The effect of genetic selection for fleece weight on urea metabolism and digestive function in Romney sheep: a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science in Animal Science at Massey University
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Date
1987
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Massey University
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Abstract
The study was undertaken to investigate the effect of 30 years
selection for greasy fleece weight on rumen metabolism, apparent
digestibility and nitrogen metabolism in the NZ Romney sheep.
Previous studies had recorded a lower plasma urea concentration in
the fleece weight selected (Fwt) animals as compared to the randomly
selected control (C) animals, so most emphasis was placed on
determining whether the lower plasma urea concentration in the Fwt
sheep was accompanied by a lower irreversible loss (IRL) .
Eight C and eight Fwt yearling rams (mean liveweight 42kg) were
housed inside in individual metabolism crates. The study was divided
into two almost identical experiments - the first one when the
animals were fed on a chaffed meadow hay diet and the second on a
lucerne chaff diet. Each experiment consisted of a two week
adaptation period, a ten day digestibility period and a final period
of six days during which 14c-urea was infused intravenously for
fourteen hours. At the end of the lucerne chaff experiment the
animals were slaughtered and the rumen contents weighed and
subsampled.
The Fwt animals had a one mM lower plasma urea concentration
when fed on both diets. However there was no difference in the
plasma urea IRL, urinary urea excretion or urea recycling to the
digestive tract between the Fwt and C sheep when fed on either diet.
Relative to the C sheep, the Fwt animals had a higher creatinine
clearance rate (63 vs 50 ml/min; P<0.05) when fed the meadow hay
diet, and a higher urea clearance rate when fed both diets (meadow
hay diet 42 vs 32 ml/min, P<0.10; lucerne chaff diet 72 vs 60 ml/min,
P<0.05).
When fed the meadow hay diet there were no differences between
the Fwt and C animals in voluntary feed intake (VFI) (Fwt animals 44
vs 42 gDM/kgw0 · 75;d C animals) or apparent digestibility (DM
digestibility 55% for both genotypes). However when fed on the
lucerne chaff diet the Fwt animals had a higher VFI than the C
animals (102 vs 94 gDM/kgw0 · 75 /d) and a lower apparent dry matter
digestibility (56.1% VS 61.3%; P<0.05). These changes were
associated with an increase in the rumen Fractional Outflow Rate
(FOR) of lignin in the Fwt sheep (3.00 vs 2.58 %/h; P<0.05). There
was a higher molar proportion of acetate present in the rumen fluid
of the Fwt animals than in the C animals (68.6 vs 64.0%; P<0.10).
These results confirm that the Fwt animals have a consistently
lower plasma urea concentration over a range of nutritional levels
(0.6X maintenance and 1.5X maintenance) than the C animals, but there
was no difference in urea metabolism as measured by plasma urea IRL
and urea excretion. The urea and creatinine clearance rates suggest
that selection for fleece production may have altered kidney
function, but that the expression of these differences is related to
the nutritional level. The greater FOR in Fwt sheep fed at the high
level of nutrition (i.e. lucerne chaff) may mean that the amino acid
flow at the duodenum (undegraded dietary plus microbial protein) is
greater than in the C animals and this could be a factor contributing
to the superior wool production of the Fwt sheep.
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Keywords
Romney marsh sheep, Urea, Digestion, Physiology, Metabolism