ENGLISH ABSTRACT: A series of experiments were conducted to determine the effect of the replacement of oat
silage by wheat straw in low forage diets for high producing dairy cows. In the first trial, 28
multiparous Holsteins were randomly assigned to two dietary treatments, namely silage and
straw after completing a covariate period coinciding with the first 40 days of lactation and
remained on these diets for the remainder of experimental period from 41 to 252 days. In
order to evaluate the effect of substitution during different stages of lactation, the
experimental period was divided into four phases. Diets were formulated to support similar
levels of milk for both treatment groups and for all phases. The substitution of oat silage by
wheat straw had a negative effect on dry matter intake, milk production and composition, as
well as on feed and nitrogen efficiency, but had no effect on body weight and body condition.
A rumen fermentation trial was also done, where six multiparous cannulated cows were
randomly allocated in a changeover design to the same dietary treatments used in the
animal performance trial. Despite a relatively low peNDF content and high NFC content of
the diets, the mean rumen pH and time spent below pH 5.8 indicated that cows were actually
not subjected to subclinical rumen acidosis, probably due to effective chewing activity.
Forage source had no significant effect on rumen fermentation end products or mean daily
rumen pH. However, the time spent below pH 5.8 was significantly lower for cows in the
silage treatment than for those in the straw treatment, which could be related to a longer
chewing time and more effective chewing activity that was observed on the straw treatment.
Cows on the straw treatment sorted less against the medium sized particles, which was not
expected, since straw inclusion is normally associated with a higher degree of sorting. In a
third experiment, the effect of the replacement of silage by wheat straw was evaluated in
terms of pre and postpartum animal performance. Dry cow diets were formulated with the
objective to restrict intake, thus preventing cows from consuming metabolizable energy in
excess of their requirements. Both treatments were successful in controlling intake and the replacement of oat silage by wheat straw in dry cow diets had no apprent effect on
postpartum animal performance. Using the data from the lactation experiment, the adequacy
of the CPM Dairy model to predict animal performance from low forage diets was also
evaluated. The model was highly accurate in predicting adjusted milk production of cows
consuming oat silage as basal forage, with a mean bias of only 0.05 kg milk/d, but it overpredicted
milk production of cows receiving diets with wheat straw as the sole forage source,
with a mean bias overprediction of 2.3 kg milk per day during the corresponding period of
positive energy balance. Overprediction of energy balance from straw was confirmed in the
dry cow experiment where a statistical analysis of the residuals between predicted and
actual body condition score change indicated an overprediction on the straw treatment.
AFRIKAANSE OPSOMMING: Die invloed van die vervanging van hawerkuilvoer deur koringstrooi in diëte vir
hoogproduserende melkkoeie is in ‘n reeks proewe nagegaan. In die eerste proef is 28
volwasse Holsteinkoeie ewekansig aan ‘n kuilvoer- of strooigebaseerde behandeling
toegeken na die voltooiing van ‘n kovariaatperiode van 40 dae. Ten einde die invloed van
die behandeling tydens verskillende stadia van laktasie te evalueer, is die proefperiode in
vier fases verdeel. Diëte is geformuleer om vergelykbare melkproduksies vir beide
behandelings tydens al die fases te ondersteun. Vervanging van hawerkuilvoer deur
koringstrooi het ‘n negatiewe invloed op droëmateriaalinname, melkproduksie, melksamestelling,
voeromset en stikstofbenutting gehad, maar geen invloed op ligaamsmassa- en
kondisieverandering nie. In ‘n rumenfermentasieproef is ses volwasse, gekannuleerde
Holsteinkoeie in ‘n omswaai-ontwerp aan twee diëte, soortgelyk aan dié wat tydens die
laktasieproef gebruik is, in toegewys. Ten spyte van ‘n relatief lae peNDF-inhoud en ‘n
relatief hoë NFC-inhoud van die diëte was daar, te oordeel aan die gemiddelde rumen pH
en tyd waartydens die rumen pH onder 5.8 was, geen aanduiding van subkliniese rumenasidose
nie, waarskynlik as gevolg van ‘n langer kou-tyd en meer doeltreffende kouaktiwiteit.
Ruvoerbron het skynbaar ook geen invloed op rumenfermentasie-eindprodukte en
gemiddelde rumen pH gehad nie. Die periode waartydens rumen pH laer as 5.8 was, was
egter korter vir koeie in die kuilvoerbehandeling as vir dié in die strooibehandeling. Dit kan
waarskynlik aan meer doeltreffende herkou-aktiwiteit toegeskryf word. Aangesien strooiinsluiting
in herkouerdiëte normaalweg met ‘n groter mate van voerseleksie geassosieer
word, was die kleiner mate van seleksie teen mediumgrootte partikels met die
strooibehandeling verrassend. In ‘n derde proef is die invloed van van die vervanging van
kuilvoer met koringstrooi in terme van pre- en postpartum diereprestasie ondersoek.
Droëkoeidiëte is geformuleer met die doel om inname te beperk en koeie sodoende te verhoed om meer metaboliseerbare energie in te neem as wat hulle benodig. Albei
behandelings was doeltreffend om inname te beheer en die vervanging van hawerkuilvoer
met koringstrooi het geen invloed op postpartum diereprestasie gehad nie. Deur van die
data van die laktasieproef gebruik te maak, is die toereikendheid van die CPM Dairy model
om diereprestasie vanaf lae ruvoerdiëte te voorspel, ook bepaal. Die model was hoogs
akkuraat in die voorspelling van melkproduksie in die geval van die kuilvoerbehandeling,
maar dit het melkroduksie met 2.3 kg/dag oorskat in die geval van die koringstrooibehandeling.
Hierdie oorberaming van energiebalans is tydens die droëkoeiproef bevestig.
