ALBERT

Description: SUMMARYThis study sought to determine whether increases in live weight (LW) of steers implanted with Compudose 200®, a growth promotant that contains 17β-oestradiol, arose in part from increases in resistance of those steers to cattle ticks and gastro-intestinal nematodes (worms) to which the steers were exposed while they were growing. Half of a group of interbred Hereford × Shorthorn (HS) and interbred Brahman × HS (BX) steers was implanted with Compudose 200 on two occasions 200 days apart. Half of each of the implanted and nonimplanted groups in each breed was treated every 3 weeks to control cattle ticks and worms. All animals grazed together and were regularly monitored for parasite burdens and LW changes. Both breeds responded positively to Compudose 200, to which BX responded most, and to treatment to control parasites. Resistance to either ticks or worms was unaffected by the presence of the implant so that, within each breed, increases in LW gain in response to the implant were similar regardless of parasite control. Thus the increases in LW gain resulting from the use of Compudose 200 arose directly from the action of the implant on metabolism, not indirectly through any effect on resistance to parasites.

Description: SUMMARYA study was made of the effect of increased rectal temperature on digestibility, nitrogen balance and water metabolism in Brahman x British (BX), Africander x British (AX) and Shorthorn x Hereford (SH) steers fed a fixed amount of lucerne hay.An increase in rectal temperature of c. 1·4 °C caused a decrease in food intake and nitrogen balance and was associated with an increase in dry-matter digestibility in all breeds. Both dry-matter and nitrogen digestibilities were consistently higher in the Zebu crossbreds (BX and AX). Water intakes and ‘water balance’ (water intake – faecal and urinary water) were higher in all breeds when rectal temperatures were elevated. Water intakes were highest in the BX at both normal and elevated temperatures and increased most in the AX with an increase in temperature. ‘Water balances’ were similar in all breeds at normal body temperatures (4–5 1/day) but were lowest in the SH at elevated temperatures. This could partly explain the lower heat tolerance of the SH steers observed under these experimental conditions.

Description: SummaryCalves from three breeds, Brahman, Hereford × Shorthorn (HS) and Brahman × HS (BX), were divided equally into two groups, one of which was treated every 3 weeks from birth onwards to control ticks and gastrointestinal helminths, and one of which was untreated. Mortalities, growth rates and levels of resistance to environmental stresses that affected both mortality and growth under grazing conditions were recorded for all animals up to weaning (6 months) and for all males up to 15 months of age. The Brahmans were the most and the HS were the least resistant to environmental stresses, each of which was shown to depress growth in proportion to its magnitude and to contribute to the high mortalities of the HS. All breeds responded positively to parasite control with the greatest response in both survival and growth in the HS breed and the least response in the Brahman breed.Samples of males from the various breed-treatment groups were taken into pens where they were protected from environmental stresses and fed both low-quality pasture hay and high-quality lucerne hay ad libitum. Measurements were made of fasting metabolism, maintenance requirement, voluntary food intake and gain, variables related to the growth potential of each animal. The HS animals had the highest whilst the Brahmans had the lowest values for each variable.However, despite their low growth potential, the Brahmans had the highest growtli rate, and the HS, despite their high growth potential, had the lowest growth rate, when growth was measured in the presence of all environmental stresses. When parasites were controlled, growth rates were highest for the BX, the breed with intermediate growtli potential, and did not differ between the HS and Brahmans. These interactions arose because of the different contributions of resistance to environmental stresses and growth potential to growth rate measured at the different levels of environmental stresses. The relevance of these interactions to breed evaluation and cross-breeding is considered.Growth potential and resistance to environmental stresses were negatively correlated both between and within breeds, though the latter was biased by the effects of compensation. The influence of these relationships on the likely outcome of selection for increased growth rate, both between and within breeds, is discussed.

Description: SummaryBy comparing growth rates of Brahman (B), Hereford × Shorthorn (HS), their reciprocal F1, hybrid (F1BX), their Fn hybrid (FnBX) and an F1 Charolais × Brahman hybrid (Fl CH x B) in environments that differed in their levels of stresses that affected growth, it was shown that heterosis for growth realized in any environment arose because of heterosis in its underlying determinants, namely growth potential and resistance to environmental stresses. Growth potential of the F1 BX was similar to that of the better parent (HS) whilst resistance to environmental stresses was similar to or approached that of the more resistant parent (B). This combination of high growth potential and high resistance to environmental stresses enabled the F1 BX to outgain both parents at all levels of environmental stress above zero. However, some or all of the heterosis in both growth potential and resistance to environmental stresses was lost in the Fn BX. Thus, although previous selection for increased live-weight gain should have favoured the Fn BX, they realized lower live-weight gains than the Fl BX in all environments and lower live-weight gains than the parental breeds in all but intermediate environments.Because the breeds differed in both determinants of growth, the magnitude of estimates of heterosis for realized growth was dependent on the environment in which it was measured. A figure depicting this interaction was constructed.Comparative estimates were also made of the rate of approach to sexual maturity of bulls of each breed. The F1 BX had similar values to the better parent (HS) for both scrotal circumference and plasma testosterone concentrations. However, the Fn BX had values that were intermediate to those of the parental breeds.Generally, gains of the -F, CH × B exceeded those of all other breeds in all environments but their rate of approach to sexual maturity was slower than that of the F1 BX.

Description: SUMMARYSince 1966 a closed line of Hereford × Shorthorn cattle (selected line) has been selected principally for growth rate under conditions of moderate to high environmental stress. Responses in growth rate were measured over the years 1970–5 as differences from an unselected control line. Birth weight declined but live weight at all other ages increased.Groups of animals were taken from each line in 1976 and 1977 and used to determine the reasons for the differences in growth rate. Growth rate of the groups was measured when they were exposed to several different levels of stress, namely, the plane of nutrition, high ambient temperature, infection with bovine infectious keratoconjunctivitis(BIK) or gastro-intestinal helminths. The selected line was shown to be more heat tolerant, to have lower maintenance requirements and greater resistance to infection with BIK and helminths and, in consequence, always had higher growth rates in the presence of these stresses. However, they did not have superior growth rate at low levels of these stresses. Thus, selection had not increased the components associated with growth potential, and the improvement in growth rate was achieved entirely through increases in resistance to environmental stresses which affected growth rate.The application of these findings to selection for growth rate, particularly in harsh environments, is discussed.

Description: SummaryThe incidence of lantana poisoning and its effect on mortality, growth rate and fertility were recorded from 1975 to 1982 in seven lines of Bos taurus and Bos indicus cattle and their crosses.Two Bos taurus Hereford × Shorthorn (HS) lines had higher incidences of poisoning and higher mortalities after poisoning than the Bos indicus based breeds.During the period of poisoning, live–weight gains of affected animals of all breeds were reduced but subsequent recovery was rapid. Lantana poisoning had no detectable effect on the fertility of animals affected at least 10 months prior to their first mating, irrespective of breed.Parasite control influenced the incidence of poisoning and mortality in genetically similar groups of HS cattle. This observation, when combined with the between-breed relationships between the incidence of lantana poisoning and susceptibility to environmental stresses suggests that the marked breed differences in susceptibility to lantana poisoning stem from differences in resistance to other environmental stresses, including cattle ticks. This offers little scope for selection of animals that are resistant to lantana poisoning per se but has practical significance in lantanainfested regions.

Description: SUMMARYSynovex-H®, a combination of testosterone propionate and oestradiol benzoate, was implanted on three occasions at about 90-day intervals into Hereford × Shorthorn (HS), Brahman (B) and BX(B × HS) heifers to try to increase live weights and hence pregnancy rates at first breeding. Half of both the implanted and nonimplanted heifers of each breed was treated every 3 weeks to control cattle ticks and gastro-intestinal helminths. Treatment to control these parasites increased live weights, the increase depending on the breed. Implanting with Synovex-H also increased growth, the magnitude of the increase depending on both breed and treatment to control parasites. When parasites were present in significant numbers in the environment, the implanted groups had higher tick and helminth burdens and, in consequence, their live weight gains in response to the implant were lower than those of the nonimplanted group. The reason for the increase in susceptibility to parasites of the implanted group was not sought. It is suggested that it is associated with the presence, either alone or in combination, of the androgen and oestrogen components of Synovex-H.By the start of the 10-week breeding season, 290 days after the last dose of Synovex-H was implanted, previous advantages of the implanted groups in live weight had been eroded to the point where the differences between groups were no longer statistically significant. Subsequent fertility both as maidens and as first-calf heifers was generally significantly lower in the previously implanted groups of all breeds. Calves born to implanted heifers were not as heavy as those born to nonimplanted heifers both at weaning and 120 days after weaning. The particular regimen of use of Synovex-H followed in this study cannot be recommended if heifers are to be used for breeding.