Since the early studies of Jones & Tisdale (1921), temperature has been recognized as influencing symbiosis and as affecting both nodule formation and nitrogen fixation (Roughley & Dart, 1969).

In recent years, processors in Tasmania have introduced Russet Burbank as the most desirable potato cultivar for processing into French fries. Malformed tubers, collectively known as second growth, have been observed in some commercial Russet Burbank crops with the main defect being knobby tubers. Second growth of potato tubers has been shown to occur when the plant suffers a period of stress during the growth of the tubers such as drought, high and low temperatures, restricted soil aeration, interference with water and carbohydrate translocation and fertility imbalance (Sparks, 1958; Bodlaender, Lugt & Marinus, 1964; Iritani, 1981; Holder & Cary, 1984) or a combination of both drought and high temperature stress (Ohms, 1968). If stress from these factors is relieved, the renewed growth of the tuber is often confined to the eyes, producing knobby tubers (Moorby, 1978). Moderate water stress during early tuber bulking can lead to pear-shaped tubers (pointed at the basal end) whilst water stress during the latter part of the season can lead to tubers pointed at the apical end (Iritani, 1981). Knobby tubers are undesirable because knobs are easily broken off leading to yield loss and the exposure of tissue to infection. Severely pointed tubers cannot be processed efficiently into French fries.

O'Kelly (1985) has presented evidence which suggests that at the same feed intake Brahman cross cattle synthesize more microbial lipid than British cattle with a resultant increased efficiency of utilization of metabolizable energy for maintenance. The lipid content of bacteria associated with both liquid and solid fractions of rumen contents is also higher in Brahman than in Hereford cattle (J. C. O'Kelly & W. G. Spiers, unpublished). The reasons for these breed differences are unknown. A step in investigating the problem could be to determine whether microbial lipid content in Hereford cattle can be increased by altering rumen fermentation patterns in association with changes in the microbial population. Ionophores alter rumen fermentation by increasing the production of propionic acid relative to other volatile fatty acids and also inhibit Gram-positive bacteria by facilitating the formation and transport across cell membranes of lipid-soluble complexes with various cations (Bergen & Bates, 1984). However, there are no reports on the effects of ionophores on microbial lipid metabolism in cattle on restricted feed intakes typical of those in tropical areas.

Birth weight and pre-weaning performance are recognized as important components in determing economic returns from beef cattle. The accuracy of ranking animals based on their breeding values, and hence the effectiveness of selection, will be increased when allowance is made for non-genetic sources of variation. Furthermore, the proper evaluation of identifiable sources of variation in calf weaning weight aids in more accurate appraisal of genetic differences in mothering ability among beef cows.