The luteolytic effect of prostaglandin F2α (PGF2α) during the confirmed luteal phase of the oestrous cycle was evaluated in ten Boran and ten Boran × Friesian cross-bred heifers. Following injection with 25 mg Lutalyse, animals were bled every 6 h for 96 h and plasma progesterone (P4) determined by the ELISA technique. Borans had significantly (P < 0·05) smaller corpora lutea (12·01±0·72 ν. 17·03±2·10 mm) and responded faster to PGF2α injection (65·57±1·40 ν. 78·27±2·18 h) than the cross-bred heifers. However, there was no significant difference in either the initial P4 values (6.24±0·98 ν. 8·00±1·71 ng/ml) or the rate of its decline following PGF2α injection between the two breeds. Values declined sharply to basal levels (11% of the initial pretreatment values) within 48 h in both breeds. All ten cross-breds and eight Borans showed standing oestrus within a week of treatment. However, oestrus was better synchronized (P < 0·05) in Borans than cross-breds. It was concluded that PGF2α is effective for oestrus synchronization in both breeds, but oestrus occurred earlier and was more precise in Borans than in cross-breds.

The effects of two diets formulated to be either synchronous or asynchronous with respect to the hourly supply of energy and nitrogen on rumen fermentation and microbial protein synthesis were studied in sheep.

In Expt 1, the in situ degradation characteristics of nitrogen (N), organic matter (OM) and carbohydrate (CHO) fractions were determined in winter wheat straw, winter barley, malt distillers dark grains rapeseed meal and fishmeal. The feeds exhibited a large range in degradability characteristics of the nitrogen and energy-yielding fractions.

A computer program was developed based upon the raw material degradation characteristics obtained from the above studies. The program was used to formulate two diets with similar metabolizable energy (9·5 MJ/kg DM) and rumen degradable protein contents (96 g/kg DM) but to be either synchronous (diet A) or asynchronous (diet B) with respect to the hourly rate of release of N and energy. The program was used to predict the hourly release of N, OM and CHO and the molar production of volatile fatty acids (VFA).

In Expt 2, the two diets were fed to four cannulated sheep at the rate of 1 kg/day in four equal portions, in two periods, using a change-over design. Rumen ammonia concentrations followed the predicted rate of N degradation. A maximum concentration of 10·5 and 7 mM for diets A and B respectively was achieved within the first hour of feeding which then fell to 7 and 3 mM respectively. Rumen VFA proportions were more stable for the synchronous diet (A) than the asynchronous diet (B) and were more stable than predicted for both diets. True ruminal degradation of OM and CHO was similar for both diets and close to that predicted, although fibre degradability in diet A was 30% lower than predicted due to a reduction in both cellulose and hemicellulose digested. Microbial protein production was estimated simultaneously with L-[4,5–3H]leucine and a technique based on cytosine. Estimates varied with marker but mean values indicated a 27% greater production of microbial N (g N/kg DM I) with the synchronous diet (A) and an average improvement in microbial protein efficiency (g N/kg OM truly degraded or CHO apparently degraded) of 13%, although neither difference was significant. There was evidence of a greater recycling of N in the animals and a significantly lower content of rumen degradable protein when the sheep were fed the asynchronous diet (B).

The results are consistent with the view that synchronizing the rate of supply of N and energyyielding substrates to the rumen micro-organisms based upon ingredient in situ degradation data can improve microbial protein flow at the duodenum and the efficiency of microbial protein synthesis.

Four silage diets were prepared from tropical grass and legume mixtures and fed to intestinally fistulated sheep to determine the sites and extent of digestion of the different silage components in the digestive tract. Pangola grass (Digitaria decumbens) was ensiled either alone or with the addition, 33% (w/w), of leucaena (Leucaena leucocephala cv. Cunningham), gliricidia (Gliricidia sepium) or cowpea (Vigna unguiculata cv. Caloona). All forages were grown at the University of Queensland farm in 1985. All silages were well preserved as indicated by low pH (< 4·1), ammonia N (< 9% total N), acetic acid (< 13 g/kg dry matter (DM)) and ethanol (< 19 g/kg DM) contents, and high DM (> 290 g/kg) and lactic acid (> 41 g/kg DM) contents. The inclusion of leucaena and gliricidia significantly increased the total N content of the silages. The addition of legumes at ensiling significantly increased rumen ammonia N (pangola 59 mg N/l, legume diets 110–115 mg N/l) and the proportion of propionic acid in rumen fluid. Sheep given pangola + leucaena silages had significantly higher DM intakes (66·3 g/kg W0·75 per day) than sheep given pangola and pangola + cowpea silages (52·4 and 53·2 g/kg W0·75 per day respectively). The inclusion of gliricidia significantly depressed silage intake (43·1 g/kg W0·75 per day). There were no significant effects of diet on DM (551–609 g/kg) and organic matter (550–612 g/rkg) digestibility. Sheep given pangola + leucaena silages had higher N intakes, N balances, rates and efficiencies of microbial N synthesis and total non-ammonia N (NAN) flows to the small intestine than did sheep given the other silages. Apparent degradabilities of silage N in the rumen were similar for all diets (0·491–0·559 g N/kg N intake), but sheep given pangola + leucaena silages had lower true (corrected for endogenous N) N degradabilities (0·648 g N/kg N intake) than those given the other diets (0·751–0·775 g N/kg N intake). The values obtained in this experiment were compared with those reported for temperate grass silages, and it was concluded that whilst the digestibility of tropical grass/legume silages was low, intakes and rumen fermentation characteristics were similar to those found for temperate grass silages.

Nitrogen balance and flow of nitrogen along the gastrointestinal tract were determined in growing wether sheep fed a dried grass pellet diet with or without intraruminal propionic acid infusion. Plasma free amino acid and glucose concentrations tended to be higher in peripheral blood of animals receiving propionate (P < 0·10). Overall total nitrogen and amino acid digestibilities and apparent digestibilities of each fraction along the digestive tract were not affected by infusion of propionic acid. The contribution of microbial amino acids to total amino acid flow in the duodenum measured using purine bases as the microbial marker averaged 0·49 and was not affected by propionic acid infusion. It is suggested that changes in circulating amino acid levels arise from reduced utilization of luminal amino acids by gastrointestinal tissues due to the increased availability of alternative energy-yielding substrates.

An experiment was conducted at Toorak Research Station, Julia Creek, in semi-arid northern Australia (141° E, 21° S) during 1990 to determine the relationship between placental and fetal weight in sheep after placental growth had been restricted by chronic heat stress during mid-pregnancy. Placental and fetal weight were measured in single bearing ewes housed either in a thermoneutral environment throughout pregnancy, or in a heated room between the 30th and 80th days of pregnancy followed by a thermoneutral environment until the 140th day of pregnancy. The placental weight of the heat-treated ewes was lower than that of the control ewes at the 80th (P < 0·05) and 140th (P < 0·01) days of pregnancy. Fetal weight and dimensions did not differ significantly between control and heat-treated ewes at the 80th day of pregnancy, although fetal weight (P < 0·01) and fetal dimensions (P < 0·05) for the previously heat-treated ewes were lower than those for the control ewes at the 140th day of pregnancy. Placental and fetal weight at the 140th day of pregnancy were correlated (P < 0·05) with the rectal temperature of ewes measured at 08.00 and 16.30 h during the period of heat-treatment, but not with the change in rectal temperature between 08.00 and 16.30 h. It was concluded that restricted placental growth in heat-treated ewes retarded fetal growth during late pregnancy even in the absence of heat treatment, and it is suggested that selection of ewes which can maintain normal rectal temperatures during periods of heat stress would produce lambs of normal birthweight in a hot climate.

Oestrus and ovulation data from 104 adult Corriedale ewes, recorded daily from February 1988 to May 1990 in Argentina, were analysed to estimate repeatabilities of and phenotypic correlations between onset and cessation of the breeding season, duration of the anoestrus and ovulation rate.

The repeatability of dates of onset and cessation of the breeding season and duration of the anoestrus was 0·48±0·080, 0·49±0·075 and 0·50±0·081, respectively.

The duration of the anoestrus was significantly correlated both with onset (r = 0·24; P < 0·05) and with cessation (r = –0·47; P < 0·01) of the previous and subsequent breeding seasons, respectively.

There was no correlation between dates of onset and cessation of the breeding or non-breeding seasons.

Repeatability estimates of ovulation rate were 0·27 ± 0·034, 0·34 ± 0·081 and 0·43 ± 0·094 using the normal, logit and probit scales, respectively. Phenotypic correlations between ovulation rate and the components of the breeding and non-breeding seasons were all non-significant.

Five Suffolk crossbred ram lambs were administered ritodrine, a β2-agonist drug, by continuous intravenous infusion at 0·5 μg/kg per min for 9 days to determine effects of prolonged β-agonist infusion on metabolism and on the responsiveness of the animals to short-term infusions of adrenaline. Consistent with other investigations, this study showed that initially the drug caused rapid increases in heart rate and plasma concentrations of free fatty acids (FFA), lactate and insulin, but decreased plasma growth hormone. There was no effect on plasma glucose or pancreatic glucagon concentrations. After 24 h of ritodrine infusion, these effects were attenuated as tachyphylaxis to the drug developed. On days 2–9 of the infusion, heart rate remained significantly higher than preinfusion values, but plasma glucose concentration was significantly lower than before infusion. Despite the rapid disappearance of these ‘side-effects’ of the drug and the apparent normality of most metabolic parameters, short-term infusion of adrenaline (50 ng/kg per min for 60 min then 500 ng/kg per min for a further 60 min) after 9 days of ritodrine infusion showed that the responses of lactate, FFA and heart rate to adrenaline were significantly attenuated in comparison with the responses determined before β-agonist infusion. This result, together with similar earlier observations on fetal lambs, implies that selective attenuation of β-receptor mediated cardiovascular, metabolic and endocrine responses to the endogenous catecholamines could play an important role in determining how β-agonist drugs act to modify carcass development.

Rumen fluid was removed from four sheep 6 h after feeding, and the fluid was centrifuged to remove the micro-organisms. Perchloric acid (PCA) was added to the supernatant fluid to precipitate soluble proteins, which were again removed by centrifugation. The PCA extract was neutralized with KOH and the precipitate was removed by centrifugation. The supernatant fluid was hydrolysed with 6 MHC1 at 110 °C for 24 h, then dried by rotary evaporation, and the amino acid composition of each extract was analysed by ion-exchange chromatography. The recovery of amino acids was 98%, except for methionine, cysteine and tryptophan, which were destroyed. The recovery of amino acids from added Trypticase was 92% of the peptide mixture as amino acids. The free amino acid composition of extracellular rumen fluid was low and variable in both amount and composition (0·36, S.D. 0·49, μmol/ml). The concentration of amino acids released by acid hydrolysis of the PCA extract, presumed to be derived from peptides, was larger and its composition was less variable (1 ·02, S.D. 0·30, μmol/ml). Aspartic acid and histidine were enriched in peptides in comparison with the amino acids present in the feed or in rumen particulate material. Glycine and proline contents were higher in peptides that in particulate material. In contrast, the contents of isoleucine, leucine, tyrosine and phenylalanine tended to be lower in peptides than in the other materials. It was concluded that extracellular degradation-resistant peptides had a composition that was different to microbial protein and to the feed. The peptides appeared to be enriched for amino acids which previous studies with pure peptides had shown tend to make peptides more resistant to degradation.