The effects of leaf shear breaking load on the feeding value of perennial ryegrass for sheep : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Massey University, Palmerston North, New Zealand
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1992
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Massey University
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Abstract
Reducing physical resistance has been thought to be a key factor to increase
efficiency of masticatory breakdown of forage, which may lead to faster rumen
fractional outflow rates (FOR) and consequently to increased voluntary feed intake
and hence improved feeding value (FV). Two selections of perennial ryegrass
(PRG) were selected for low (LS) and high leaf shear breaking load (HS) based on
the maximum load required to shear across the leaf, i.e. leaf shear breaking load
(LSBL). The series of experiments were conducted to investigate the effects of
LSBL on the FV of PRG for sheep.
LSBL, morphological and anatomical parameters were measured on the LS and HS
PRG selections grown under the optimum climatic conditions. LSBL for the LS
PRG selection was approximately 41 % lower the HS PRG selection. However,
the LS selection had shorter leaf lengths, narrower leaf widths and narrower leaf
cross-sectional area (CSA) than the HS selection. Therefore, in leaf shear strength
(LSS), the LS selection was estimated to be approximately 27 % less resistant to
shear than the HS selection per unit of CSA. The lower LSS in the LS selection is
due to the lower concentration of sclerenchyma tissues in leaf CSA compared with
the HS selection. However, the differences in the total shear load required to
breakdown a unit dry weight of leaves to 1 x 1 mm·particle size, namely, leaf index
of masticatory load (IML) between the selections were influenced by the
differences in m orphological characteristics of leaves between the two PRG
selections.
Comparisons were made between the LS and HS PRG selections in the efficiency
of mastication by sheep on particle breakdown. There were no major effects of
reduced LSBL in PRG on the efficiency of mastication during eating and during
rumination. Although the LS PRG selection was approximately 29 % lower in
LSBL than the HS PRG selection, the difference for the two PRG selections in
IML was almost nil.
Effects of LSBL in PRG on rumen fractional outflow rate (FOR) and apparent
digestibilities were investigated in sheep fed at restricted feed intake levels. There
were no effects of reduced LSBL on FOR, although the LS PRG selection was
approximately 39 and 12 % lower than the HS PRG selection in LSBL and IML,
respectively. The digestibility of the cellulose fractions was approximately 16 %
lower in the LS PRG selection than the HS PRG selection. The leaf morphology in
PRO may affect the efficiency of fibre digestibility.
Two field trials were conducted to test the hypothesis that LSBL in PRG improves
FOR and leads to higher voluntary feed intake, and hence achieves improved live
weight gain and wool production, namely FV. Although the LS PRG selection had
25-30 % lower LSBL than the HS PRG selection, live weight gain and wool
production of sheep were not improved by reduced LSBL. FOR in sheep showed
no indications of difference and voluntary feed intake was similar between the
animals grazing the LS and HS PRO selections. The lack of difference in IML
between the LS and HS PRG selection can be considered as a main reason for this.
The hypothesis, that reduced LSBL in PRG would improve its FV, was therefore
rejected.
In conclusion, there were no major effects of reduced LSBL in PRG on efficiency
of masticatory particle breakdown, and consequently, FOR, feed intake and hence
FV in sheep. This is due to the lack of selection effect of PRG in IML. IML is a
determining fac tor for the efficiency of mastication both during eating and
rumination. The selection of PRG for a lower IML will, therefore, be necessary in I
order to increase efficiency of masticatory particle breakdown, FOR and hence FV
of PRG.
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Keywords
Forage, Perennial ryegrass, Sheep feeding and feeds, Ryegrass digestion