1. An analysis has been made of the production records of over 1400 heifers got by A.I. bulls at five non-Board centres. The A.I. heifers, when compared with other heifers in the same herd in the same year, showed no significant increase in yield but a significant increase in fat content.

2. The difference in yield between A.I. and contemporary non A.I. heifers was not significantly different in herds at all levels of production. This would suggest that only a small proportion of the differences in yield between the herds in the sample was genetic in origin. In fat percentage, there was some evidence, though not very conclusive, that rather more of the inter-herd variation was genetic.

3. Significant differences were found between the progeny groups by different sires in both milk yield and fat content. The standard deviation of the variation due to the breeding value of the sire was 37 gal. and 0·13% fat respectively.

4. Methods of interpreting progeny tests made under A.I. conditions are discussed theoretically. It is concluded that the comparison with heifer contemporaries in the same herd is most satisfactory for milk yield and the simple average is best for fat content.

5. The relevance of these results to the genetic structure of breeds is discussed. They are in good agreement with the conclusions from pedigree analysis that the genetic differences between the dominant group of herds and the mass of commercial cattle in each breed is not great.

1. The proportion of stained spermatozoa in a ram semen-nigrosin-eosin mixture has been found to increase with the time of exposure to the stain. Five minutes is considered to be a suitable interval for practical use.

2. The value obtained for the proportion of dead cells can be considerably affected by the selection of fields that are easy to count; fields should be chosen at random.

3. The variation between, counts on the same smear is much larger than that expected for binomial proportions. This may be due partly to the ‘clumping’ of the dead spermatozoa and partly to the difficulties of interpretation caused by the presence of many partially stained cells.

4. A significant difference was found between the estimates of the proportion dead obtained by the two experimental workers; this is thought to be due to a difference of interpretation of ‘stained’ and ‘unstained’.

5. Tables are given by which to determine the approximate accuracy of counts made on a range of numbers of spermatozoa, smears and subsamples, separate tables being necessary for ram, boar and bull semen.

Ethiopia is one of the countries most vulnerable to the impacts of climate variability and change on agriculture. The present study aims to understand and characterize agro-climatic variability and changes and associated risks with respect to implications for rainfed crop production in the Central Rift Valley (CRV). Temporal variability and extreme values of selected rainfall and temperature indices were analysed and trends were evaluated using Sen's slope estimator and Mann–Kendall trend test methods. Projected future changes in rainfall and temperature for the 2080s relative to the 1971–90 baseline period were determined based on four General Circulation Models (GCMs) and two emission scenarios (SRES, A2 and B1). The analysis for current climate showed that in the short rainy season (March–May), total mean rainfall varies spatially from 178 to 358 mm with a coefficient of variation (CV) of 32–50%. In the main (long) rainy season (June–September), total mean rainfall ranges between 420 and 680 mm with a CV of 15–40%. During the period 1977–2007, total rainfall decreased but not significantly. Also, there was a decrease in the number of rainy days associated with an increase (statistically not significant) in the intensity per rainfall event for the main rainy season, which can have implications for soil and nutrient losses through erosion and run-off. The reduced number of rainy days increased the length of intermediate dry spells by 0·8 days per decade, leading to crop moisture stress during the growing season. There was also a large inter-annual variability in the length of growing season, ranging from 76 to 239 days. The mean annual temperature exhibited a significant warming trend of 0·12–0·54 °C per decade. Projections from GCMs suggest that future annual rainfall will change by +10 to −40% by 2080. Rainfall will increase during November–December (outside the growing season), but will decline during the growing seasons. Also, the length of the growing season is expected to be reduced by 12–35%. The annual mean temperature is expected to increase in the range of 1·4–4·1 °C by 2080. The past and future climate trends, especially in terms of rainfall and its variability, pose major risks to rainfed agriculture. Specific adaptation strategies are needed for the CRV to cope with the risks, sustain farming and improve food security.

1. In an examination of Park Grass soils extending over 3 years, and in shorter studies of other grassland soils, the fresh soil always contained more ammonia than nitrate nitrogen. The levels of both were low, and in spite of minor fluctuations were sufficiently constant to suggest the existence of equilibrium conditions in the nitrogen cycle. Climatic factors had no appreciable influence except on one very acid plot; here frost or drought, severe enough to kill the herbage, allowed ammonia to accumulate. Otherwise, treatment had little effect, but liming tended to give higher ammonia and lower nitrate levels. The equilibrium levels of both ammonia and nitrate were higher in old grassland soils than in land newly put down to grass.

2. “Mineralizable” nitrogen, produced by incubating the fresh soils under standard conditions, showed a seasonal rhythm the opposite of the annual temperature rhythm, tending to a maximum in winter and early spring and a minimum in summer and early autumn. This was related to the addition and decay of organic residues in the soil. An abnormally dry summer caused a temporary rise, especially on the more acid plots. An extremely acid plot, on which a mat accumulated in the field, produced as much mineralizable nitrogen on incubation as more normal soils.

3. Ammonia and nitrate production on incubation differed greatly with different treatments, apparently as a result of the influence of treatment on soil reaction. The more acid soils, with pH less than 6.0, produced chiefly ammonia, while the less acid soils produced chiefly nitrate. There was a significant correlation between reaction and percentage nitrification.

Experiments in three dry years, 1993/94, 1994/95 and 1995/96, on a medium sand at ADAS Gleadthorpe, England, tested responses of six winter wheat cultivars to irrigation of dry-matter growth, partitioning of dry matter to leaf, stem and ear throughout the season, and to grain at final harvest. Cultivars (Haven, Maris Huntsman, Mercia, Rialto, Riband and Soissons) were selected for contrasts in flowering date and stem soluble carbohydrate. Maximum soil moisture deficit (SMD) exceeded 140 mm in all years, with large deficits (>75 mm) from early June in 1994 and from May in 1995 and 1996. The main effects of drought on partitioning of biomass were for a decrease in the proportion of the crop as lamina in the pre-flowering period, and then earlier retranslocation of stem reserves to grains during the first half of grain filling. Restricted water availability decreased grain yield by 1·83 t/ha in 1994 (P<0·05), and with more prolonged droughts, by 3·06 t/ha in 1995 (P<0·001) and by 4·55 t/ha in 1996 (P<0·001). Averaged over the three years, grain yield responses of the six cultivars differed significantly (P<0·05). Rialto and Mercia lost only 2·8 t/ha compared with Riband and Haven which lost 3·5 t/ha. Losses for Soissons and Maris Huntsman were intermediate. In the two years with prolonged drought, the biomass depression was on average greater for Haven (6·0 t/ha) than for Maris Huntsman (4·2 t/ha) (P<0·05). Thus, the grain yield sensitivity of Haven to drought derived, in part, from a sensitivity of biomass growth to drought. Harvest index (HI; ratio of grain to above-ground dry matter at harvest) responses of the six cultivars to irrigation also differed (P<0·05) and contributed to the yield responses. The smallest decrease in HI of the six cultivars with restricted water availability was shown by Rialto (−0·033); this partially explained the drought resistance for this cultivar. The largest decrease was for Maris Huntsman (−0·072). The cultivars differed in flowering dates by up to 9 days but these were poorly correlated with grain yield responses to irrigation. Stem soluble carbohydrate at flowering varied amongst cultivars from 220 to 300 g/m2 in the unirrigated crop; greater accumulation appeared to be associated with better maintenance of HI under drought. It is concluded that high stem-soluble carbohydrate reserves could be used to improve drought resistance in the UK's temperate climate, but that early flowering seems less likely to be useful.

A grazing experiment, conducted for 22 weeks in 1992/93 at Aorangi Research Station, AgResearch Grasslands, Manawatu, New Zealand, compared the productivity of weaned lambs grazing Lotus corniculatus (birdsfoot trefoil) and lucerne (Medicago sativa). Effects of condensed tannins (CT) in lotus were evaluated by studying the responses of lambs to twice daily oral supplementation with polyethylene glycol (PEG). A rotational grazing system with restricted feed allowance was used. Measurements were made of pre- and post-grazing herbage mass, the composition of the feed on offer and diet selected, voluntary feed intake (VFI), liveweight gain (LWG), carcass growth, wool growth and the concentration of metabolites in rumen fluid. For both lotus and lucerne swards, the diet selected was mainly leaf. Lotus contained 34 g total CT/kg dry matter in the diet selected, whilst there were essentially no CT in lucerne. Compared to lambs grazing lucerne, lambs grazing lotus had slightly lower VFI, and higher LWG, carcass weight gain, carcass dressing-out percentage and wool growth. PEG supplementation had no effect on these measurements or upon the composition of rumen fluid in lambs grazing lucerne. However, in lambs grazing lotus, PEG supplementation reduced wool growth (10·9 ν. 12·1 g/day), slightly reduced LWG (188 ν. 203 g/day), increased rumen ammonia concentration, and increased the molar proportions of /.so-butyric, isovaleric and n-valeric acids and protozoa numbers in rumen fluid. PEG supplementation did not affect carcass gain, carcass fatness or the molar proportion of rumen acetic, propionic or n-butyric acids in lambs grazing lotus. It was concluded that the principal effect of CT in growing lambs grazing lotus was to increase wool growth without affecting VFI, thereby increasing the efficiency of wool production, that the greater rate of carcass gain of lambs grazing lotus than those grazing lucerne was mainly caused by factors other than CT and that CT did not affect the rumen fermentation of carbohydrate to major volatile fatty acids.

It is impracticable to give a detailed summary of the many findings of.this experiment; attention is confined, therefore, to the major aspects and the general principles emerging.

1. By quantitative control of the plane of nutrition, twenty closely inbred pigs have been made to conform to four major variations in the shape of the growth curve from birth to 200 lb. live weight. A high rate throughout (High-High), a high followed by a low rate (High-Low), a low followed by a high rate (Low-High) and a low rate throughout the period (Low-Low) afforded comparison between animals of the same weight but different age and between animals of the same weight and age but with differently shaped growth curves. The relative effects of the treatments upon the development of body proportions and anatomical composition have been studied.

Oilseed rape yields on farms have not increased in several countries, including the UK, since the mid 1980s. This may be because the yield potential for the environment in these countries has been reached, or due to a lack of genetic improvement, or due to changes in the environment of the growing crop caused by crop management practices. The present paper investigates which of these factors may be causing the yield of farm crops in the UK to remain at 3 t/ha. The yield potential for the UK that would be possible by combining the best characteristics that have been observed with the best crop management is estimated to be significantly greater than average farm yields at 6·5 t/ha (90 g/kg moisture content). In order to achieve 6·5 t/ha, a crop would have a flower cover of less than 0·4 to facilitate the production of 130000 seeds/m2. Seed filling must last 46 days and have a solar radiation use efficiency of 0·75 g of seed/MJ. A tenth of the yield must come from stem reserves and the seed weight must be 5·0 mg. All of these characteristics have been achieved; therefore the challenge lies in combining these traits within the same crop. The ultimate yield potential for water retentive soils in the UK is estimated at 9·2 t/ha. This would require new characteristics to be bred into the crop and represents a long-term target. In the UK, new oilseed rape varieties are introduced each year and the yield of these varieties under optimum growing conditions is estimated to have increased by 62 kg/ha/year between 1978 and 2005. Lack of genetic improvement is therefore unlikely to explain the halt in farm yields. There have been trends for farm crops to be grown in shorter rotations, established using minimal cultivations rather than ploughing and to receive less nitrogen fertilizer. Sulphur applications have increased, but probably have not kept pace with the reduction in deposition from the atmosphere. Fungicide applications to farm crops are less than applied to the variety testing system and are unlikely to offer complete disease control. It is concluded that a combination of these crop management factors has caused the halt in yield improvement on UK farms.

By providing both spatial and temporal information remote sensing may function as an important source of data for site-specific crop management. This technology has been used for nitrogen application strategies to obtain optimum yield and grain quality. Here, the objective was to use early repeated remotely sensed multi-spectral data to predict grain yield and quality for winter wheat (Triticum aestivum L.) and spring barley (Hordeum vulgare L.). The crops were sown with two different seeding rates and a wide range of nitrogen strategies were applied. Multi-way partial least squares regression (N-PLS) was used to predict grain yield and protein content. The results were compared with unfold-PLS1 and PLS1 using reflectance data from the last measurement day. Both single reflectance wavelengths and selected vegetation indices were used simultaneously. The results reveal that all models can make a good prediction of yield in both crops with unfold-PLS1 and N-PLS as the best. However, estimation of grain protein content at harvest was very poorly determined in barley, as no relation between the reflectance measurements and barley protein content was obtained. The relation between reflectance measurements and protein content was slightly better in wheat, where especially N-PLS improved the prediction of grain protein content. The overall conclusion of the present experiments is that data from repeated measurements of reflectance used in multi-way partial least squares regression before heading improved the prediction of grain yield and protein content in wheat and barley.

The fertility of a stock of red deer kept under farming conditions on hill land over a period of 7 years has been assessed. The stock consisted of five cohorts of hinds born in successive years from 1971 to 1975. The fertility of hinds was related to their body weight by the general relationship P = 1 – exp(– 0·085(W– 52)) where Pis the probability that a hind will calve and W is her weight in kg at the time of the rut. When body weight was taken into account in this way, age had no effect on hind fertility. Fertility was, however, poor in one year following a rut which occurred in bad weather. Separation of calves from hinds at the time of the rut had no effect on fertility, and under the farm conditions there was no increase in fertility of hinds which had been barren the previous year above that expected from their body weight. Date of calving was slightly skewed, but 63 % of all calves were born in the first 20 days of June. Smaller hinds calved later; 1 kg body weight increased date of calving by about 0·3 days. Evidence of stag infertility was obtained and it was shown that one stag could effectively mate with 28 hinds such that they mostly calved within a 20-day period. It was possible to run 60 hinds with three stags and obtain calving rates which were similar to those for the hind population as a whole. There were no differences in fertility between matched groups of hinds exposed to one or two stags. The results are compared with those obtained by observation of wild stocks in Scotland.

White clover (Trifolium repens L.) and red clover (T. pratense L.) are the most important legumes of temperate pastures. The former is used largely in systems based around sheep or cattle grazing and is grown together with a companion grass. Breeding aims to optimize the white clover contribution to the sward. This means that yield per se is not the aim but rather to take full advantage of the benefits of white clover; in particular, nitrogen fixation, high protein content, digestibility, mineral content and high intake. The objective is an agronomically and, as far as possible, nutritionally balanced sward, thus persistence of white clover and yield stability over a number of years are key goals. A considerable focus of germplasm improvement has therefore been overcoming biotic and abiotic stresses to clover performance. The former include not only pests and diseases but also the impact of the ruminant animal and the competitive interaction with the companion grass, while abiotic stress could be loosely defined as ‘winter hardiness’ and ‘summer survival’ depending on the site. In recent years the focus of many breeding efforts has shifted to give more consideration to the effects of variation within white clover germplasm on animal performance and the environment. Beneficial effects on productivity have been known for many years, but recent studies of the impact of forage diets on meat and milk quality have opened up new opportunities for improvement. Diffuse pollution of nitrogen and phosphorus from agricultural sources is high on the environmental protection agenda of many governments. Breeding efforts are now being made to reduce the contribution of clovers to both direct (leaching) and indirect (through animal returns) pollution. In particular, recent insights into mechanisms affecting protein breakdown in the rumen and silo offer new prospects for breeding interventions to reduce environmental impacts.

Molecular marker methods are being developed in white clover and the transfer and use of resources and information accumulating in the model legumes Medicago truncatula and Lotus japonicus is likely to be a major route by which the power of genomic approaches is translated into forage legume improvement. Hybrids of white clover and related species have been developed to introgress key traits; namely, drought tolerance, grazing tolerance of large leaf types and enhanced seed yield, for which only limited genetic variation is present within the white clover gene pool.

Red clover is less persistent than white clover, is typically cut three or more times in a season and is used to make silage for winter feed. Although it is often grown with a companion grass, monocultures are common and yield per se as well as persistency and pest and disease resistance are major breeding aims. Fewer agronomic studies and less germplasm improvement have been carried out in this species and molecular studies are not as well advanced although, as with white clover, future developments are likely to benefit greatly from a close relationship to model legumes. Red clover brings considerable benefits in terms of animal production and meat and milk quality. These aspects, alongside approaches to reduce nitrogenous pollution from the silo, represent considerable opportunities for variety development.

Four groups of eight lambs, 4 months of age and reared parasite-free from birth, were used to investigate the effect of the intestinal parasite Trichostrongylus colubriformis on the intake and efficiency of utilization of food, using balance trial and comparative slaughter techniques. One group (CI) was killed initially as control. Two further groups (ALC and ALI) were offered ad libitum a complete ruminant diet. The ALI group was dosed orally each day for 14 weeks with 2500 infective larvae. Sheep in a further group (PF) were individually paired to members of the ALI group, and given the same amount of food as their pair. Balances of nitrogen, Ca and P and the digestibility of dry matter and energy were determined for the ALI and PF groups during weeks 6–7 and 12–13 and after 98 days of infection the sheep were killed. The bodies of these and the CI and ALC sheep were analysed for water, fat, nitrogen, Ca and P contents.

Parasitism reduced food intake over the whole experiment by 9%, but had no effect on the digestibility of energy or nitrogen. At the first trial the N balances of the ALI group were inferior to those of the PF group due to increased urinary N excretion, but there was no difference at the second trial. Ca and P balances of the ALI group tended to be inferior to those of the PF group at both trials. The weight gain of the ALI sheep was only 50 % of that of the PF sheep with the same digestible energy and protein intakes. The protein content of the gain in empty body weight of the ALI sheep was 80 g/kg compared with 112 and 124g/kg in the ALC and PF groups, respectively. Possible causes of the poor protein anabolism are discussed.

The gross efficiency of utilization of metabolizable energy for growth was calculated to be 13·3 % in the ALI sheep, compared with 26·2 and 24·2 % respectively, in the ALC and PF groups. It was not possible to determine whether this was due to change in maintenance requirement or in efficiency of fattening.

The Ca and P content of the skeletons of the ALC and PF sheep increased by 55 % during the 98-day period, while no net change occurred in the ALI sheep. It was concluded that this was caused by an induced mineral deficiency resulting from parasitic damage in the small intestine.

In four series of experiments Merino ewes were exposed to ambient temperatures of about 44 °C and water vapour pressure of 33 mmHg for 9 h daily, and to 32 °C and 18 mmHg for the remaining 15 h daily, during the middle third, the final third or the final two-thirds of pregnancy. Birth weight and the weight of the placenta were considerably reduced by the treatments and the reductions were considerably more than could be accounted for by the partial loss of appetite produced by heating.

There was a close inverse relationship between birth weight and the elevated rectal temperature of heated ewes in several series; but the absence of foetal dwarfing in ewes with elevated rectal temperatures due to daily heating for only 9 h at 44 °C indicates that the elevated rectal temperature of the ewe is not the main cause of dwarfing.

Contrary to previous suggestions, the dwarfed lambs were not proportional miniatures, for the head components, the body lengths, the kidneys and adrenal glands were disproportionately large in heated lambs, while the liver, thyroid and thymus glands and the biceps femoris muscle were disproportionately small, and the ratio of secondary to primary wool fibres was very much reduced. There were cavities in the white matter of the cerebral hemispheres of heat-dwarfed lambs.

The hypothesis that foetal dwarfing is due to stunting of the placenta was examined, but conflicting evidence was obtained in the different series. However, it appears that under some circumstances a heat-stunted placenta is capable of considerable growth in the absence of heating during the final third of pregnancy, when the placenta is normally shrinking, and that placental shrinkage in late pregnancy may be greatly accelerated by the application of heat.

Neither thyroxine nor a preparation of ovine growth hormone, injected into heated ewes, prevented foetal dwarfing; the injections of growth hormone appeared to increase foetal mortality.

A computer simulation model of canopy development in a crop of winter wheat is described. The principal features of the model are the simulation of the emergence, growth and senescence of individual leaves and the production of tiller groups (cohorts) during a defined phenological period, with their survival depending on cohort age and shoot population density.

Comparison is made between the model output and early- and late-sown crops from three seasons. The behaviour of the model in response to changes in leaf senescence and tiller production is discussed for crops sown in 1978.

Areas of sulla (Hedysarum coronarium) and pasture (Lolium perenne/Trifolium repens/Holcus lanatus) were grazed by young sheep (29·5–34·8 kg initial liveweight) in four experiments, and effects upon body growth, wool growth and rumen metabolism were measured.

Sulla contained 40–50 g condensed tannins (CT)/kg DM, whilst the pasture contained small amounts of CT (2–6 g/kg DM). After chewing during eating, a lower proportion of total CT was readily extractable and greater proportions were protein-bound and fibre-bound. Nutritional effects attributable to CT were assessed by oral administration of polyethylene glycol (PEG), which binds and inactivates CT, to half the animals grazing each forage. Rates of body growth were consistently higher for lambs grazing sulla than those grazing pasture, with the CT concentration in sulla being neither stimulatory or inhibitory to body growth or voluntary feed intake (VFI). The action of CT reduced carcass fatness in sheep fed both diets, in the one experiment where this was measured. CT present in both sulla and pasture decreased rumen ammonia concentration and decreased molar proportions of iso-butyrate and iso and n-valerate. During spring and early summer, when wool growth rates were highest, CT present in both pasture and sulla increased wool growth rate; when wool growth rates were low during winter, CT had no effect upon the wool growth of sheep grazing either forage. Numbers of protozoa and molar proportions of n-butyrate in rumen fluid were increased by CT in sheep grazing sulla but not pasture.

It was concluded that the higher rates of body growth and VFI in lambs grazing sulla was most likely to be due to its very high ratio of readily fermentable: structural carbohydrate.

1. Certain characteristics of bull semen have been examined at four artificial insemination centres in England. One hundred and twenty-one samples of semen, comprising 168 ejaculates from seventysix bulls of seven breeds, were examined. The semen from 119 tested samples was subsequently used to inseminate 4604 cows.

The following semen characteristics were studied: volume of ejaculate; concentration of spermatozoa; incidence of dead spermatozoa; incidence of morphologically abnormal spermatozoa; initial fructose concentration; resistance of spermatozoa to temporature shock; methylene-blue reduction; O2 uptake; fructose utilization; visual evaluation of motility; impedance change frequency; and fertilizing capacity (conception rate).

A clinical examination was made of the reproductive organs of each bull.

2. Variations in the concentration of living spermatozoa and in the incidence of dead spermatozoa were found to account for most of the variations in the metabolic activity of semon: they also largely explained differences in physical activity as assessed by visual estimation of motility or by impedance change frequency.

3. Although the rates of methylene-blue reduction, O2 uptake and fructolysis showed a close correlation with the numbers of living spermatozoa present, O2 uptake and fructolysis per living cell decreased with increasing cell concentration.

4. Variations in metabolic activity showed no correlation with variations in the morphological characteristics of the spermatozoa, except in the case of spermatozoa with bent tails. These forms occurred in some samples as a result of staining treatment, but their incidence was positively correlated with the O2 uptake and fructolysis per living cell.

5. There was evidence that highly active spermatozoa are more resistant to temperature shock than less active spermatozoa.

1. The free air in the pores of the soil to a depth of 6 inches is very similar in composition to the atmospheric air but it differs in two respects:

(a) It contains more CO2 and correspondingly less oxygen, the average in 100 volumes being 0–25 volume CO2 and 20–6 of oxygen against 0–03 volume CO2. and 20–96 oxygen in atmospheric air.

(b) It shows greater fluctuations in composition.

Usually the sum of the CO2 and oxygen is only slightly less than in atmospheric air but at periods when nitrates rapidly increase there is a perceptible falling off of oxygen, and a still greater one in waterlogged soils.

In ruminants, the urinary excretion of purine derivatives (PD) reflects the absorption of microbial purines and can be used as an index of microbial protein supply. The objective of this study, carried out in Aberdeen, 1992, was to examine whether PD concentrations in spot urine or plasma samples vary diurnally during a given feeding regime and if they reflect differences in daily PD excretion induced by varying feed intake. Sixteen sheep were offered ad libitum one of four diets (fresh weight basis, the rest of each diet being minerals and vitamins): (1) 99·9% lucerne (pelleted); (2) 50% hay, 30% barley, 9% fishmeal and 10% molasses; (3) 72% straw, 7% molasses and 20% molassed sugarbeet pulp; and (4) 97% barley. Measurements were made for 1 week after a 2-week adaptation period. Urine was collected daily on days 1–4 and hourly on days 5–7. Hourly urine collection was achieved using a fraction collector. Plasma samples were collected hourly from 09·00 to 17·00 h on day 4. Feed intake varied considerably (347–1718 g DM/day) between diets and between animals. Daily excretion of PD (7·1–22·6 mmol/day) was linearly related to DM intake (r = 0·85, n = 16), and so was the microbial N supply (3·9–19·5 g N/day) estimated from daily PD excretion (r = 0·87). In hourly urine samples, the ratio of PD:creatinine concentrations showed no significant difference between sampling times, and was linearlycorrelated with the daily PD excretion (r = 0·92). Similarly, plasma PD concentration also showed little diurnal fluctuation. Glomerular filtration rate (GFR) increased with feed intake. Plasma PD was not well correlated with daily PD excretion in urine (r = 0·57). The tubular load of PD (plasma PD × GFR) was better correlated with the daily excretion (r = 0·80). It appears that when sheep are fed ad libitum, PD in spot urine may provide a practical indicator of microbial protein supply status.

Acetic acid bacteria were isolated from maize silages and from samples of maize silage exposed to air. The isolates apparently belonged to the genus Acetobacter. By inoculating maize silage with strains of acetic acid bacteria isolated from silage and by monitoring the development of the microbiota of samples exposed to air it was demonstrated that acetic acid bacteria can be responsible for the onset of aerobic deterioration of maize silage. However, acetic acid bacteria and yeasts often developed simultaneously in uninoculated silage samples exposed to air. In all experiments ethanol was oxidized to acetic acid followed by a rapid oxidation of lactic and acetic acids when ethanol was depleted.

Since Monteith (1977) proposed that growth can be analysed in terms of the amount of energy intercepted by the leaf surface and the efficiency of its use, there has been much interest in the measurement of light interception in potato crops (Scott & Wilcockson, 1978; Milford et al. 1980; Allen & Scott, 1980). This can be measured directly using tube solarimeters placed horizontally above and below the canopy. However, this method suffers from three major drawbacks: (a) in an experiment with many plots it becomes expensive in terms of equipment, (b) a single solarimeter samples only a very small part of the plot canopy, and (c) the solarimeter cannot distinguish between living leaves and other material. Thus, when the crop lodges and/or starts to senesce, visual observations suggest that the area over the solarimeter may not be representative of the plot as a whole.