SummaryFor seed crops other than cereals there have been comparatively few comprehensive field studies which have analysed the physiological basis of the effects of genotype and agronomic treatments on growth, development and yield. Milbourn & Hardwick (1968), working on vining peas (Pisum sativum L.), have recently shown that pod growth rate and the yield of seeds are not simple functions of leaf area index and that sourc es of materials other than the leaves are important in affecting the yields of pods and seeds. Furthermore, Flinn & Pato (1970) have shown that the pods themselves can contribute to assimilation in the field pea (Pisum arvense L.). Field experiments carried out at Cambridge in 1968 and 1969 have produced evidence on the relative importance of leaves and other photosynthetic tissues in determining the yields of pods and seeds in another seed crop, namely oilseed rape. The experiment in 1969 is described and discussed below.

Experiments designed to assess differences between treatments in pasture productivity as measured by the production of animals are complicated by the stocking rates chosen. The paper attempts to derive a simple model relating production per animal and production per unit area to stocking rate for meat animals, based on biological considerations.A method is proposed which would allow meaningful grazing experiments to be carried out employing only one stocking rate, thus considerably reducing the expense of this type of experimentation.The model is applied to some recent data obtained by Hodgson (1966) and Appleton (1967, personal communication).Experiments designed to evaluate the effects of various treatments on the productivity of pasture, as measured in terms of animal output, have been notoriously difficult to conduct and to interpret. Several experiments were conducted and many hypotheses put forward in the 1940s and 1950s (see McMeekan, 1960;Wheeler, 1963) but it became apparent that meaningful conclusions about the effect of any treatments would be very difficult without accounting for the strong interaction of such treatments with stocking rate. Furthermore, the first obvious method of overcoming the effect of stocking rate in experimental design-that of comparing two or more treatments at a common stocking rate-became completely untenable. A significant step forward in the approach to this problem was made by Mott (1960), who attempted to construct a general model to describe the relation of animal output from pasture to stocking rate. This approach was overtly empirical, based on the development of a broad qualitative model from curves based on actual results. Subsequent authors, including Cowlishaw (1962) and Hodgson (1966) who worked with dry sheep and Riewe (1961) who worked with young beef cattle, have published results that cannot easily be reconciled with Mott's hypothesis.The chief conclusion that has been drawn from these studies is that grazing experiments should be conducted at several widely different stocking rates before the results can be used for any general advice. Such a procedure inevitably entails a considerable increase in the costs of such experiments.The present authors consider that the complexity of the subject is such that solutions will be obtained only if it can be reduced to a number of relatively simple propositions, and in the present paper an attempt is made to derive a simple model relating to the conversion of grass into meat based on the biological relationships involved. An approach similar to the initial part of the argument presented here was adopted by Peterson, Lucas & Mott (1965), but they do not show how the hypothetical relationships can be applied to practical grazing situations, whereas in the present paper a model, based on certain assumptions, is proposed and applied to the problem of comparing treatments in grazing experiments; the fit of the hypothesis with published work is discussed.At the risk of over-simplification, the position at a single moment of t...

SUMMARYA total of 64 nine-week-old pigs were used in two experiments to study the influence on feed intake, growth and carcass quality of dietary restriction at various ages or live weights. Two diets of different energy content were given throughout each experiment, which was divided into four periods on either an age or weight basis. Feed was restricted or offered ad libitum during each of the first three periods, all pigs being fed ad libitum during the last period.Compensatory intake following a period of feed restriction occurred at most stages of growth but was most marked in the early stages. Compensation when it occurred took several weeks to develop fully and persisted for some time, often after an intervening period of ad libitum feeding. Increased gains resulted from increased intake although some of the early compensatory gains could not be fully explained by increased intake.There were few appreciable differences between the pigs on the highand those on the low-energy diets. However, in the early stages the latter did not show the same response in intake following restriction as did the former (but in experiment I the latter were considerably lighter).No effects on carcass quality were demonstrated under the conditions of these experiments.

1. Twenty-seven Large White pigs from three litters were involved in a 33 factorial experiment in which they were given three pelleted diets containing 0, 10% and 20% of dry sawdust during each third of an experimental period ranging from 29·5 to 118 kg live weight. The effects of these treatments on dry-matter intake and rate of growth in each sub-period and on the final carcass quality of the pig were studied.2. The results confirmed a previous finding that pigs developed a compensatory increase in intake in response to diets containing increasing amounts of sawdust, but that in small pigs this operates very poorly and leads to much retarded growth on the high sawdust diets.3. The compensatory intake mechanism took some time to develop after the pig had been put on to the sawdust diets, the delay being much longer in the smaller pig. Similarly, when the pigs were returned to high energy diets some time elapsed before intake returned to normal. During this time lag the pigs grew faster, as with compensatory growth, than when they had not previously been subjected to low energy diets.4. It is concluded that long-term observations must be made in order to investigate the nutrition of pigs fed ad libitum.5. Although energy intake can be restricted by giving low energy diets for a short period, this does not seem to improve carcass quality.