'n Beoordeling van prestasie-evalueringsprosedures van Dorperramme onder ekstensiewe bestuurstoestande

Von Schauroth, Erich Dieter Friedrich (Stellenbosch : Stellenbosch University, 2007-03)

Thesis (MScAgric)--University of Stellenbosch, 2007.

Thesis

ENGLISH ABSTRACT: AN ASSESSMENT OF PERFORMANCE EVALUATION PROCEDURES OF DORPER RAMS UNDER EXTENSIVE MANAGEMENT CONDITIONS. During 1988 to 2000, Dorper rams (n=2565) maintained on the Kalahari Experimental Farm during performance test periods of approximately 180 days, were subjected to evaluation according to specific body measurements and breed standards. The respective body measurements included live weight, shoulder height, body length, body width, scrotal circumference and average daily weight gain (ADG). The measurements were recorded at the beginning and end of each evaluation period. The rams were evaluated visually by breed inspectors and classified according to breed standards. Live weight recorded at the beginning of evaluation periods, was moderate to high, and positively correlated with most of the body measurements recorded in the study. The exception was ADG, which was negatively correlated (-0.25) with live weight. Live weight recorded at the end of evaluation periods was moderately and positively correlated with scrotal circumference (0.57), and highly correlated with body width (0.76). Average daily weight gain was high and positively correlated with live weight (0.63) recorded at the end of the evaluation periods. Large variations within test periods, that could be attributed to age differences and variation between the respective groups of rams, necessitated correction of the data for the effect of year/season. The rams were divided into above- and below average groups according to their weight at the beginning of the test period. Analysis of the growth patterns of rams during the study period, using the adjusted end data, indicated that 75% of the rams remained in their respective groups. This finding supports the above-mentioned positive correlation of 0.58 that was reported for live weight at the beginning and end of evaluation periods. Body measurements recorded at the beginning of the evaluation periods were in most cases moderately to high and positively correlated (P<0.0001) with measurements recorded at the end of the evaluation periods. The exception was ADG, which was low and negatively correlated with the respective body measurements that were recorded at the beginning of evaluation periods. After each body measurement was quantified by the sum of squares, it became evident that year/season had the greatest influence on the body measurements recorded at the end of an evaluation period. Year/season influenced live weight, body width, shoulder height, scrotal circumference and ADG, with the largest effect on live weight and the least influence on ADG (R2 = 31.62). After correction for year/season, the contribution of the respective measurements to live weight recorded at the end of evaluation periods were still in the same order. After a step-wise procedure for adjusted live weight at the end of evaluation periods was performed, it was found that body width contributed the most to live weight. Average daily gain made the third largest contribution, i.e. with respect to body width (largest) and body length (second largest), to corrected live weight recorded at the end of evaluation periods. The high correlations reported between the respective body measurements necessitated the analysis of the data for multi-colinearity to determine whether the partial contribution of the respective body measurements would differ from the initial values. The partial contribution of the respective body measurements, however, was not affected by the analysis. Regression analysis indicated that the respective body measurements and weights recorded at the beginning of evaluation periods decreased significantly (P<0.0001) over the entire period (1988-2000). The largest annual decreases were reported for ADG (- 2.5%) and body width (-1.1%), respectively. The decreases in ADG and body width are indications that Dorper sheep decreased in size during the period of this study. The value of visual appraisal methods therefore need to be seriously considered to determine whether this is a valuable management tool to assess the production performance of Dorper rams under extensive conditions. The influence of corrected body measurements and ADG, i.e. after selection by breed inspectors and according to breed standards, was plotted on scatter plots. The plots indicated that the breed inspectors selected larger and heavier rams throughout. It is however, still the case in the Dorper industry that rams that perform below average are sometimes classified and used as stud breeding material. Body width and shoulder height were the body measurements that received the highest and lowest approval as visual selection criteria, respectively. A possible reason for this is that the tendency of a smaller Dorper sheep can be attributed to the decreasing shoulder height, as the latter is an indication of body frame size. Visual appraisal should therefore be used in conjunction with scientific methods. Optimum size for the Dorper should be established.

AFRIKAANSE OPSOMMING: ‘N BEOORDELING VAN PRESTASIE-EVALUERINGSPROSEDURES VAN DORPERRAMME ONDER EKSTENSIEWE BESTUURSTOESTANDE Dorperramme (n=2565) is vanaf 1988 tot 2000 te Kalahari Proefplaas in prestasietoetstydperke van ongeveer 180 dae onder ekstensiewe toestande volgens sekere liggaamsparameters en rasstandaarde geëvalueer. Die onderskeie liggaamsmates het lewende gewig, skouerhoogte, liggaamsbreedte, liggaamslengte, skrotumomtrek en gemiddelde daaglikse toename (GDT) ingesluit. Die parameters is aan die begin en einde van elke toetsperiode gemeet. Die ramme is visueel deur rasinspekteurs geëvalueer en volgens rasstandaarde geklassifiseer. Die lewende gewig van die ramme wat aan die begin van die toetsperiodes aangeteken is, was matig tot hoog en positief met meeste van die liggaamsparameters gekorreleerd. Die uitsondering was GDT, waar ʼn negatiewe korrelasie (-0.25) gevind is. Die lewende gewig van die ramme aan die einde van die toetsperiodes was matig en positief met beide skrotumomtrek (0.57) en die hoogste met liggaamsbreedte (0.76) gekorreleerd. Gemiddelde daaglikse toename was hoog en positief (0.63) met lewende gewig aan die einde van die toetsperiode gekorreleerd. Groot variasie binne toetsperiodes, wat aan ouderdomsverskille en variasie tussen toetsgroepe toegeskryf kan word, het genoodsaak dat die data vir die effek van jaar/seisoen gekorrigeer word. Ramme is volgens begingewig in bo- en ondergemiddelde groepe gerangskik. Groeipatrone van die ramme is met gekorrigeerde einddata ondersoek en gevind dat 75% van ramme gedurende die toetsperiode in hul onderskeie groepe gebly het. Hierdie bevinding verleen steun aan die positiewe korrelasie van 0.58 wat vir lewende gewig aan die begin en einde van toetsperiodes hierbo gevind is. Die liggaamsmates wat aan die begin en einde van die toetsperiodes gemeet is, was in die meeste gevalle matig tot hoog en positief (P < 0.0001) met mekaar gekorreleerd. Die uitsondering was GDT, wat laag en negatief met liggaamsmates, wat aan die begin van toetsperiodes aangeteken is, gekorreleerd was. Die seisoenseffek het die grootste bydrae tot lewende gewig aan die einde van die toetsperiodes gemaak, d.i. nadat elke meting deur die somme van kwadrate gekwantifiseer is. Dit is gevolg deur liggaamsbreedte, -lengte, skouerhoogte, skrotumomtrek en GDT (R² = 31.62%). Nadat daar vir jaarseisoen gekorrigeer is, was die bydrae van die onderskeie parameters tot lewende gewig aan die einde van toetsperiodes nog in dieselfde volgorde. Nadat ʼn stapsgewyse prosedure vir gekorrigeerde lewende gewig aan die einde van toetsperiodes gedoen is, is gevind dat liggaamsbreedte die grootste bydrae tot lewende gewig aan die einde van toetsperiodes gemaak het. Gemiddelde daaglikse toename het die derde grootste bydrae, d.i. na liggaamsbreedte en –lengte tot gekorrigeerde lewende gewig aan die einde van toetsperiodes gemaak. Vanweë die hoë korrelasies wat tussen die onderskeie liggaamsmates gevind was, is daar ook vir multi-kollineariteit getoets om vas te stel of die parsiële bydraes van die onderskeie liggaamsmetings verander. Dit het egter onveranderd gebly. Regressie-analises het getoon dat liggaamsmates geneem aan die begin en einde van die toetsperiodes betekenisvol (P<0.0001) oor die hele tydperk (1988 – 2000) afgeneem het. Die grootste jaarlikse afnames was vir GDT (-2,5%) en liggaamsbreedte (-1,1%) bereken. Die negatiewe waardes wat verkry is, toon dat die Dorper oor tyd kleiner word. Die toepaslikheid van die visuele seleksiemetodes moet dus ernstig bevraagteken word. Die invloed van gekorrigeerde liggaamsmates en GDT op seleksie volgens rasstandaarde wat deur die rasinspekteurs gedoen is, is toe op puntediagramme aangetoon. Dit het getoon dat die rasinspekteurs deurentyd groter en swaarder ramme vir stoetseleksie selekteer. Daar is egter steeds ramme wat ondergemiddeld presteer en wat aan die Dorperbedryf as stoetramme beskikbaar gestel word. Liggaamsbreedte was die liggaamsmate wat die hoogste voorkeur van rasinspekteurs gekry het. Die liggaamsmate wat die minste deur die rasinspekteurs in ag geneem is, was dié van skouerhoogte. ʼn Moontlike verklaring vir laasgenoemde is dat die tendens van die kleinerwordende Dorper hieraan gekoppel kan word, omdat skouerhoogte 'n aanduiding van raamgrootte is. Visuele beoordeling behoort in samewerking met wetenskaplike metodes gebruik te word om sodoende ʼn optimum grootte vir die Dorper daar te stel.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/19888
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