In experiments harvested in 1985 and 1986 the grain yields of 61 F1 hybrids among winter wheat varieties and advanced breeding lines were 5·9% greater than the yields of the best parents. In a trial with 430 hybrids in 1986, the hybrids yielded 3·6% more grain than the best parents. Among these 430 hybrids heterosis for yield was greatest for those from the lowest yielding parents. This result is taken to indicate that among these genotypes most genes for high yield have been fixed in the highest yielding parents.

The hybrids had slightly fewer ears/m2, but more grains per ear and heavier grains than the highest yielding parents. They yielded more straw as well as more grain.

The results are compared with those from other studies and it is concluded that yield advantage of F1 hybrids so far tested is not generally sufficient to justify their introduction into U.K. agriculture.

The growth and development of maize and soya bean when grown in mixtures in different proportions and arrangements were studied at Samaru (11° 11' N, 38° E) and Yandev (07° 23' N, 09° 10' E) in the savannah belts of Nigeria. The crops were sown in both pure and mixed stands. The pure-stand densities were 513 plants/m2 for maize and 20·15 plants/m2 for soya bean. Mixtures were made in three proportions: additive (100:100), containing 100% of the sole crop population of each crop; semi-additive (67:67), containing 67% of each crop's sole population; and replacement (67:33 and 33:67), containing 67% of one crop's population to the other's 33%. The mixtures were formed either on the same ridge (in-ridge) or on different ridges (alternate ridge).

Maize grain yield at Samuru was not significantly reduced in mixtures except where its proportion was reduced to 33% of its sole crop density. Seed yields in soya bean were significantly reduced in all mixtures, and when associated with maize density of not greater than 67%, soya bean was capable of 50% of its sole crop yield. At Yandev, late planting coupled with inadequate rainfall in the month following planting depressed seed yields in both crops, the sole crop yields being only 1·53 t/ha in maize and 0·24 t/ha in soya bean. Also maize appeared to be adversely affected in the mixtures at this location, the yields being significantly reduced in the mixtures irrespective of planting ratios.

All mixtures, except 100:100 at Yandev, proved beneficial by producing land equivalent ratio (LER) values greater than one, while 67:67 proportions made higher gross returns than any of the sole crops at Samaru. In this experiment moderately high populations of each crop were found to be necessary to obtain beneficial yields from the mixtures. In this regard semi-additive mixtures, with 67% of each crop's pure stand population, appeared most appropriate.

Incidence of maize streak was significantly lower in alternate ridge mixtures; otherwise the crop in the mixtures seemed to be little affected by pattern of plant arrangement; while soya bean was quite sensitive, its flowering and podding being significantly delayed in in-ridge mixtures at Yandev. The legume yields in this mixture were also slightly, though not significantly, adversely affected. Thus alternate ridge arrangement was more favourable.

Growth curves fitted to factorial data can be modelled using an extrinsic time variate or using the mean responses within the levels of a subset of factors. Where factors can be partitioned into a set of ‘treatment’ factors and a set of ‘environment’ factors, fitting growth curves to the mean effects of environments allows the effects of treatments to be assessed relative to a uniform background growth rate. This leads to a test of a null hypothesis of equal treatment effects in all environments, given that the mean growth rate and stage of development in all environments is equal. The approach is exemplified using data from a glasshouse tomato crop experiment testing variety, nutrient and sowing date factors. Variety and nutrient treatment effects were of direct interest but sowing dates were intended to generalize results by providing a range of growing environments. Treatment effects are analysed by modelling running cumulative yield totals by growth curves and regressing variety and nutrient growth variates on the mean growth variate within each sowing date. In the discussion the case of more than one environmental factor is considered.

The frost hardiness of 5-month-old seedlings of 12 white clover cultivars was examined at – 4, – 8, – 12 and – 16°C and in a subsequent study the frost hardiness of 6-month-old seedlings of 190 experimental lines and 23 cultivars and ecotypes was determined at – 12°C. There were large differences among cultivars and lines in frost tolerance based on the percentage of plants damaged and the percentage of leaves killed. The most frost-hardy were the cultivars Podkowa and Undrom and ecotypes collected from Kaikoura and Nelson Lakes. There was no significant, correlation between the percentage germination of cultivars at 4 °C and their subsequent frost hardiness.

Large-leaved, erect cultivars tended to be more frost sensitive than small-leaved, prostrate cultivars. Frost-tolerant cultivars and lines tended to be acyanogenic. Selection for low winter growth did not increase frost tolerance. However, lines derived from crosses between genotypes of cold-hardy lines selected for rapid germination at 4 °C were more frost-hardy than lines from genotypes selected in a similar way that had been crossed with unselected Huia genotypes.

Field experiments comparing 13 winter wheat varieties representing very old, old, intermediate and modern groups were carried out over three seasons. The experimental sites were on soils of high fertility and weeds, diseases and pests were controlled by applications of proprietary agrochemicals. Lodging was prevented by supporting the plants with coarse-mesh netting.

The average yield of all varieties over the three harvest years 1984, 1985 and 1986 was 7·7 t/ha (at 15% moisture content). Compared with the very old varieties which were grown by farmers during the 19th century, the modern varieties gave 59% more grain, had 14% more ears/m2 and 30% more grains per ear, but a similar mean grain mass. The modern varieties yielded slightly more biomass (total above-ground dry matter at harvest), were much shorter and reached anthesis about 6 days earlier than the older ones. In 1984, when the yields were greater than in 1985 and 1986, the yield advantage of the modern varieties was more in percentage and absolute terms than in the other years. Also, in 1984, the differences between the very old and the modern varieties in biomass was the greatest.

The genetic gain in yield measured in these experiments is compared with those estimated for other countries and reasons for the variation are discussed.

Experiments are described in which a range of seed beds was created in each of 5 years. The weather in these years produced wet, dry and capping seed-bed conditions. The time course of sugar-beet seedling emergence on each seed bed was recorded each year and the differences were considered in terms of soil physical characteristics: much of the year-to-year variation was accounted for by considering thermal time above 3 °C. The differences in emergence between seed beds were large only when conditions were dry, but in all years it was advantageous to level the seed bed in autumn or winter. Seed beds with a dense soil below the seed and fine aggregates above gave the most suitable conditions for rapid and successful emergence.

In an experiment, carried out over 3 years, two groups of dairy cows offered ad libitum access to grass silage received 8 kg/day concentrate differing only in phosphorus content (low P concentrate: 4·0–4·5 g/kg dry matter (D.M.); high P concentrate: 6·0–6·5 g/kg D.M.) during the winter feeding period of approximately 6 months. During the summer period all animals were grazed together on perennial ryegrass pastures. There were no consistent significant effects of P content on any of the variables measured. The mean calving indices were 371 (S.E. = 3·71, n = 122) and 379 (S.E. = 4·28, n = 95) days and overall conception rates were 0·70 and 0·68 for the low and high P treatments, respectively. Mean plasma P concentrations during the winter period were significantly reduced (P > 0·01 or greater) in years 2 and 3 of the study on the low P treatment. In 1 year milk yield was significantly higher (P > 0·01) on the low P treatment although the fat content was significantly reduced (P > 0·05) but these effects were not recorded in the other 2 years.

The post-weaning growth, frame size and body condition of Hereford (H × H), Brahman × Hereford (B × H), Simmental × Hereford (S × H) and Friesian × Hereford (F × H) heifers were evaluated on three pasture systems in each of five years. One group of heifers was grown from weaning (at 8 months of age) to 18 months on the high-pasture system, which included supplementation with grain, while the other two groups were grown to 30 months on pastures of medium or low quality. Pasture effects were large compared with differences between years, with average daily gains between weaning and 18 months of 552, 275 and 97 g/day on high, medium and low pastures, respectively. Genotype had a significant effect on all measurements. Pasture × genotype interaction was significant in most instances. While crosses grew faster and were heavier than H × H on all pastures, S × H were heaviest by 18 months on high pasture, with B × H heaviest at all ages up to 30 months on both medium and low pastures. There was little difference in live weight between B × H and F × H on high pasture, or between S × H and F × H on medium pasture, whereas F × H was heavier than S × H on low pasture. There were concomitant differences in measures of skeletal size, but when adjusted allometrically to a common live weight, pasture effects were removed, while differences between genotypes remained. At a common live weight F × H had the largest height, length and pelvic area, but shared the lowest body condition with S × H. Pelvic height was greatest among B × H heifers, but this cross had similar body length, condition and subcutaneous fat cover to H × H. The pattern of genotype × pasture interaction observed in this study was similar to that observed by Darnell, Hearnshaw & Barlow (1987) for growth rate among contemporary steers located in different environments.

The interaction between Phytophthora infestans and potato viruses X (PVX) and Y (PVY) was studied in the potato cultivar Kufri Chandramukhi. Prior infection by PVX and PVY reduced the susceptibility of the plants to P. infestans. The reduction in susceptibility was more marked when leaflets were inoculated with P. infestans 72 h after inoculation with PVY. The same type of reaction was observed in whole plants when the interval between PVY and P. infestans inoculation was 30 days. No significant differences were observed when the sequence of inoculation was reversed.

Ninety-six young rabbits (48 males, 48 females) each weighing 08 kg were offered one of six diets to appetite (n = 8 per treatment) or at one of four restricted feeding levels (n = 2 per treatment). All animals were culled at ca. 2 kg body weight and carcass analysis was carried out. Nutrient retention was estimated by the comparative slaughter technique.

Five of the experimental diets (A–E) were formulated to contain increasing concentrations of a poor-quality grassmeal (0, 100, 300, 500 and 700 g/kg, respectively), the remainder of the diet consisting predominantly of ground oats and soya-bean meal. A sixth diet (F) was based on a common laboratory rabbit diet (SGI). All diets contained similar digestible crude protein (DCP): digestible energy (DE) ratios of ca. 13·1 g DCP/MJ DE. Acid detergent fibre concentrations in diets A–F were 146, 158, 213, 264, 321 and 180 g/kg dry matter (D.M.), respectively.

When feeding to appetite, growth rates were depressed at the highest inclusions of grassmeal (diets D and E). Growth rates (GR) and voluntary food intakes (VFI) for diets A–F were: GR (g/day): 40, 41, 43, 36, 30 and 40; VFI (g D.M./day): 93, 103, 120, 137, 159 and 111, respectively. The depression in growth rate on diets D and E was associated with a reduction in total daily DCP and DE intake. The data suggested that, when the DE content of the diet falls below 10·5 MJ/kg D.M., the young rabbit is unable to compensate further by increasing daily VFI. Males grew significantly faster than females (40 cf. 37 g/day) and females had fatter carcasses at slaughter (301 cf. 274 g fat/kg carcass D.M.).

Relationships between daily N and energy intake and their retention in the body of the growing rabbit were calculated using data from both restricted and ad libitum-fed animals. Daily energy retention (ER, kJ) was related to daily DE intake (DEI, kJ) by the equation: ER = –227 + 6 x DEI (b ranged from 0·407 to 0·502, according to diet).

The estimated maintenance energy requirement of the growing rabbit ranged from 351 to 434 kJDE/kg0·75 per day and the average value across all diets was 377 kJ DE/kg0·75 per day.

Daily N retention (NR, g) was related to daily digestible nitrogen intake (DNI, g) by the equation: NR = –0·398 + 6 x DNI (b ranged from 0·589 to 0·667, according to diet).

The estimated N requirement for maintenance ranged between 0·464 and 0·525 g DN/kg0·75 per day and the average value across all diets was 0·497 g DN/kg0·75 per day.

The fate of abomasal and intravenous doses of [35S]cystine and [35S]methionine was investigated in sheep to determine their relative efficacy as supplements for wool growth.

Administration of L-[35S]cystine and L-, DL- and D-[35S]methionine via the abomasum produced similar patterns of 35S labelling in blood plasma. Only 10–16% of the administered 35S was excreted in urine over 7 days and 22–32% of the dose was incorporated into wool over 21 days. All isomers were utilized by the sheep with similar efficiency and it was concluded that D- and L-methionine and L-cystine are equally effective as supplements for wool growth at optimal dose rates. Abomasal supplementation with a large amount of methionine (10 g/day) resulted in the excretion of a high proportion of the 35S in urine (55% of the dose over 7 days) and low incorporation into wool.

A comparison of the extent of incorporation of 35S into wool from intravenous doses of L-[35S]cystine and L-[35S]methionine in the same sheep, and in sheep from flocks selected for high and low wool production, showed that incorporation from methionine averaged 80% of that from cystine. It was concluded that, at moderate (2–3 g/day) methionine availability, methionine is utilized for synthesis of wool with about 80% of the efficiency of cystine, conversion of methionine to cysteine occurring via the transulphuration pathway.

Measurements of skeletal size were made at 2–3-month intervals on 30 Border Leicester × Merino castrate male (wether) sheep between 2 and 27 months of age. Fifteen sheep were fed ad libitum on a high-quality diet and the other 15 half the average amount consumed by the first group, age for age. The ad libitum group grew faster and were larger in all body dimensions on each occasion, except for leg length at 27 months which showed no statistical difference between groups. When the groups were compared over the live-weight range common to both (16–44 kg) the unrationed animals were consistently wider at the shoulders but smaller in leg and chest dimensions.

The relationship between each body component and age is described by a Mitscherlich equation and the relationship with live weight by a linear equation in which both variables are log transformed. Separate relationships were determined for each sheep and tested for differences within and between groups.

The Southdown is a terminal sire breed producing about 205 registered rams for commercial use in Britain each year. It is strongly associated with its native area of Sussex, with an hierarchical breed structure very similar to that of 30 years ago. The number of breeding ewes has declined from 5475 in 1957 to (minimum estimate) 1202 in 1985.

French and New Zealand imports in the last 20 years have resulted in present-day British Southdown breeding rams being, on average, only 76% of British descent. If further importations are to be made, semen from rams without foreign blood should be taken for storage first, in the interests of genetic conservation.

The required production of Southdown rams for commercial purposes is achieved with a rather small population size. It would be misleading to describe the Southdown as a ‘rare breed’ without taking account of its commercial function and breed structure.

The analysis of Kennedy et al. (1985) was applied to 150 individuals from each of six plots containing mixtures of the perennial ryegrass (Lolium perenne L.) cultivar Grasslands Nui and the resident ryegrass population at Rawhiti, Central Wairarapa, New Zealand. Four alleles (A, B, C and D) were identified at the phosphoglucoisomerase-2 allozyme locus in the two ryegrass populations. Using the maximum likelihood method, non-linear models were fitted to calculate the percentage contribution of Grasslands Nui to the mixtures. The contribution of Grasslands Nui to the six evaluation plots ranged from 5 to 44%, with S.E.S of 11–17%. The lowest contribution occurred under set stocking with sheep and a low fertilizer regime. The persistence of Grasslands Nui under contrasting grazing managements and fertilizer regimes in this dry environment, and aspects of enzyme electrophoresis application to field trials, are discussed.

Volatilization of ammonia from urea applied to rice on a soil of pH 9·3 was measured during 1982 and 1983 at Karnal. For three moisture regimes (continuous submergence, and irrigation 1 day and 4 days following subsidence of ponded water) mean ammonia losses varied from 28·2 to 31·8 kg N/ha. For urea applications between 40 and 120 kg N/ha, mean ammonia losses varied from 26.1 to 33.7 kg N/ha. Ammonia losses were highest on the second day following urea application and dropped to near zero 7–8 days after application.

The potential of selected cultivar mixtures to decrease lodging and consequently improve yields in winter barley was examined in field experiments at two locations over 2 years. The experiments were designed to identify growing conditions and varietal characteristics associated with mixture effects, i.e. the deviations from arithmetic pure cultivar means, and to compare the latter with effects of growth regulators. Pathogens were controlled chemically.

Use of mixtures tended to reduce initial lodging. With increasing lodging, desired mixture effects prevailed in mixtures of cultivars highly susceptible with those highly resistant to lodging. On average, the yield advantage of mixtures compared with pure stands was small; it was greater when lodging decreased yield of a pure cultivar. Reductions in lodging prevailed in two-component mixtures; adding more components was of no advantage.

When lodging developed steadily, effects of mixtures on grain yields were comparable to those of growth regulators. When lodging was caused suddenly by a thunderstorm, only growth regulator applications resulted in yield improvements. Growth regulators resulted in yield advantages in two of the four trials only.

Field plot experiments were carried out for 3 years at four sites to study the effect of date of application of ammonium nitrate/calcium carbonate (CAN) and urea (U) on perennial ryegrass production in spring. Fertilizer (70 kg N/ha) was applied at weekly intervals for 10 weeks from 1 February. Herbage was cut on the same day at all sites, 3–4 weeks after the last N application. CAN and U at 50 kg N/ha were immediately re-applied and a second cut of herbage was taken after 4–5 weeks. From meteorological data, the dates after 1 February when soil temperature at 100 mm depth increased to 5·5 °C and the dates when cumulative average daily air temperatures < 0 °C from 1 January reached 200 °C were calculated for each site and year.

The date of application for maximum dry matter (D.M.) yield at the first cut differed with site and year, but for 11 of the 12 site/years was in February. The number of dates of application resulting in optimum yield (i.e. at least 90% of the average maximum yield response to CAN or U) varied also with site and year from one to six. Soil and air temperature predictive systems implied a precision in choice of application date that was unjustified and were no more successful at predicting the optimum application date than a simple date range. The first fertilizer application had a pronounced positive residual effect on D.M. yields at the second cut. Less precision on date of the first application was required to obtain optimum cumulative yields over both cuts than optimum yield at the first cut only.

Differences in performance between CAN and U were only significant for three of the 120 fertilizer applications at the first cut. On these occasions, all in one year at two sites, U gave higher yields than CAN. Correlations were sought between D.M. yield response and growth period, air temperature, long-term rainfall and short-term rainfall for CAN and U separately. Factors relating to rainfall had no significant effect on response to U but response to CAN showed a significant negative correlation with short-term rainfall. The short-term weather forecast may therefore be another criterion to be considered in deciding when to apply N in early spring.

In a field study at exchangeable sodium percentage (ESP) values of 11 and 32, subjecting a wheat (Triticum aestivum L.) crop to waterlogging for 1, 2, 4 and 6 days at the time of first irrigation (25-day-old plants), significantly reduced tillering and plant height, delayed ear emergence and resulted in 8, 17, 27 and 39% reduction in grain yield, respectively. Waterlogging decreased oxygen diffusion rate (ODR), restricted root growth and reduced ion uptake, especially of N, P, K., Ca, Mg and Zn, and led to higher absorption of Na, Fe and Mn. However, the effects of waterlogging were more pronounced at higher alkalinity values. The results indicated that, to ensure optimum wheat yield in alkaline soils, waterlogging must be avoided.

Experiments were conducted on the seed rates of the components of oat-vetch mixtures combined with varying rates of nitrogen fertilizer in a rainfed area of north-east Portugal in 1982–3 and 1984–5.

Forage oats grown alone showed a very high yield response to N and rainfall ranging from 30 to 18·2 t D.M./ha, but had a very low crude protein concentration (37–54 g/kg D.M.) and a poor mineral nutrient composition.

The increased use of vetch in the seed mixture had a buffering effect on dry-matter (D.M.) yield but this depended very much on crop N nutrition. For N-deficient conditions vetch inclusion increased the D.M. yield and had a strong positive effect on total forage crude protein (CP) and mineral composition. When crop N nutrition was good, very high D.M. yields (> 12 t D.M./ha) were possible using a high proportion of oats, with vetch making a reduced contribution to the yield and nutritive value of total forage.

Seed rates suggested in the literature for Mediterranean conditions are adequate for low N inputs and low available soil N, but not for high D.M. yields which can only be obtained by using a higher proportion of oats and adequate N fertilizer.