2. LA SAGA/ FUGA DE J.B (1972)
2.1. LA IMAGINACIÓN DE BASTIDA
NS, P>0. 1 ; (*) , P<0 . 1 0; *, P<0.05.
SED2.85
7.44 4.470.1 1 0
0.01 9
0.053
0.050
0.1 1 0
SIGNIFICANCE OF DIFFERENCE NS " " " NS " •(
") .... N ....122
L4 DISCUSSION.
'olyethylene glycol ( 1 .7g/g CT) displaces cr from cr-protein complexes (Jones and 1angan, 1977) and completely binds available cr (Barry and Forss, 1983). Consequently, ,hen PEG binds cr in the rumen, preventing it from binding to dietary protein, plant
rotein can be degraded to rumen NH3. Therefore, the increase in rumen NH3 concentration
, . " . 0 ....
·
fter 3 days of intraruminal PEG infusion, indicates that in the present study PEG infusions .•
ffectively rendered CT unreactive in the rumen .
1 sheep fed Lotus pedunculatus, the presence of cr (55g!kgDM) had a major effect on
lcreasing the entry rate of cystine into plasma, the flux of cystine to productive processes nd maintenance and the conversion of plasma methionine to cystine. This has potentially :gnificant benefits for productive processes such as wool growth.
he lower IRL of plasma sulphate in control sheep (ie ... with the cr operating) compared to EG sheep was a result of a lower rate of oxidation of plasma methionine (P<O.05) and ystine (P<O.05), and a lower rate of sulphate entry (P<O.05) into plasma from sources
ther than the oxidation of SAA. The lower sulphate entry rate into plasma in control sheep las a direct effect of a lower (P<O.OOI ) absorption of sulphide from the rumen (represented y rumen reducible-S IRL; Table 5), which is the main source of sulphate entry into plasma e'ig. 3; Kennedy et al., 1975; Kennedy and Milligan, 1978). These observations indicate lat the presence of reactive cr reduced rumen protein degradation and the reduction of AA to sulphide in the rumen of control, compared to PEG sheep. The changes in sulphate letabolism (Table 5) in control sheep compared to PEG sheep resulted in less sulphur
eing excreted in urine (p<O.05) and recycled to the digestive tract and tissues (P<O.05).
be recycling of plasma sulphate to the digestive tract and tissues in sheep fed Lotus edunculatus (this study; Table 5) was lower than for sheep with a similar plasma sulphate �L, but fed lucerne pellets (Kennedy and Milligan, 1 978). However, in sheep fed Lotus edunculatus a higher proportion of plasma sulphate was excreted in the urine than in
1eep fed lucerne pellets (Kennedy and Milligan, 1 978; Table 5). The difference in the
artitioning of plasma sulphate IRL between recycling and excretion in the urine was
robably due to more extensive degradation of protein in the rumen of sheep fed fresh
otus pedunculatus than sheep fed lucerne pellets.
he plasma methionine and cystine IRL reported in this study are higher than published ata (Table 6) although, this was probably due to the higher S-intake of sheep in this study.
1 control compared to PEG sheep there was a 4 1 % reduction in the rate of plasma
lethionine oxidation and a 79% increase in the rate of transulphuration of methionine to
., . ,��, .. ' . " \ . ' f' .. . ,. . ,. : . I · ;. · , • ' . . .. ',.: · -. . . " . � :. ',.
:ystine in post-hepatic blood. Together with those changes, there was a 36% decrease in )lasma cystine oxidation (P<0.05) and a 76% increase in cystine entering the plasma from he digestive tract and whole body protein turnover (P<0.05). In digestion studies, control :heep fed Lotus pedunculatus containing 55gCf/kgDM, had a higher apparent absorption )f methionine from the small intestine (+23%; P<0.00 1), whilst cystine apparent absorption .vas not significantly (+4%; P>0.05) affected (Chapter 2). In contrast, in control sheep the �ntry rate of methionine into post-hepatic blood was not significantly different, whilst the !ntry rate of cystine was increased (P<0.05), compared to PEG sheep. Radcliffe and Egan )978) reported that the liver was a major site of transulphuration, so it is probable that the ilcreased methionine absorbed from the small intestine in control sheep, was
rransulphurated in the liver and appeared in post-hepatic blood as cystine.
In control sheep, decreased SAA oxidation, increased transulphuration of methionine to
cystine and increased cystine entry rate, resulted in a 1 10% increase in plasma cystine flux to maintenance and productive processes (P<0.05) compared to PEG sheep. This suggests
that there was a higher metabolic requirement for cystine than methionine in control
compared to PEG sheep.
Waghorn et al. (1987) reported that the presence of CT in Lotus corniculatus
(22gCT/kgDM) increased apparent absorption of EAA from the small intestine by 62%. This effect of CT was probably responsible for the increased N-retention in sheep fed CT containing forages compared to sheep fed comparable forages which did not contain CT (Egan and Ulyatt, 1 980; John and Lancashire, 1 98 1 ; Waghorn et al., 1 987). A possible consequence of the increased protein deposition, would be reduced oxidation of limiting amino acids such as that reported for SAA in the present study.
The sulphur content of clean dry wool is 2.7-4.2% (Reis 1965 a, b) and consists of 26 residues of cysteine for every one residue of methionine (Marshall and G illespie, 1977). This suggests wool growth is a productive process with a very high demand for cysteine. The increased flux of cystine to productive processes and maintenance in control sheep could therefore be expected to result in an increased rate of wool growth. It has recently
been demonstrated that the wool growth of control sheep grazing sulla (Hedysarum
coronarium) containing CT (48g/kgDM) was 1 2% higher than that of similar sheep grazing sulla but given a daily drench of PEG (T.N.Terrill et at., unpublished).
Presumably, when methionine absorption from the small intestine (3g!d) exceeds the transulphuration capacity of the liver (Pi sulewski and Buttery, 1985), the increased methionine entry into plasma would also stimulate the rate of transulphuration of
124
methionine than cysteine, this would explain why post-ruminal infusions of methionine, cystine or cysteine (2-3g!d) are equally effective at increasing wool growth (Reis, 1979).
The high cysteine content of wool and the well documented observation that wool growth responds to post-ruminal supplementation of cystine or cysteine (Reis, 1 979) suggests cysteine is a limiting amino acid for wool growth. The apparent flux of sulphur away from methionine, to plasma cystine and the increased flux of plasma cystine to productive processes and maintenance reported in this study for control sheep, also supports this suggestion. The presence of CT in forage diets would appear to provide one practical means of increasing SAA absorption from the small intestine, and the subsequent flux of cystine to productive processes and maintenance. It has been well documented that increasing the dietary CT concentration from 5 to 95gCT/kgDM increases non-ammonia nitrogen (NAN) flow to the small intestine in sheep fed Lotus species (Barry et al., 1986; Waghorn et aI., 1989). However, in control sheep fed Lotus corniculatus (22gCT/kgDM), Waghorn et al. (1987) demonstrated a greater increase (62%; P<O.Ol ) in EAA absorption from the small intestine than was reported for methionine (27%; P<O.OO l ) and cystine (4.5%; P>O.05) in control sheep fed Lotus peduncu[atus (55gCT/kgDM; Chapter 2). Therefore, further research is necessary to better define the relationship between CT concentration in the diet, absorption from the small intestine and changes in plasma SAA metabolism, particularly at lower CT concentrations, as it may be possible to further improve the flux of cystine to productive processes and maintenance. The dietary concentration of CT needed to maximise these processes needs to be defined.
, . " ' " . ' tII '. . ·tI' \ . . , . .
WMcNabb (this study)
Kennedy and Milligan
(1978)