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    Epidemiological studies of parasitism in sheep and reproduction in horses : a thesis presented in partial fulfilment of the requirements for the degree of Master of Veterinary Studies (Epidemiology) at Massey University
    (Massey University, 1997) Macchi, Chiara
    Anthelmintic resistance is a well-recognised problem for control of nemotodes in sheep in most countries of the world. The climatic conditions in New Zealand are particularly favourable to the survival and development of gastrointestinal nemo=atodes. As a consequence, gastrointestinal parasitism is a major impediment to profitable sheep raising in New Zealand. A random postal survey of 300 sheep farmers in the southern North Island region of Manawatu was conducted with the purpose of examining current farming and drenching practices and investigating possible risk factors in the development of anthelmintic resistance. The results of this study, reported in Chapter 2, revealed a high degree of awareness and concern about the problem of resistance, but also a lack of understanding on how grazing management strategies should be combined in order to achieve integrated control over gastrointestinal nematodes while minimising the use of anthelmintic drugs. Only 31% of respondents had performed at least one drench test on their property. Amond testing farms, prevalence of resistance approached 70% and involved benzimidazole products in all but one case. Subsequently, a trial was undertaken to investigate the economic consequences of anthelmintic resistance in growing lambs on commercial farms (Chapters 3 and 4). Five farms with a history of resistance to benzimidazole drenches were selected. The effects of three treatment strategis on nematode egg counts, bodyweight gains and susceptibility to diarrhoea were compared between groups of ewe lambs. Suppressively treated lambs performed significantly better than effectively treated lambs, which in turn performed better than ineffectively treated lambs. However, a partial budgeting analysis carried out by means of a stochastic simulation model (Chapter 4) indicated that effective treatment yielded the highest net returns. The model also showed that the range of possible outcomes oscillated substantially around the mean, reflecting the degree of uncertainty about the outcome on any single farm due to variation between farms. Chapter 5 describes a study which was carried out with the objective of evaluating two management strategies for breeding mares after foaling. Mares were examined on day 7-9 postpartum by palpation and ultrasound. Pregnancy rates, pregnancy loss rates and time from foaling to conception in two groups were compared. The statistical power of the study as determined by power analysis, was insufficient for the observed differences to reach statistical significance.
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    The measurement of plasma progesterone levels in the normal mare and its application to some equine breeding problems : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Veterinary Science at Massey University
    (Massey University, 1977) Kelly, Christopher Morton
    A radioimmunoassay system was developed to measure plasma concentrations of endogenously produced and exogenously administered progesterone in non-pregnant and pregnant mares in normal and some abnormal reproductive states. Assay sensitivity was 0.5 ng/ml, with a between assay coefficient of variation of 16.8% for a high progesterone sample, estimated over 24 assays, and 8.5% for a low progesterone sample, estimated over 15 assays; the within assay coefficient of variation was 7.3, 10.1 and 6.9% respectively for six replicates of one sample, estimated in three separate assays. Plasma progesterone concentrations of six normal non-pregnant cycling mares followed regular cyclic changes, with levels less than 0.5 ng/ml during oestrus and ranging between 8-22 ng/ml at peak values in dioestrus. The first oestrus following the winter non-breeding period was longer than the following oestrus and the period from ovulation to progesterone decline tended to be less variable than the rest of the cycle. There was a large between-mare variation in plasma progesterone concentrations in mares at both early and late stages of gestation with levels varying from 4.9 to 15 ng/ml in the former and 5.2 to 16.9 ng/ml in the latter group. No significant effect was noted between stage of gestation and progesterone concentration. A group of five mares all had plasma progesterone concentrations greater than 9.5 ng/ml within 24 hours prior to parturition; two of these mares sampled within eight hours prior to parturition had plasma progesterone concentrations of 4.3 and 3.9 ng/ml. The first post partum sample was taken within 24 hours of foaling; by this time plasma progesterone concentrations had fallen to less than 0.5 ng/ml and remained low until sampling ceased at the first post partum oestrus. Prostaglandin F2α (THAM salt) was effective in causing luteolysis in 13 mares with active corpora lutea before treatment. By three days post-injection 12 of the 13 mares had plasma progesterone concentrations of less than 0.5 ng/ml and by five days post-injection 12 of the 13 mares were exhibiting oestrus. Of the ten mares bred at the induced oestrus, seven became pregnant to that mating. Plasma progesterone concentrations were measured on 16 non-pregnant mares in anoestrus. Six of eight mares sampled early in the breeding season (September) had plasma progesterone concentrations of less than 0.5 ng/ml, the other two mares had plasma progesterone concentrations of 0.6 ng/ml. Eight of eight mares sampled later in the breeding season (November and December) had plasma progesterone concentrations greater than 0.5 ng/ml, the levels ranging from 6.2 to 13.1 ng/ml. Concentrations of plasma progesterone in normal dioestrous mares were measured half and one hourly (three mares) for 24 hours and four hourly (two mares) for 120 hours. There were large apparently random variations, with more than 100% differences being recorded between a number of consecutive samples. Plasma progesterone concentrations varied from 7.8-23.0, 3.2-21.9 and 4.2-12.9 ng/ml for the three mares sampled half hourly and hourly, and from 6.8-24.6 and 0.8-11.0 ng/ml for the two mares sampled four hourly. Radioactive progesterone, administered by venepuncture to a mare with no detectable endogenous plasma progesterone, disappeared from the plasma within 40 minutes; 85% of the injected steroid had left the plasma by 2.5 minutes post-injection. Two and 25 mg of progesterone in 16% alcohol in saline was administered by venepuncture to mares with plasma progesterone concentrations of less than 0.5 ng/ml. For the mare given 2 mg, the plasma half life of injected progesterone was 1.75 minutes for the initial "fast" component, and for the mare given 25 mg the plasma half life was 2.75 minutes. There was a second peak of plasma progesterone at from 8 to 19.5 minutes for the former and from 9.5 to 17 minutes for the latter mare. A third much smaller peak was recorded at about 50 minutes post-injection for the mare given 2 mg progesterone. A mare with no detectable endogenous plasma progesterone was administered by intramusuclar injection a total of 600 mg progesterone in arachis oil over a period of seven days. Plasma concentrations of the steroid reached a maximum of 4.3 ng/ml at one day post-treatment and were maintained at this level for only a maximum of 24 hours. A second mare, again with no detectable plasma progesterone, was administered by intramuscular injection a total dose of 2 g of hydroxyprogesterone capronate in castor oil over a period of ten days. Maximum plasma progesterone concentrations of 1.2 ng/ml, maintained for less than 24 hours, were reached nine days after the first injection. Wide variation in plasma progesterone levels within and between mares over relatively short time periods suggest that there are many difficulties in identifying "progesterone insufficiency" as a cause of embryonic absorption or abortion in this species. Moreover the short half life of this steroid in the plasma of the mare, together with the sustained high dose levels that would be required to elevate plasma concentrations of progesterone to a level equivalent to that produced by normal secretory corpora lutea, indicate that current levels of administration of this drug are likely to have little effect in overcoming such a breeding problem unless the progesterone is acting at a local level. A definitive answer in respect to this vexed question concerning the existence or not of "progesterone insufficiency" as a cause of prenatal loss in the mare, together with an appropriate method of treatment, still remains to be found.
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    Maternal investment in Kaimanawa horses : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Ecology at Massey University
    (Massey University, 1998) Cameron, Elissa Zanna; Cameron, Elissa Zanna
    Maternal investment (MI) was studied in Kaimanawa horses, a population of feral horses in central North Island, New Zealand. It is predicted that individual mares will vary their maternal behaviour so as to maximise their own and their offspring's reproductive success. Maternal investment is defined and measured by maternal input, proximal maternal costs and reproductive costs to the mother. The primary maternal input is milk. Time spent sucking is frequently used to measure milk intake based on the assumption that more sucking equates with proportionately more milk intake. A review found little support for this assumption and so I tested whether sucking time predicted milk intake by labelling the milk of thoroughbred mares with tritium and measuring its transfer to foals. No significant predictive relationship was found. Therefore sucking time cannot be used as an index of milk intake and conclusions about differential MI based on time spent sucking may be wrong. A mare's social environment is a significant modifier of her MI. Mares were more protective, and suffered reproductive costs, in bands with more than one stallion or in other circumstances where paternity is uncertain or rates of aggression are high. Despite individual differences in maternal style, mare behaviour was modified by experimental manipulations of the number of stallions in a band. In an unusual event when a mare and her adult daughter lived in the same band, both co-operated in the care of a single foal. Mare age and experience modified maternal behaviour. As mares age they become more successful at raising foals through better-targeted input, but no extra investment. I tested the Trivers-Willard hypothesis (TWH) of sex differential MI. The hypothesis predicts that mothers in better condition should produce more sons, and invest more in their sons, whereas mothers in poorer condition should favour their daughters. I argue that horses are an ideal species on which to test the TWH. We found no sex biased MI on a population level in terms of maternal input, proximal costs or ultimate costs. However, the TWH makes specific predictions about individuals, not populations. Individual mares in poor condition gave birth to more daughters and biased their MI towards daughters. Conversely, mothers in good condition gave birth to more sons and biased their investment towards sons, supporting all the predictions of the TWH. Mares alter their maternal investment in ways that conform to predictions based on maximisation of lifetime reproductive success.