The object of this paper is to review some aspects of reproductive physiology in the pig, especially in relation to conditions after farrowing when reproductive ability is normally inhibited by lactation and suckling, and when weaning at different periods after parturition may have a profound influence on the chances of successful conception and pregnancy. In undertaking this review, however, it soon became clear that very few detailed experimental studies had been carried out in sows during the period immediately following parturition. It was considered best, therefore, to compare some of the situations which exist in pigs at other reproductive stages with what little is known about the sow at the post partum stage and during lactation.

The potential advantages of early weaning have been listed by Dr Braude in his introduction. In practice, however, piglet mortality can reduce or eliminate these potential improvements in number of piglets per sow per year, and in overall efficiency of feed energy utilization for piglet production (Table 1).

The rearing and economic results from normal weaning systems are highly variable; this is also true for early weaning, not only because it is a new system, but because thesystem depends more upon the farmer and much less upon the sow. Farmers like early weaning particularly because of its labour-saving aspects. In the first part of the paper I shall give a survey of the causes of differences in rearing results we have obtained from different farms and rearing centres. The second part will examine what is currently the major problem of early weaning systems, the subsequent reproductive performance of the sow.

I think it is extremely important that when we are making comparisons of the advantages and disadvantages of artificial rearing, as compared with other systems of rearing, that the comparisons should be made on a valid basis.

There are three ways in which the systems can be compared: firstly, they can be compared on a basis of the biological targets of the system. These represent a limit to production in the appropriate systems. Secondly, they can be compared on a basis of maximum performance under laboratory conditions, or in those situations in which scientific personnel have clear control over the system being undertaken. Thirdly, systems can be compared on the basis of the best performance achieved under purely practical conditions. This point can be illustrated by three examples:

1. Number of pigs born per sow per year

Since gestation length cannot be affected, the important factors involved in the number of pigs born per sow per year are: number of pigs born, length of lactation period and the interval between weaning and service.

The principal objective to date in packaging dried forages into wafers, cobs or pellets has been to improve handling and storage. Over 80% of the dried forage produced in the UK is still used as a component in pig and poultry rations, for which a milled and pelleted product is the most suitable. However there is evidence that the different forms of package are not of the same nutritive value to ruminants. With the increasing interest in the use of dried forages for ruminant feeding it is important to examine packaging in these dual rôles, as both mechanical and nutritional treatments.

Processing roughages generally causes marked changes in the particle size of the feed and this review is restricted to the effect of this on nutritive value; processing roughages by chemical methods is not considered. Where possible experiments with milking cows given processed artificially dried forages are discussed, but unfortunately relatively little work has so far been reported with these feeds. It is therefore necessary to refer to results from a much larger body of American information on processing field-cured lucerne, and to the few European studies with milled hays and straws. Much of the work has been done with small numbers of cows and often without reporting the particle size of the processed roughages. On the occasions this information was given it shows that, between experiments, the average particle size of pelleted hay varied enormously from 1·35 mm to 0·22 mm. The average particle size of the feed was the size of mesh through which half the total weight of particles passed (Gill, Campling and Westgarth, 1966). Processing a feed into pellets therefore does not imply a common particle size.

A roughage begins to be processed as soon as it is cut, receives further processing during preservation and possibly more after it has been preserved. At each stage, the processing has a potential or an actual effect on the nutritive value of the feed, but in this review we shall concern ourselves solely with processes applied after preservation has taken place. Roughages are processed mainly with the intention of increasing their energy value and their acceptability to the animal (although there may be other nutritional aims such as reducing the solubility of protein, or non-nutritional aims such as facilitating transportation or storage). Most of these processes are mechanical, but some—such as heat treatment—are better classified as physical and others —such as delignification with sodium hydroxide—are chemical.

Animal Production from Grassland was to have been the theme of the Summer Meeting in 1971 but, owing to circumstances prevailing in Northern Ireland at that time, most reluctantly the Society had to cancel the Meeting. However, because of the importance of the theme selected for that meeting, it was decided to invite the authors to present their papers at a Special Topic Session at the 1972 Winter Meeting. In introducing this group of papers, there are two points which should be made.

The factors involved in meeting the beef requirements of our expanding population range from biological and economic to social in nature. It was decided to restrict this paper to those biological and economic factors which are directly involved in achieving the targets of beef production within low cost systems.

In general the energy supply is the main factor limiting the output of milk by the dairy cow from most food sources. Under Northern Ireland conditions grass produces a greater energy output per unit area than any of the other major crops grown there. Furthermore Lowe (1970) has estimated the relative production costs per unit of dry matter for cereals and root crops to be twice and eight times respectively those incurred with grass. Therefore it is apparent that both on a basis of energy output per unit area, and per unit cost input, grass must continue to provide the maximum possible proportion of the food supply for the dairy industry within Northern Ireland.

Artificial drying is undoubtedly the most efficient method of grass conservation. The nutrient losses incurred in the drying process are low and, in contrast to other conservation techniques, grass drying is consistently efficient throughout the season and between seasons. However, both capital and running costs of a drying plant are high and inevitably the cost of production of dried grass is also high. Where there is a large proportion of small farms, as is the case in Northern Ireland, it appears to be out of the question for a farm unit to have a grass dryer because of costs. It is more than likely that under these conditions grass drying will be either undertaken on a cooperative or commercial basis, in the latter case the grass so used being a cash crop.

The traditional method of handling excreta from housed farm animals involved the provision of bedding material, normally straw, to absorb the urine and provide additional carbohydrate material for the composting process which took place during storage. Modern systems of housing generally provide for the collection and storage of the excreta in the form of slurry. This reduces labour costs and should ensure practically complete collection of both faeces and urine, thereby reducing risks of polluting water courses by drainage direct from the farmyard.

The most important metabolic disease associated with intensive grassland production and utilization in Northern Ireland is grass tetany. Grass tetany or hypomagnesaemic tetany was recognized in the early decades of this century in Europe and probably appeared in the British Isles in the early 1920's. There has been a steady increase in incidence over the years, and this would seem to be related to an increase in productivity, both in terms of output from the animal and from pasture.

The potential value of carcass chemical analysis in a variety of animal research studies is not only in the specific information it alone can provide, but in particular as a means of substantiating and augmenting data obtained from other experimental approaches. For example, it may be used in conjunction with in vivo nutrient balance studies and with physical dissection in carcass quality assessment studies. Realization of the full potential value of carcass chemical analysis depends entirely, however, on the ability to produce, from the initial highly heterogeneous carcass material, a fully representative sample that is sufficiently homogeneous to enable valid analyses to be made using the desired analytical techniques. Macro-analytical techniques may be acceptable when the numbers of samples involved are relatively small and /or when the analytical resources available are not critical. Under such conditions a lower degree of homogeneity will be acceptable in the material to be analysed than that required when micro-analytical techniques are used.

Under this very broad title we have selected three aspects of current and recent work which have a bearing on our understanding of the interpretation of findings dealing with muscle-weight distribution. The first is a justification of the anatomical approach, the second a confirmation of, and elaboration on, some earlier work, and the third a suggestion for some future thinking.

Since 1968, the Meat and Livestock Commission has been carrying out a considerable amount of tissue separation work on samples of cattle and sheep arising from its livestock improvement schemes. The purpose of this work has been to provide data on variation in carcass composition of these animals to relate to variation in production characteristics. From the point of view of the meat market, it is important when fast growing sires are selected for use, particularly by AI, that they should be known to be capable of producing offspring which, under normal feeding conditions, are no fatter than the mass market requires. The latter is known to favour the lean end of the fatness range, although there is still a market for fatter carcasses which are believed to have superior eating characteristics, but may require fat to be trimmed from them before and after they reach the plate. In order to describe market requirements more clearly than at present, the data being collected on variation in carcass composition and its prediction are being used to develop carcass classification schemes for cattle and sheep.

For the purpose of the present discussion I will define commercial cutting as the cutting of carcasses into commercial wholesale or retail cuts, and the trimming of some fatty tissue and bone from these to commercial specifications. I assume that this will be done by a commercially trained butcher. I also assume that the cutting and trimming will be done in commercial premises.