were the lowest, as can be expected for nestlings and adults respectively. Fatty tissues and bones had the highest values of metal accumulation, especially Sr, and this is congruent with results from previous studies. It was concluded that most concentrations compared well with those reported for other avian species, but concentrations of Cr, Ni and Pb in the dead vultures were generally above values characteristic of heavy metal poisoning. The values reported for each of two wild populations of P. africanus nestlings and adults, and for two other vulture species could serve as base-line data for future comparative studies.

Index selection of beef cattle for growth and milk production using computer simulation modelling
Dzama, K.Walter, J.P.Ruvuna, F.Sanders, J.O.Chimonyo, M. 65-75

Abstract: The Texas A&M University (TAMU) Beef Cattle Production model was expanded to include basic concepts of quantitative genetics. The traits simulated were birth weight, yearling weight, mature weight and milk production. The progeny inherited attributes from both the sire and the dam. The incorporation of genetic concepts into the model allowed for the introduction of variation between individuals and generations. This was achieved by interfacing the original model with stochastic genetic subroutines including a restricted selection index for desired genetic change. The index included birth weight and yearling weight. In addition, a function for estimating dystocia was also added. The model was used to simulate selection strategies for a small-to-moderate size breed of cattle and a large-size breed using a restricted selection index.
There was an increase in both birth and yearling weight after 20 years of selection in the small-to-moderate size breed, and there was also an increase in dystocia. Antagonistic selection to decrease birth weight and increase yearling weight was simulated for the large breed. Birth weight and dystocia problems declined while yearling weight increased for all classes of animals. In both experiments correlated responses were recorded for mature size and milk production. It was concluded that the modified TAMU Beef Cattle Production model offers breeders an opportunity to compare different selection strategies and evaluate different breeding plans.

An evaluation of Panicum maximum cv. Gatton: 1. The effect of stocking rate and period of absence on the production of sheep
Relling, E.A.Van Niekerk, W.A.Coertze, R.J.Rethman, N.F.G. 77-83

Abstract: The aim of this study was to evaluate the livestock production potential of Panicum maximum cv. Gatton cultivated under dry-land conditions and grazed at various stocking rates after various periods of re-growth. Treatments included a high stocking rate, fast rotation treatment (A), a medium stocking rate, medium rotation treatment (B) and a low stocking rate, long rotation treatment (C). The initial availability of dry matter differed between treatments and was lowest for treatment A and highest for treatment C. The same trend was observed for terminal availability of dry matter. The initial leaf:stem ratio did not differ between treatments A and B, but was lower for treatment C. The terminal ratio did not differ between treatments A and C but was higher for treatment B. Pasture height differed between treatments and was lowest for treatment A and highest for treatment C both at the beginning and end of the period. The sheep subject to treatment C had the highest average daily gain and there was no difference between treatments A and B. Although treatment B resulted in the best individual performance, the highest stocking rate yielded the highest livestock production per ha.

An evaluation of Panicum maximum cv. Gatton: 2. The influence of stage of maturity on diet selection, intake and rumen fermentation in sheep
Relling, E.A.Van Niekerk, W.A.Coertze, R.J.Rethman, N.F.G. 85-91

Abstract: The aim of the study was to evaluate the nutritional value of Panicum maximum cv. Gatton for sheep production in three different seasons and at three different stages of maturity. More mature pasture, compared to younger pasture, resulted in lower quality being selected. This was reflected in lower N values, lower digestibility and higher NDF, ADF and ADL values in selected material. Digestible organic matter intake (DOMI) also decreased with maturity. Grazing of more mature pasture resulted in lower rumen ammonia nitrogen (NH₃-N) levels and higher acetic:propionic acid ratios. This trend was noted in all seasons. The nutritional value of the pasture in summer was high enough to meet at least the maintenance requirements of sheep in all stages of maturity evaluated. The nutritional value of the young and medium mature autumn pasture was also high enough to meet at least the maintenance requirements, but at the mature stage intake was too low. Intake in winter at all stages of maturity, was too low to meet the maintenance requirements of sheep.

An evaluation of Panicum maximum cv. Gatton: 3. The partial digestion by sheep of organic matter, nitrogen and neutral detergent fibre of herbage at three stages of maturity during summer, autumn or winter
Relling, E.A.Van Niekerk, W.A.Coertze, R.J.Rethman, N.F.G. 93-99

Abstract: The aim of this study was to quantify the partial digestion of organic matter (OM), nitrogen (N) and neutral detergent fibre (NDF) in sheep grazing Panicum maximum cv. Gatton pastures at three stages of pasture maturity during summer, autumn or winter. As pasture progressed from the young to the mature stage, OM digestibility decreased for all seasons, but OM disappearance in the rumen was unaffected. OM disappearance from the small intestine decreased with increasing pasture maturity during summer, and OM disappearance from the large intestine decreased with increasing pasture maturity during winter. N disappearance from the small intestine decreased with increasing pasture maturity during all seasons and N disappearance from the large intestine decreased with increasing pasture maturity in summer. Non ammonia nitrogen disappearance from the small intestine was decreased with increasing pasture maturity only for mature autumn pasture. Total tract N disappearance decreased with increasing pasture maturity during summer and autumn. NDF disappearance from the rumen decreased with increasing pasture maturity during autumn and winter, while NDF disappearance from the large intestine was unaffected. NDF disappearance from the total tract decreased with increasing pasture maturity during all seasons. It was concluded that P. maximum cv. Gatton pasture would best be utilized at the younger stages of development and that nutritional value would be higher in summer followed by younger autumn and winter pastures.

The effect of flavophospholipol (Flavomycin ^®) on milk production and milk urea nitrogen concentrations of grazing dairy cows
Van der Merwe, B.J.Dugmore, T.J.Walsh, K.P. 101-105

Abstract: Milk production and milk composition responses to supplementation of Holstein-Friesian cows grazing kikuyu pasture in summer and a combination of annual ryegrass and maize silage in winter with 64 mg/d flavophospholipol were determined. There was no difference in the average milk yield over the first 100 days of lactation between the control group (19.8 kg/day) and the Flavomycin supplemented group (20.5 kg/day). Milk production over the full lactation of 300 days was also not influenced by Flavomycin addition (control: 5 525 kg; treatment: 5 627 kg). There was no difference between the control and treatment group in average butterfat percentage (3.55% vs. 3.62%), butterfat yield over 300 days (195.3 kg vs. 201.2 kg), average protein percentage (3.12% vs. 3.14%) or protein yield over 300 days (170.2 kg vs. 178.5 kg). Flavomycin addition only reduced the milk urea nitrogen (MUN) concentrations during the third week of April (treatment: 16.0 ± 0.8 mg MUN/dl; control: 18.6 ± 0.4 mg MUN/dl) and the second week of September (treatment: 16.0 ± 1.2 mg MUN/dl; control: 19.6 ± 0.9 mg MUN/dl). Cows grazing nitrogen-fertilized pastures displayed great variation in weekly and monthly milk urea nitrogen concentrations which frequently exceed 18 mg MUN/dl.

The effect of selection on genetic parameter estimates
Van Dyk, R.Neser, F.W.C.Kanfer, F.H. 107-114

Abstract: A simulation study was carried out to investigate the effect of selection on the estimation of genetic parameters for butterfat production in dairy cattle. It was found that selection leads to a substantial overestimation of fixed effects and variance components and an underestimation of heritability estimates. This effect was attributed to the fact that all information on all animals in the analysis should be available to compensate for the effect of selection.

Direct and correlated responses to selection for total weight of lamb weaned in Merino sheep
Olivier, W.J.Snyman, M.A.Olivier, J.J.Van Wyk, J.B.Erasmus, G.J. 115-121

Abstract: This study was conducted to evaluate direct and correlated selection responses to selection for total weight of lamb weaned per ewe joined (TWW), number of lambs born (NLB), number of lambs weaned (NLW) and weaning weight (WW). Data from the Grootfontein Merino stud and the Carnarvon Merino flock were used. Estimated heritabilities for TWW, NLB, NLW and WW were 0.19, 0.23, 0.17 and 0.21 for the Grootfontein Merino stud and 0.21, 0.19, 0.16 and 0.30 for the Carnarvon Merino flock. High genetic correlations (0.89-0.98) were obtained between reproduction traits in both flocks, while the genetic correlations between WW and TWW, NLB and NLW were moderate to high (0.32-0.78). Phenotypic correlations between WW and reproduction traits were low (0.04-0.19) and phenotypic correlations between reproduction traits were high (0.71-0.96). In the Grootfontein Merino stud, approximately the same selection response (gain per generation) for TWW could be expected from direct selection (9.03 kg) as from indirect selection for NLB (9.20 kg). In the Carnarvon Merino flock, the highest selection response for TWW (6.37 kg) would be achieved by direct selection for TWW. The estimated selection responses indicate that direct selection for TWW would be the most suitable selection criterion for improving reproductive performance in flocks with a high reproduction rate where an increase in the number of lambs would be undesirable.

Environmental and genetic aspects of survival and early liveweight in Western Australian Merino sheep
Cloete, S.W.P.Greeff, J.C.Lewer, R.P. 123-130

Abstract: (Co)variance estimates for birth weight, pre-weaning lamb survival and weaning weight were obtained for Merino lambs derived from 16 bloodlines. Between bloodline variance ratios (± s.e.) amounted to 0.10 ± 0.04 for lamb birth weight, 0.053 ± 0.036 for lamb survival (logit scale) and 0.18 ± 0.07 for lamb weaning weight. Direct additive genetic variances for weaning weight - expressed as a ratio of the total phenotypic variance within bloodlines (h², direct heritability) - were estimated at 0.18 ± 0.02 for birth weight, 0.094 ± 0.020 (logit scale) for pre-weaning survival and 0.30 ± 0.02 for weaning weight. Corresponding maternal genetic variance ratios (m²) were estimated at 0.15 ± 0.02 for birth weight and 0.08 ± 0.02 for weaning weight. It was not significant in the case of lamb survival. Maternal permanent environmental variance ratios (c²) amounted to 0.08 ± 0.02 for birth weight, 0.044 ± 0.018 for lamb survival (logit scale) and 0.07 ± 0.02 for weaning weight. These results were discussed with reference to sheep production.