Milk yield was determined by the weigh-suckle-weigh method over 2 years (1983 and 1984 calvings), for a total of 305 purebred Hereford (H×H) and first-cross Brahman×Hereford (B×H), Simmental×Hereford (S×H) and Friesian×Hereford (F×H) cows grazing three pasture systems at Grafton, New South Wales, Australia. The age of the cows ranged from 6 to 11 years. The data were used to evaluate different estimates of milk yield and to examine the effects of milk yield on growth of calves up to weaning.

Of all the cow traits studied, average lactation milk yield (average of early, mid and late-lactation milk yields) had the highest correlation coefficient with calf 210-day weight (r=0·73) and pre-weaning average daily gain (ADGtotal, r=0·73), and explained >50% of the variation in the calf traits. However, milk yield was also moderately correlated with other cow traits (liveweight and body condition score). Thus, to predict calf performance, milk yield data may not be required if detailed data on other cow traits are available. This is supported by the finding that differences in the coefficients of determination (R2) between models for calf 210-day weight and ADGtotal which included average lactation milk yield and other cow traits (highest R2=69%) and models which included other cow traits but no milk yield estimate (highest R2=57%) were <13%.

Cow breed rankings for average lactation milk yield were similar to those for calf 210-day weight and ADGtotal. On high quality pasture, S×H and F×H cows produced the most milk (S×H, 7·5 kg/day; F×H, 8·3 kg/day; B×H, 5·7 kg/day; H×H, 5·5 kg/day) and weaned the heaviest calves (S×H, 255 kg; F×H, 252 kg; B×H, 215 kg; H×H, 217 kg), while on low quality pasture, B×H and F×H cows produced the most milk (B×H, 4·2 kg/day; F×H, 3·7 kg/day; S×H, 2·9 kg/day; H×H, 2·7 kg/day) but B×H cows weaned the heaviest calves (B×H, 180 kg; F×H, 168 kg; S×H, 159 kg; H×H, 124 kg).

The effects of exogenous purine supply on plasma concentration and urinary excretion of purine derivatives (PD), which include allantoin, uric acid, xanthine and hypoxanthine, were studied. Five sheep, totally nourished by intragastric infusion of volatile fatty acids and casein, were given an abomasal infusion of a mixture of adenosine and guanosine at three levels (5·0, 10·0 and 20·0 mmol/day) each in four infusion patterns (as one, two, or four 3-h infusion periods per day or infused continuously, P1 to P4 respectively). Urine was collected hourly over 24 h, and plasma samples were collected hourly for 7 h from the start of purine infusion. Both the plasma concentration of PD and its urinary excretion changed rapidly in response to the start and termination of purine infusion with a lag time of 2–3 h. Individual sheep differed considerably in the relative proportions of the different purine derivatives in the urine. The endogenous urinary PD excretion averaged 176 (±S.E. 28) μmol/kg W0·75 per day. Daily PD excretion in urine increased with the amount of purine infusion, but at each level the output decreased significantly in a gradient from P1 to P4. The response of total PD excretion (Y, mmol/day) to daily purine input (X, mmol/day) was predictable and followed the equation: Y=0·81 (±0·02)X+3·07(±0·81) × (u e−0·27X/u +1−u), where u is the duration over which the daily input was infused expressed as a proportion of 24 h. The recovery of the infused purines averaged 81%. When the infusion was given continuously, plasma PD concentration was relatively stable and was linearly correlated with the purine entry rate and daily PD excretion in the urine. The estimated glomerular filtration rate was 150 (±S.E. 16) litres/day and the estimated tubular transport maximum was 2·6 (±S.E. 1·3) mmol/day. It is concluded that due to the rapidity in the response of plasma and urinary PD to changes in the exogenous purine supply, spot measurements of PD in urine or plasma can be of no value for the estimation of exogenous purine uptake unless the purine supply is relatively constant throughout the day. Daily PD excretion in urine related to the exogenous purine uptake in a predictable and reproduceable manner and provides a reliable index for the estimation of the exogenous purine uptake.

The association between the rate of protein degradation and the components of the calpain system in lambs fed at sub-maintenance, maintenance and supra-maintenance levels of nutrition was investigated. Weights of the cold carcass, liver, kidney, pluck and m. semitendinosus increased with nutritional level (P<0·05). The rate of protein degradation in the hind-limb was determined using an in vivo arterio-venous method. Blood flow, protein gain and protein synthesis across the hind-limb increased with nutritional status (P<0·05). There was an increase in the amount of protein synthesised per unit of RNA (mg RNA/g protein) with improved nutritional status. The rate of protein degradation across the hind-limb increased between the sub-maintenance and maintenance treatments (P<0·05) but there was no further increase above maintenance (P>0·10). The activity of the components of the calpain system was determined after separation on a DEAE-Sepharose column with a stepwise gradient with increasing NaCl concentrations. Although there were no significant effects of nutritional status on the components of the calpain system, there was a negative association between the rate of protein degradation and the activity of μ-calpain (R2=0·61; P<0·005) and a weak positive association between calpastatin activity and protein gain (R2=0·44; P<0·05).

Dairy cows grazing a tropical grass pasture fertilized with 300 kg N/ha and with a 7-year history of phosphorus fertilizer at either 0 or 45 kg P/ha were given a P supplement in a 2 × 2 factorial experiment at Kairi Research Station, Queensland, Australia. Milk yield, fat-corrected milk yield, yields of milk fat, protein and lactose, and protein content of milk were increased (P<0·05) with P fertilizer. There was no response in milk yield or any component of milk to the provision of a P supplement. It is concluded that the milk response recorded in this experiment was due to P fertilizer leading to additional pasture on offer and increased pasture intake. The lack of response to additional P in the form of a supplement indicates that these pastures can supply adequate P for cows producing 20 kg/day even after 8 years without P fertilizer.

The effect of supplementation on the feeding and ruminating behaviour of sheep fed on low quality roughage was assessed at the Shimane University Animal Farm, Japan, in 1993. Four Japanese Corriedale wethers were used for four periods in a 4×4 Latin square design. The initial 10 days of each period were for adaptation by the sheep to their new environment and diets, followed by 5 and 7 days for rumination studies and digestibility trials, respectively. The four diets were: rice straw alone (RSA), rice straw+soyabean meal (RSS), barley straw+molasses meal (BSM) and barley straw+molasses meal+soyabean meal (BSS). Animals on the RSA and BSM diets spent less time eating (305 and 278 min/day respectively) and ate at a slower rate (1·8 g DM/min) than those on the RSS and BSS diets. Although there were no significant differences between treatments (P>0·01) in the number of rumination periods per day, animals on the RSA and BSM diets spent a longer time ruminating per 100 g NDF intake and exhibited slower chewing rates than those on the other diets. Cyclic rate and rumination indexes were both higher for RSA than for other treatment diets. The results show that sheep on the RSA and BSM diets spent less time eating and that those on RSA spent significantly (P<0·05) more time ruminating than those on supplemented diets. Animals on RSA and RSS generally regurgitated more boli but there was no significant difference (P>0·05) in chewing rates between the diets. However, generally, animals on RSA, in addition to regurgitating more boli, also had lower chewing rates, indicating more efficient rumination activity. The judicious use of small amounts of readily degradable nutrients, especially protein, will improve the rumination behaviour of sheep fed on low quality roughage diets, most probably through an improvement in microbial activity. This will result in a higher fibre digestion rate and an increase in the production of ruminal fermentation by-products, compensating for the loss in rumination efficiency.

Sixteen 2-year old female sheep were fitted with ruminal and duodenal cannulae at Johnstone Memorial Laboratory, Lincoln University during 1989–90. They were offered, at 2 hourly intervals, a pelleted concentrate diet (900 g/day) and chaffed lucerne hay (100 g/day). In a split-plot experiment they were infused, intraruminally and at four rates, with potassium (providing 16, 26, 36 or 46 g K/kg food DM/day) and magnesium (providing 1·3, 1·8, 2·3 or 3·1 g Mg/kg food DM/day) within a Latin square design and with the liquid and solid phase markers 51chromium EDTA and 141cerium chloride. Net absorption of Mg before and after the duodenum was estimated from dietary intake, duodenal flow and urinary and faecal excretion of Mg.

Increasing K intake resulted in a decline in net absorption of Mg from the entire digestive tract, supporting data in the literature. Increasing K intake from 16 to 46 g/kg DM decreased urinary Mg excretion by between 0·14 and 0·30 g/day, the extent of which was independent of the level of Mg intake. At high K intake Mg absorption from the rumen was reduced, the amount absorbed ranging from 0·07 g Mg/day at intakes of 1·3 g Mg/day and 46 g K/kg DM/day to 0·66 g Mg/day at intakes of 3·1 g Mg/day and 16 g K/kg DM/day. However, at high K intake, and when Mg absorption from the rumen was reduced, net Mg absorption from sites distal to the rumen was increased to an extent which suggested compensatory absorption. Increase in K intake was associated with a consistent reduction in plasma Mg concentration which was independent of Mg intake. Increases in Mg intake resulted in increases in Mg absorption and plasma Mg concentration at all rates of K intake in direct proportion to rate of intake. The reduction in Mg absorption from the rumen at high K intake was associated with an increase (0·3 units) in pH of rumen digesta.

Annual variation in the feeding value of barley straw (Hordeum vulgare L. subsp. vulgare) is economically significant in the Mediterranean region. The relationship between the feeding value for sheep of several winter-planted barley cultivars and monthly meteorological data was analysed at Tel Hadya, northwest Syria (mean annual precipitation 330 mm) in 11 years. Indicators of feeding value included contents of neutral detergent fibre, acid detergent fibre and nitrogen (11 years), voluntary straw intake by sheep (10 years), voluntary digestible organic matter intake (9 years), in sacco dry matter loss and lignin content (8 years), in vitro digestibility (7 years) and in vitro gas production (6 years). Monthly meteorological data included total precipitation, mean minimum and mean maximum air temperatures.

Conditions likely to decrease grain yield tended to increase the feeding value of straw. Low mean minimum temperature in February, low precipitation in January, February, March and April, and high mean maximum temperature in March, April and May increased one or more indicators of straw feeding value (by both bivariate and multiple regression, P<0·05). Precipitation before January affected grain and straw yield but had little effect on the feeding value of straw.

Genotype × year interactions had a meteorological component; the superior feeding value of the straw of locally-adapted cultivars v. exotic cultivars was only seen in years with cool or wet springs. This has implications for the genetic selection of crops with better straw feeding value.

Models based on monthly precipitation and temperature, calibrated for locations in the Mediterranean region, can predict the nutritive value of straw and aid the planning of rations for small ruminants.

Groups of nine or ten cows were assigned, after calving, to treatments in which they were (i) machine milked three times daily (M3), (ii) machine milked six times daily (M6) or (iii) suckled three times daily in addition to being machine milked three times daily (S). Treatments were administered during the first 6 weeks postpartum. On one day, at weeks 1 and 6 postpartum, blood samples were collected from all cows at 30-min intervals between 06.00 and 13.00 h and these were analysed for plasma osmolality and plasma concentrations of Na+, K+ and Cl−. Milk yield was significantly higher in suckled cows than in cows milked six times daily, but significantly lower in cows milked three times daily. In cows milked six times daily, and to a greater extent in suckled cows, there was a reduction in plasma osmolality and monovalent ion concentrations (Na+, K+ and Cl−), which could increase the susceptibility of the cows to water intoxication. Moreover, suckling or milking the cows six times daily was associated with increased fluctuations in plasma osmolality and plasma Cl− concentrations. The decrease in plasma osmolality and ion concentration and the increased variation in plasma osmolality and Cl− were probably related to increased water intake and may be indicative of a severe challenge to homoeostasis regulation.

Rumen and caecal digesta were collected, under anaesthetic, from eight sheep offered either hay, pelleted concentrate or pasture at the Johnston Memorial Laboratory, Lincoln University during 1991. Subsamples of digesta were incubated at 39°C for 1 h after adjustment of pH within the range 0·5–12 by the addition of H2SO4 or NaOH. The samples were centrifuged at 30000 g for 30 min and magnesium (Mg) concentration measured in the 30000 g supernatant fraction and in total digesta to assess Mg solubility. In rumen digesta, Mg solubility declined from 0·86 at pH 5 to 0·30 at pH 7 and differences in response between diets were small. Magnesium solubility in caecal digesta was generally higher than in ruminal digesta, and particularly at pH values >6. At pH 7 the difference was twofold. Moreover, differences were observed between diets in the rate of decline in solubility in caecal digesta with increasing pH. At pH 5, 0·90 of Mg from hay and concentrate diets was soluble compared with only 0·8 for pasture. At pH 7, Mg solubility in caecal digesta from hay and concentrate-fed animals was almost double that from pasture-fed animals (0·64 and 0·62 v. 0·36, respectively). The implications of the findings for Mg homoeostasis in ruminants are discussed.

Leaves from the tropical tree legume calliandra (Calliandra calothyrsus) were harvested and fed either fresh frozen (F) or dried (D) as a supplement (200 g dry matter (DM)) to sheep (n=4) given barley straw ad libitum in metabolism cages in a controlled-environment animal house. Sheep given these diets were intra-ruminally infused (0·5 litres/day) with either water or a solution containing 100 g/l polyethylene glycol (PEG). These treatments were administered in a randomized complete block design, and the results statistically analysed. Drying significantly increased the cell wall (cellulose, hemicellulose) and lignin contents, but decreased the condensed tannin (butanol-HCl method) content (F=35·7, D=21·7 g/kg DM). Both drying and PEG infusion significantly (P<0·05) increased straw organic matter (OM) intake, but the effects of drying plus PEG were not additive. Organic matter digestibility of the diets was significantly (P<0·05) increased by drying (F=433 g/kg, D=486 g/kg), but not by the infusion of PEG. Total nitrogen (N) digestibility was significantly (P<0·05) increased by drying (F=207 g/kg, D=316 g/kg) in the absence of PEG, and by PEG for both frozen (−PEG=207 g/kg, +PEG=392 g/kg) and dried (−PEG=316 g/kg, +PEG=434 g/kg) calliandra. PEG infusion significantly increased the proportion of feed N degraded in the rumen (−PEG=0·41, +PEG=0·52) and rumen ammonia concentrations (−PEG=63, +PEG=103 mgN/l), but had no significant effect on the rate or efficiency of microbial protein synthesis in the rumen. PEG infusions significantly increased the amount and proportion of N intake absorbed from the lower digestive tract, but had no significant effect on overall N balance. Drying decreased the tannin losses (as % intake) occurring in the rumen (F=37·3%, D=24·2%), and PEG infusion increased these losses for both frozen (−PEG=37·3%, +PEG=72·5%) and dried (−PEG=24·2%, +PEG=68·7%) calliandra supplements. It was concluded from these studies that the presence of tannins in fresh (frozen) calliandra depressed feed utilization, and that drying was an effective means of improving the nutritive value of calliandra when fed as a supplement to low quality straws.

This group, which is concerned with the applications of mathematics to agricultural science, is sponsored by the Biotechnology and Biological Sciences Research Council. It was formed in 1970, and has since met at approximately yearly intervals in London for one-day meetings. The twenty-ninth meeting of the group, chaired by Professor R. H. Ellis of the Department of Agriculture, University of Reading, was held in the Wellcome Meeting Room at the Royal Society, 6 Carlton House Terrace, London on Friday, 21 March 1997, when the following papers were read.