Thesis (PhD (Agric))--University of Stellenbosch, 2005. / ENGLISH ABSTRACT: It is generally accepted that feed intake and growth (gain) are the most important economic
components when calculating profitability in a growth test or feedlot. Feeding cost of animals is a
major determinant of profitability in livestock production enterprises. Genetic selection to improve
feed efficiency aims to reduce the cost of feeding in beef cattle production and thereby improve
profitability.
The objective of this study was to define a clear selection objective to enable South African beef
breeders and especially the feedlot industry to select for post-weaning growth or feedlot
performance and to identify factors influencing profitability in a feedlot environment.
Because of the recording of individual feed intake and weight gain values in the South African
Agricultural Research Councils' centralized growth tests, it was also possible to calculate a
phenotypic value for feedlot profitability (R-value) for each bull tested in a centralized growth test.
(Co)variances, using multitrait as well as random regression models, for and between feedlot
profitability, weaning weight and other production, reproduction and efficiency traits were
estimated. Residual feed intake (RFI) and feed conversion ratio (FCR) as efficiency traits were also
compared to growth (average daily gain (ADG), weaning weight (WW) and shoulder height
(SHD)), reproductive (scrotum circumference (SCR)) and profitability (feedlot profitability) traits
measured in growth tests of young Bonsmara bulls.
Consequently, a single post-weaning growth selection index value based on the economic and
breeding values of different selection criteria related to feedlot profitability was composed.
(Co)variance components, heritabilities and genetic correlations for and between initial weight
(lW), final weight (FW), total feed intake (FI) and shoulder height (SHD) were estimated through
the use of multitrait restricted maximum likelihood (REML) procedures. These breeding values
(EBV s) were then used in a selection index to calculate a single economical value for each animal. This economical value is an indication of the gross profitability value or gross test value (GTV) of
the animal in a post-weaning growth test.
The heritability estimate of 0.36 for R-value, obtained from the multitrait analysis, shows that this
trait is genetically inherited and that it can be selected for. The heritability for R-value obtained
from the single trait random regression model varied between 0.57 and 0.62. The genetic
correlations between the R-value and the other traits, obtained from the multitrait analysis, varied
from negligible to high. The heritability estimated for FCR was 0.34 and for RFl 0.31 with a
genetic correlation estimate of 0.75 between the traits. The estimated genetic correlation between
profitability (R-value) and FCR and RFl were -0.92 and -0.59, respectively. The genetic
correlation estimate of -0.92 between FCR and R-value is largely due to the part-whole relationship
between these two traits. This is also shown in their genetic trends. The genetic correlations and
expected correlated responses between RFl and FCR with R-value suggest that indirect selection for
R-value through the direct selection for FCR and/or RFl will result in slower genetic progress in Rvalue
than direct selection for R-value. However, where the R-value cannot be calculated and/or
where direct selection for R-value is not possible, it would be better to select indirectly for R-value
through the use of FCR rather than RF!. Consequently, a regression equation was developed (with
an R2 of 0.82) to estimate a feed intake value for all performance-tested Bonsmara bulls which were
group fed and whose feed intakes were unknown. These predicted feed intake values made it
possible to calculate a feedlot or post-weaning growth profitability value (R-value) for all tested
bulls even where individual feed intakes were unknown. Subsequently, an R-value for each bull was
calculated in a favourable economic environment (FEE), an average economic environment (AEE)
and in an unfavourable economic environment (VEE). The high Pearson and Spearman correlations
between the EBV s based on AEE and the other two environments suggested that the average
economic environment could be used to calculate EBVs for R-value or feedlot growth profitability.
It is therefore not necessary to change the carcass, weaner or feed price on a regular basis to account
for possible re-rankings based on R-value EBVs.
Heritabilities for lW, FW, Fl and SHD were 0.41, 0.40, 0.33 and 0.51, respectively. The highest
genetic correlations between these traits were the 0.78 (between lW and FW) and 0.70 (between Fl
and FW). GTV values varied between -R192.l7 and R231.38, with an average of R9.31. The
Pearson correlations between EBVs (for production and efficiency traits) and GTV range from
-0.51 to 0.68. The lowest correlation (closest to zero) was 0.26 between the Kleiber ratio (KLB) and
GTV. Correlations of 0.68 and -0.51 were estimated between average daily gain (ADG) and GTV
and feed conversion ratio (FCR) and GTV, respectively. The heritabilities of the different traits
included in the selection index suggest that it is possible to select for a GTV. The selection index can benefit feedlotting In selecting offspring of bulls with high GTV values to maximize
profitability.
The Pearson and Spearman correlations between the R-value EBVs and the index values (GTV)
were very high (0.97). This high correlation of 97% indicates that it is not important which method
is used to calculate a genetic post-weaning growth of feedlot profitability value. The selection index
value is, however, more simplified than the feedlot profitability with less assumption. Therefore, it
is recommended that the post-weaning selection index value be used as a selection objective in
breeding programmes to improve post-weaning growth profitability rather than the more complex
feedlot profitability value. / AFRIKAANSE OPSOMMING: 'N GENETIESE ANALIESE VAN DIE BIOLOGIESE EN EKONOMIESE
DOELTREFFENTHEID VAN NASPEENSE GROEI IN VLEISBEESTE IN DIE VOERKRAAL: Dit word algemeen aanvaar dat voerinname en groei die twee ekonomies mees belangrike
komponente in die berekening van 'n naspeense groei- of voerkraalwinsgewindheidswaarde is.
Voerkostes is 'n bepalende faktor van winsgewindheid in enige lewendehawe boerderypraktyk.
Seleksie om voerdoeltreffendheid te verbeter, verminder dus die voerkostes in vleisbeesproduksie
en gevolglik 'n verhoging in die winsgewindheid.
Die doelwit van die studie was om faktore te identifiseer wat 'n invloed op winsgewindheid in die
voerkraaiomgewing het asook om 'n duidelike seleksiedoelwit te formuleer wat die Suid-
Afrikaanse vleisbeesteiers en veral die voerkraalbedryf instaat sal kan stelom vir naspeense groeidoeltreffendheid
ofvoerkraalwinsgewindheid te selekteer.
As gevolg van die aantekening en rekordhouding van weeklikse individuele voennnames en
gewigstoenames van alle prestasiegetoetsde bulle, in 'n gesentraliseerde groeitoets deur die Suid-
Afrikaanse Landbou Navorsingsraad getoets, was dit moontlik om vir elk van hierdie bulle 'n
fenotipiese voerkraalwinsgewindheidswaarde (R-waarde) te kon bereken.
(Ko)variansies is vir en tussen voerkraalwinsgewindheid, speengewig en ander produksie-,
reproduksie- en doeltreffendheidseienskappe bereken deur van meereienskap en ewekansige
regressie modelle gebruik te maak. Twee doeltreffendheidseienskappe naamlik residuele
voerinname (RFI) en voeromsetverhouding (FCR) is ook met groei (gemiddelde daaglikse toename
(ADG), speengewig (WW) en skouerhoogte (SHD)), reproduksie (skrotumomvang (SCR)) en
winsgewindheidseienskappe (voerkraalwinsgewindheid (R-waarde)) vergelyk, om sodoende te
bepaal watter een die mees geskikte eienskap is om indirek vir voerkraalwinsgewindheid of groei, gebaseer op teeltwaardes en ekonomiese waardes vir die verskillende eienskappe, wat 'n
invloed op naspeense groei winsgewindheid het, gestruktueer.
(Ko)variansiekomponente, oorerflikhede en genetiese korrelasies vir en tussen begingewig (lW),
eindgewig (FW), voerinname (FI) en skouerhoogte (SHD) is bereken deur van 'n meereienskap
(REML) ontleding gebruik te maak. Hierdie teelwaardes (EBVs) is vervolgens in 'n seleksie-indeks
gebruik om 'n enkele ekonomies of voerkraal-winsgewindheids seleksie-indekswaarde (GTV) vir
elke dier te bereken. Hierdie ekonomiese waarde is 'n aanduiding van die bruto
winsgewindheidswaarde ofbruto toetswaarde (GTV) van die dier in 'n naspeentoets.
Die oorerflikheid, vanuit die meereienskapontleding vir R-waarde beraam, was 0.36. Hierdie
oorerflikheid dui daarop dat die eienskap oorerflik is en dat dit wel moontlik is om daarvoor te
selekteer. Die ooreflikhede van R-waarde, voorspel vanuit die enkeleienskap ewekansige regressieontleding
varieer tussen 0.57 en 0.62. Die genetiese korrelasie tussen R-waarde en ander
eienskappe, vanuit die meereienskap ontleding beraam, varieer tussen weglaatbaar klein tot hoog.
Die oorerflikheid van FeR was 0.34 en van RFI 0.31 met 'n genetiese korrelasie van 0.75 tussen die
twee eienskappe. Die genetiese korrelasie tussen R-waarde en FeR, en R-waarde en RFI was
onderskeidelik -0.92 en -0.59. Die rede vir die hoë negatiewe genetiese korrelasie tussen R-waarde
en FeR van -0.92 is omdat dieselfde komponente in die berekening van die twee eienskappe
gebruik is. Dit word ook in die genetiese tendense weerspeël. Die genetiese korrelasies en verwagte
gekorreleerde responsies tussen R-waarde en FeR, en tussen R-waarde en RFI dui daarop dat
stadiger genetiese vordering verkry sal word in R-waarde deur direkte seleksie vir beide FeR en
RFI as wat verkry sal word deur die direkte seleksie vir R-waarde. Wanneer 'n R-waarde egter nie
bereken kan word nie of waar dit nie moontlik is om direk vir R-waarde te selekteer nie, sal
vinniger genetiese vordering in R-waarde gemaak word deur die direkte seleksie vir FeR as vir
RF!. 'n Regressievergelyking is geformuleer (met 'n R2 van 0.82) om vir alle prestasiegetoetsde
bulle, waar bulle in 'n groep gevoer is en individuele voerinnames onbekend is, 'n
voerinnamewaarde te voorspel. Hierdie voorspelde voerinnames maak dit moontlik om vir elke
prestasiegetoetsde bul ,'n naspeengroei- of voerkraalwinswaarde (R-waarde) te bereken, al is hulle
individuele voerinnames onbekend. Vervolgens is drie verskillende R-waarde vir vleisproduksie vir
elke bul bereken naamlik, in 'n gunstige ekonomiese omgewing (FEE), 'n gemiddelde ekonomiese
omgewing (AEE) en 'n ongunstige ekonomiese omgewing (VEE). Die hoë Pearson en Spearman
korrelasies tussen die EBVs vir R-waarde, bereken in die AEE en die EBVs in die ander twee
ekonomiese omgewings, dui daarop dat die AEE gebruik kan word om EBVs vir naspeense groeiof
voerkraalwins te bereken. Dit is dus nie nodig om op 'n gereelde grondslag die karkasprys,
lewendige speenkalfprys of die voerprys te verander nie. Oorerflikhede, vanuit die meereienskap ontledings VIr lW, FW, FI en SHD verkry, was
onderskeidelik 0.41, 0.40, 0.33 en 0.51. Die hoogste genetiese korrelasies tussen die eienskappe
was 0.78 tussen lW en FW en 0.70 tussen FI en FW. GTV indekswaardes varieer tussen -Rl92.17
en R231.38 met 'n gemiddelde waarde van R9.31. Die Pearson korrelasies tussen die EBVs van
produksie- en doeltreffenheidseienskappe en GTV het tussen -0.51 en 0.68 gevarieer. Die
korrelasie naaste aan zero, van 0.26, was die korrelasie tussen GTV en die Kleiber-verhouding. Die
korrelasies tussen GTV en ADG, en GTV en FeR was onderskeidelik 0.68 en -0.51. Die
oorerflikhede van die verskillende eienskappe wat in die seleksie-indeks ingesluit is, dui daarop dat
die indekswaarde weloorerflik is en dat seleksie hiervoor wel moontlik is. Hierdie indekswaarde
kan deur die voerkraaiindustrie gebruik word om nageslag van diere met hoë GTV waardes te
selekteer om sodoende maksimum wins uit die voerkraai te genereer.
Die Pearson en Spearman korrelasies tussen R-waarde EBVs en die indekswaardes (GTV) was
besonder hoog (0.97). Hierdie hoë korrelasie dui daarop dat dit geen verskil sal maak watter een
van die twee metodes gebruik word in die berekeninge van 'n naspeense groei- of
voerkraalwinswaarde nie. Die seleksie-indeks metode is egter minder gekompliseerd met minder
aannames as in die geval van die rekeningkundige fenotipiese benadering (R-waarde). As gevolg
hiervan, word die naspeense seleksie-indeks waardes (GTV) aanbeveel om te gebruik as 'n
teeldoelwit in telingsprogramme om naspeense groei- of voerkraaiwins geneties te verbeter, eerder
as die meer gekompliseerde fenotipiese voerkraaiwins (R-waardes) metode.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/50249 |
| Date | 03 1900 |
| Creators | Van der Westhuizen, Robert Rolfe |
| Contributors | Schoeman, S. J., Van Der Westhuizen, J., Stellenbosch University. Faculty of AgriSciences. Dept. of Animal Sciences. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 74 p. : ill. |
| Rights | Stellenbosch University |
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