Physiological responses to selection for greasy fleeceweight 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
The study was undertaken to identify physiological differences
between the Control (C) and Fleeceweight-selected (FW) lines of Romney
sheep at Massey University. These differences were examined with
a view to determining the mechanisms by which sheep of high genetic
merit attain their superior fleece production, and to identification
of potential markers of genetic merit.
Three experiments were conducted using rams and ram hoggets
from the selection lines. The first (E1) was a preliminary investigation
in which 12 C and 12 FW rams (aged 14 months) were fed freshly
cut pasture ad libitum and blood sampled by jugular venipuncture. In
the second (E2) 9 ram hoggets from each line were fed a lucerne-based
diet (at fixed intakes irrespective of liveweight) during a
stabilisation period and a subsequent treatment period .. ·in which
they received the basal diet plus one of 3 levels (0, 30, or 60g)
of formaldehyde-protected casein. In the third experiment (E3) rams
received an intravenous infusion of saline or saline plus l.Sg/day
methionine in a switchback design. During this experiment rams were
fed 110% maintenance (lucerne chaff) in a regimen designed to maintain
steady state conditions.
In contrast to literature reports, no difference was found
between the lines in plasma levels of either aspartate or alanine
amino-transferase activities (E2). FW rams did maintain greater
concentrations of reduced glutathione (GSH) in erythrocytes but
differences were not significant. Concentrations of GSH were substantially
increased by methionin~ infusion (E3).ii
In all three experiments, C rams maintained greater concentrations
of urea in the plasma than FW rams (by approximately lmM).
Supplementation of the diet with protected casein increased plasma
urea concentration but this effect was additive with that of selection
line (E2). Methionine infusion did · not affect plasma urea in FW
rams but reduced that of C rams so that the between-line difference
was reduced by 50%. Control rams were also observed to have greater
creatinine concentrations than FW rams (E3) but these were not
influenced by methionine infusion. The reduced plasma urea concentration
of FW rams may be due both to their lower rates of amino acid
deamination and to an increased glomerular filtration rate.
Fleeceweight rams also exhibited lower concentrations of
thyroxine than C rams in the two experiments in which they were
examined (E2, E3). This result is consistent with those from
Australian Merino selection lines but the physiological basis for
the difference is unknown.
It is concluded that, in view of their consistent association
with genetic merit for fleece production, and their ease of measurement,
plasma concentrations of urea, creatinine and thyroxine may
prove to be useful predictors of genetic merit for wool production.