Browsing by Author "Calitz, F. J."

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    Influence of reproduction traits and pre-weaning growth rate on herd efficiency of different beef breed types in an arid sub-tropical environment
    (South African Society for Animal Science, 2006) Du Plessis, I.; Hoffman, L. C.; Calitz, F. J.
    ABSTRACT: The efficiency of Simmentaler cross (SX), Bonsmara cross (BX), Afrikaner (AF) and Nguni (NG) cowherds to produce weaner calves under natural sweetveld conditions was investigated. The respective cowherds were selected to differ with regard to frame size (SX > BX > AF > NG). The AF cows, young (13 to 15 months old) heifers and herd had significantly lower pregnancy rates than the SX, BX and NG cows, young heifers and herds. The weaning rate for NG was significantly higher (15%) than SX, and although not significant, it was still 5 and 8% respectively higher than BX and AF. Weaning weight and pre-weaning growth parameters differed significantly between breed types (SX > BX > AF > NG). Cow efficiency was significantly lower for the AF cows than for the SX, BX and NG cows. The NG herd was more efficient than the other three herds, while the AF herd was the least efficient. Differences in herd efficiency were mainly due to differences in the reproduction rates of the respective herds. The reproduction and calf survival rates were the most important production traits that affected herd efficiency. Management practices should be adapted to maximize the reproduction rate of the females, including the young heifers, to maximise herd efficiency.
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    Spray deposition and control of botrytis cinerea on grape leaves and bunches : part 1 (table grapes)
    (South African Society for Enology and Viticulture, 2016) Brink, J. C.; Calitz, F. J.; Fourie, P. H.
    Insufficient quantity but also quality of spray deposition on susceptible grapevine tissue (i.e. target sites) and favourable conditions for pathogens could lead to control failure during high disease pressure situations. To determine deposition quantity and quality benchmarks for biologically effective spray deposits, bunches and leaves of table grapes (Waltham Cross) were sprayed at various growth stages, using different application volumes of a mixture of fenhexamid and a fluorescent tracer pigment and subsequently dusted with dry conidia of Botrytis cinerea where after infection levels on pedicels, receptacles and leaves were determined. Pigment deposition quantity and quality were determined from photos of sprayed parts taken with a digital camera under a stereo microscope and black light illumination at ×30 or ×10 magnifications and assessed with digital image and Hoerl regression analyses. The deposition quantity resulting in 75% control of B. cinerea infection (FPC75 values) was calculated from biological efficacy curves (sigmoidal regression analyses) for leaves and for each growth stage, for pedicels and receptacles. Deposition quantity and quality measurements correlated favourably with Botrytis infection. An optimal deposition value for the control of B. cinerea was determined by increasing spray volume, however by increasing spray volume and deposition quantity or quality levels past this optimum will not significantly improve disease control further. It was indicated that efficacy of agricultural chemicals could be influenced by improving both deposition quantity and quality, quantifiable by digital image analyses of fluorescent pigment deposition. FPC75 values obtained in this study can be used as benchmarks to evaluate future spray application in vineyards.
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    Spray deposition and control of botrytis cinerea on grape leaves and bunches : part 2 (wine grapes)
    (South African Society for Enology and Viticulture, 2016) Brink, J. C.; Calitz, F. J.; Fourie, P. H.
    Poor control of fruit and foliar diseases in vineyards is often attributed to insufficient spray deposition of susceptible tissue. To optimise spray deposition, a deposition assessment protocol using fluorometry, photomicrography and digital image analyses was developed to determine minimum spray deposition quantity and quality levels needed for effective B. cinerea control in wine grapes (Chenin blanc). Leaves and bunches were sprayed at different growth stages with different volumes of a mixture of fenhexamid and fluorescent pigment. Pigment deposition quantity and quality were determined from photos of pedicels and leaves taken with a digital camera under a stereo microscope and black light illumination at ×30 and ×10 magnification, respectively. After inoculation with dry airborne conidia of B. cinerea infection levels on pedicels, receptacles and leaves were determined and infection levels and deposition data were subjected to sigmoidal and Hoerl regression analyses, respectively. From these biological efficacy curves the deposition levels that affected 75% control of B. cinerea infection (FPC75 values) were calculated for leaves and for each growth stage for pedicels and receptacles. Deposition measurements on sprayed leaves and bunch parts correlated favourably with Botrytis infection levels. An increase in spray volume resulted in higher deposition quantity and improved quality values with a reduction of B. cinerea infections. However, at a certain point, deposition quality remained constant and infection levels did not decrease significantly with increasing spray volume. Susceptibility of pedicels and receptacles to B. cinerea decreased with maturity. FPC75 values can be used as benchmarks to evaluate spray application in wine grape vineyards.