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Item The development of an expert system for diagnosing reproductive problems in seasonal dairy herds : a thesis presented in partial fulfilment of the requirements for the degree of Master of Veterinary Science at Massey University(Massey University, 1997) Hayes, DavidAn expert system for investigating the reproductive performance of seasonally calving dairy herds (DairyFIX) was designed and developed. The system uses information retrieved from an on-farm information system (DairyMAN) to identify the primary causes of reduced performance and make recommendations for further action. The current performance of DairyMAN user herds and a sample of herds from the National Dairy Database were initially measured. Realistic targets for each performance indicator were then calculated using this information. The associations and interactions between the performance and diagnostic indicators used in DairyMAN were more clearly defined using multivariate statistical analyses and path models. Additional information was obtained from the literature for incorporation into the DairyFIX system. The reproductive performance studies covered one complete year (1993/94 season) and were limited to spring calving seasonal herds which are typical of the New Zealand dairy industry. Such herds are managed so that all cows calve on a synchronous annual cycle as close as possible to the "ideal" calving date in that location. A typical aim is to have calvings spread over 6 to 8 weeks with a narrow spread being strongly favoured. The herd planned start of calving (PSC) date defines the beginning of this optimal period of calving and the planned start of mating (PSM) the first day of mating that must be used to achieve the desired calving pattern. Expert system development is introduced in Chapter 1 with some historical information that supports the use of on-farm information systems. The concepts and design methods used for developing expert systems are then reviewed in brief. Examples are provided to illustrate the different levels of sophistication that can be used for developing expert systems. These demonstrate that relatively simple designs are often the most successful. Examples including Bovid and Dairy Expert are discussed in detail. The methods used to monitor seasonally calving dairy herds are then reviewed with an emphasis on the main performance and diagnostic indicators used in DairyMAN. These include the four and eight week calving rates, 21 day submission rates, non-return and conceptions rates and four and eight week in-calf rates. Empty rates can be considered the final definitive measure of performance by many dairy farmers, but the difficulties with interpreting this figure are presented. The limited mating period in seasonally calving dairy herds means that the assessment of heat detection is difficult and often inaccurate. For this reason several alternative methods for assessing heat detection efficiency and accuracy are discussed in detail. A detailed description is given in Chapter 2 of herd and individual cow performance for DairyMAN user herds and the National Dairy Database sample. These data show that the reproductive performance of New Zealand herds is often below that previously reported. Calving rates are on average above industry targets, but only with a significant level of calving induction. Removal of inductions for welfare and marketing reasons will have a significant effect on the performance of many herds. Submission rates are the earliest available measures of performance during a mating season. New Zealand herds do not, on average, achieve the necessary targets. All measures of heat detection efficiency, although imprecise, show this is not a major problem with about 6% of heats missed. This has a negative effect on submission rates. Detection efficiency is an important issue for some individual herds as the consequences of poor heat detection are dramatic. The performance levels suggest that nutritional anoestrus and the effects of a spread calving pattern are the major causes of low submission rates. Conception rates of less than 60 % are reported. These are below those often suggested as typical for New Zealand herds. Much of the previous data has been taken from small study groups that may not adequately represent all herds. The common use of non-return rates may have created expectations that cannot be achieved in average herds as these are an optimistic measure of performance. Health events such as lameness are only reported for reference. Only limited health data is recorded although DairyMAN provides the flexibility to records such data. The variability in the type and degree of recording of these data is identified as a significant problem that limits the use of the available records. Some of the health events and especially lameness may have a large effect on the reproductive performance of many herds. Path models are developed in chapter 3 as an essential prerequisite to the development of the DairyFIX expert system. The models statistically confirm most of the relationships that have been previously considered important when evaluating herd reproductive performance. A number of factors including herd size and breed are shown to be associated with differing calving rates. The four week calving rate is shown to have strong indirect effects on submission rates, conception rates and herd in-calf rates. As such, it is one of the most important variables in seasonal herds. The importance of submission rates and conception rates is confirmed. Daily per cow milk production is shown to be a useful indicator of submission rates as both of these variables are directly influenced by nutrition. The models identify some limitations with using non-return rates as measures of conception. These generally give optimistic results that do not accurately reflect true performance. Such problems are compounded if reasonably accurate measures of heat detection cannot be obtained in herds with a very restricted mating period. Although the interactions between the performance and diagnostic indicators are largely understood from previous work this is the first time they have been brought together in statistically verified path models. The use of an on-farm information system (DairyMAN) was shown to be associated with improved herd performance including daily per cow milk yield and reproductive outcomes. DairyMAN user herds had cows of the same breed and genetic capacity. So DairyMAN users were able to produce more milk with animals of equivalent genetic merit, indicating that users achieved better management of the herd through improved attention to managerial details. This was associated with their adoption of DairyMAN, but not shown by this study to be a direct consequence of it. These findings are important because there is very little information confirming that on-farm information systems or central databases give true performance gains. This is despite the historical recognition of these systems and the rapid expansion in recent years. Justifying the use of more sophisticated tools such as an expert system would be more difficult if gains were not being achieved with the current technology. Calving induction was shown to be associated with some negative effects on milk yield and reproductive outcome. The New Zealand dairy industry does not currently favour the use of this management tool, but the impact any changes in management practices would have need to be evaluated with consideration of these effects. Calving induction is typically not used as recommended in New Zealand as many of the treatments are done too late to provide sufficient economic gains through increased lactation length and increasing the number of days from calving to the planned start of mating. Regional differences in performance were identified. DairyMAN user herds in the Manawatu had inferior conception rates while herds in the Taranaki had superior reproductive performance. These observed differences suggest a need to further identify causes of these differences, if performance is to be improved in some regions. The performance of individual cows and groups was examined in detail and statistical models developed for use in DairyFIX. Breed, lactation number, days calved at the start of mating and some health events were all shown to have an important impact on performance. The inferior performance of lactation 1 and 2 groups is having a large effect on performance. Cows in lactation 3 and 4 generally have the best performance with some reduction for aged cows. These aged cows do not dramatically affect overall herd performance because they are only a small proportion of most herds Jersey cows tended to show superior calving and submission rates. A number of complex models were developed for herds that pregnancy test and those that use non-return as the measure of conception. DairyFIX was developed to achieve two primary objectives. The first was to simplify the epidemiological approach to investigating herd reproductive problems. The system automates procedures that would otherwise be followed using DairyMAN and uses a graphical diagnostic interface to achieve this. DairyFIX quickly focuses on the important aspects of any performance deficit and reduces the time required to investigate a herd problem. Although DairyMAN is a comprehensive software package, users have difficulties due to its complexity and may invest a considerable amount of time exploring the program rather than focusing on the task required. The second objective of this project was to make expertise available for the user when examining herd performance. This required the use of the more sophisticated aspects of an expert system including the development of a "knowledge base" of information. DairyFIX consists of three sections. The first simply evaluates performance and determines if any problems exist. The second part considers the effect of the major components of performance such as the calving pattern and heat detection so that the primary causes of poor performance are identified. This section uses the statistical models previously developed to estimate expected performance. The third part of DairyFIX consists of several specific interrogation procedures for each area of poor performance. This section is not necessarily required for an operational system as the user can otherwise be referred to the appropriate reports in DairyMAN. Another module within DairyFIX was designed to assess the expected performance of a herd in subsequent seasons given a predicted herd profile. The models used for this purpose are similar to those that retrospectively evaluate performance. DairyFIX simplifies the investigative task and identifies the major causes of poor reproductive performance. It is anticipated that this tool will allow more dairy farmers and veterinarians to make effective use of DairyMAN while reducing the investment in time that is currently required.Item Productive and reproductive efficiency of two Holstein Friesian lines of cows which differ genetically for live weight : a thesis presented in partial fulfilment of the requirements for the degree of Master of Applied Science in Animal Science, Massey University(Massey University, 1998) Laborde, DanielTwo lines of Holstein Friesian cows which differ genetically for live weight, the Light Line (LL) and the Heavy Line (HL), have been selected at the Dairy Cattle Research Unit (Massey University) since 1989. The aim of the current experiment was to compare the productive and reproductive performance of these two lines during early lactation. Measurements of milk production, liveweight (LW), and pasture intake were made in 1996, while reproductive data were analysed for 1992 to 1997. In experiment 1a, the milk production of the two lines was compared during the first 12 weeks of the lactation by the weekly measurement of the milk yield and the milk composition of 30 LL cows (average LW=412 kg) and 27 HL cows (average LW=445 kg), with the two groups of cows fed and managed identically. Pasture dry matter intake (DMI), calculated as pasture disappearance, was 13 to 15 kg DM a day during these 12 weeks. Although the HL produced slightly more milksolids (MS) than the LL, the difference was not significant (LL= 139 vs HL= 141 kg MS). However, the HL cows > 2 year old produced 7 kg MS more than the LL cows > 2 year old (P<0.05). The LW and body condition score (BCS) changes in cows after calving were similar for both lines, but in the heifers the LL lost 17 kg of LW during the first 5 weeks of lactation compared to the HL that maintained their LW (P<0.05). Similarly, the BCS of the LL was lower than that of the HL at 40 days postcalving (LL= 4.17 vs HL= 4.43, P<0.05) mainly due to the BCS lost by the LL heifers. In experiment 1b, the DMI and the dry matter digestibility (both estimated using the alkanes technique) of 21 LL cows ( 406 kg) and 21 HL cows (482 kg), grazing at a pasture allowance of 40 to 45 kg DM/cow/day, was measured in a ten days trial. The grazing behaviour of the two lines was also recorded during 2 days. Although the LL cows ate slightly less DM (LL= 14.3 vs HL= 15.1 kg DM/cow) and had a slightly higher MS conversion efficiency than the HL cows (LL= 120 HL= 110 g MS/kg DM eaten), the differences were not significant. When DMI was regressed on LW0.75 and MS yield, the effect of LW0.75 only approached significance (P<0.1), but the effect of MS was highly significant (P<0.001). The two lines had similar DMD (LL= 77.8% vs HL= 78.0%), gross energy conversion efficiency (LL= 44.6% vs HL= 42.3%) and net energy conversion efficiency (LL= 64.8% vs HL= 64.6%). The bite size of the HL cows (estimated from the grazing time, biting rate and DMI) was heavier than that of the LL cows (LL= 0.46 vs HL= 0.60 g DM/bite, P<0.01), but the LL cows compensated for their lighter bite size by increasing the number of bites per minutes ( LL= 55 HL= 50 bites/minute, P<0.05). The reproductive performance of the two lines was compared for the period from 1992 to 1997, and the interval Calving-Ovulation was estimated from the concentration of progesterone in milk in 1996 and 1997. The HL cows had shorter calving-ovulation intervals than the LL cows (LL= 32 vs HL= 28 days, P<0.05), but the difference in calving-first heat interval was not significant (LL= 43 vs HL= 50 days). Compared to the LL cows >2 year old, the HL cows > 2 year old tended to calve and to conceive later in the calving and mating periods, respectively, because the HL cows had a lower conception rate at first service than the LL cows (LL= 70% vs HL= 58%, P<0.05). The ovaries of 10 cows from each line (LL= 405 vs kg HL= 481 kg) were scanned daily during a complete cycle before the start of mating. Cows from the HL had preovulatory follicles with larger diameter (LL= 12.7 vs HL= 15.7 mm, P<0.05) and corpus lutea with larger areas (LL= 690 vs HL= 859 mm2, P< 0.05) than the LL cows. No differences were detected in the diameter of the first and second dominant follicles. On average, the preovulatory follicles of the HL cows achieved their maximum diameter later in the cycle compared to the LL (LL= day 18th vs HL= day 20th). The results from the current experiment show that although the HL produced slightly more MS than the LL in the longer period, the two lines of cows achieved similar levels of MS yield during early lactation independently of their LW and size. Similarly, although the LL cows had a slightly higher MS conversion efficiency than the HL cows, the differences in energy and MS conversion efficiency between the two lines were not significant. The reproductive data analysed from 1992 to 1997 suggest that the LL cows achieved a better reproductive performance than the HL cows because of their higher conception rate at first service. However, more information is required from other stages of the lactation before any definite conclusion is reached about the feed conversion efficiency of the two lines. Similarly, considering the variation in the reproductive performance of the HL between the years, reproductive data from subsequent seasons must be collected in order to verify, or disprove, the current conclusions.Item Aspects of the survival and welfare of neonatal calves : a thesis presented in partial fulfillment of the requirements for the degree of Master of Science at Massey University(Massey University, 1998) Wesselink, EricaNeonatal calves form the basis of all dairy and beef herds. There are many factors that influence calf survival and welfare and therefore the economic viability of the farm. This thesis examines four factors that have the potential to affect the survival and welfare of neonatal calves. 1. Induction of premature parturition in dairy cows results in the birth of calves that have a higher mortality rate than those born at full-term. The mortality rate of these calves decreases as their gestational age approaches full-term. The aims of the study described in chapter two were to assess the physical and physiological state of calves born at varying gestational ages. Sixty-eight induced calves that were born up to five weeks prematurely were examined and the following observations and measurements were recorded: body weight, crown rump length, rectal temperature, the number of deciduous incisors erupted and palpable, breathing rate, heart rate, gum colour, hoof membranes, serum gamma glutamyl transferase activity and a general assessment of coordination. It was found that calves born prematurely had lighter body weights, shorter crown rump lengths, lower rectal temperatures, fewer deciduous incisor teeth erupted and that they breathe more slowly. It was concluded that welfare of calves born more than 3 weeks prematurely (<259 days gestational age) were more likely to die or become ill because of a reduced ability to maintain their body temperature and difficulty breathing and feeding effectively. 2. Although many premature calves die on the farm a few are kept for rearing and the remainder are sent for slaughter. While the calves remain on the farm their welfare and survival depends on them being fed colostrum or milk at appropriate volumes and intervals. Calves that are sent for slaughter may go without food for up to 30 hours. Although the metabolic responses of full-term calves to feeding and fasting have been examined those of premature calves have not. In chapter three the responses of premature calves to different feeding frequencies (100-120 ml colostrum /kg body weight split into either 2 or 4 feeds of equal volume over ten hour period) and to 30 hours fasting were examined. The calves were either four or ten days old at the start of the experiment and varied in gestational age at birth. The effects of feeding frequency and fasting were assessed using the changes in plasma glucose, β-hydroxybutyrate, triglycerides, urea, total plasma protein concentration, packed cell volume and the rectal temperature. It was found that premature calves tended to have lower plasma glucose and higher β-hydroxybutyrate concentrations than those born at full term indicating a greater dependence on lipids to provide their energetic requirements. The wide variation in these parameters was primarily determined by the gestational age of the calves at birth. Calves born further from full-term apparently depend on lipid catabolism to a greater degree. The premature calves gained no apparent energetic advantage when fed a similar volume of milk in four feeds as compared with twice within ten hours. There was considerable variation in the ability of the calves to tolerate 30 hours of fasting. The availability of energy in fasted calves (as indicated by plasma glucose concentration) was greater in those fed 50 ml colostrum/kg bodyweight rather than 25 ml/kg at the onset of fasting. Calves born at an earlier gestational age had a reduced ability to tolerate 30 hours without food. A decrease in rectal temperature during fasting was seen in some of the more premature calves. This period of fasting did not cause dehydration as indicated by the absence of significant increases in packed cell volume or total plasma protein concentration. The calves' postnatal age did not affect their responses to either feeding frequency or fasting. 3. The fetal membranes of twin calves often fuse where they abut resulting in varying degrees of placental vascular anastomosis. The gross anatomy of placentas from 18 twin-bearing cows, and the PCV changes of their calves between birth and 24 hours were examined. Six classes of vascular anastomosis were identified. The extent of anastomosis varied from none, to extensive. The PCV decrease over the 24 hours following birth differed to a greater extent between twins from placentas exhibiting a high degree of anastomosis. It was concluded that these differences were likely to have been due to blood transfer within the joined placental vessels of the calves during birth. 4. The intake of colostral immunoglobulins gives neonatal calves the ability to resist disease. However, the capacity to absorb immunoglobulins decreases rapidly between 6 and 24 hours after birth. In the study described in chapter five it was found that failure of passive transfer of immunoglobulins, as indicated by low gamma glutamyl transferase activity, occurred in approximately 45% of the 74 dairy calves from which blood samples were taken. Calves that had not received colostrum were not easily distinguished from those that had on the basis of obvious physical or behavioural features. The behaviour of 21 cows and their calves was observed between birth and the time the calves were separated from their dams. The amount of time the calves spent with their dams ranged from 74 minutes to 1492. Thirty-three percent of calves had not sucked within this time. Of those calves that did suck, 79% did so within 6 hours of their birth. There were no obvious behaviours on the part of the cows or their calves that influenced the time between birth and first sucking or the amount of time spent sucking by each calf.Item The microbiology of bovine semen and the antimicrobial activity of bovine seminal plasma : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Microbiology at Massey University(Massey University, 1977) Schollum, Linda MayMicrobial populations in semen from 18 bulls used for artificial breeding, varied between 30 and 560,000 organisms/cm3, with a mean of 3,300/cm3. Wide day to day fluctuations occurred in the microbial content of semen and there was no correlation between the total microbial counts, the proportion of these organisms sensitive to bovine seminal plasma inhibition and the measureable amount of antibacterial activity in a given semen sample. All semen samples had antibacterial activity, the amount being relatively constant for a given animal; marked differences occurred between breeds. Sixty-eight percent of the bacteria isolated from semen (representing ten genera) were inhibited by seminal plasma. Together with other data, this supports the view that most semen micro-organisms are transient contaminants from outside the reproductive tract, derived mainly from the preputial cavity. The most common organisms in bovine semen were species of Bacillus, Corynebacterium and Micrococcus, but twelve other bacterial genera and two yeast species were also represented among the isolates. Most semen microorganisms were sensitive to several antibiotics, with pseudomonads, coliforms and yeasts proving the most resistant. The control of these organisms is important since they may grow in extended semen and have been implicated in bovine genital disorders. Chloramphenicol and streptomycin are routinely used in semen extenders and have been highly effective in controlling bacterial growth; penicillin has been less effective. At least three antimicrobial activities have been observed in bovine seminal plasma: (i) an antiviral activity, (ii) a heat labile (60°C for 60 minutes) antibacterial activity and (iii) a heat stable antibacterial activity. Research has centered on the heat stable fraction which is sensitive to proteolytic enzymes but not to periodate treatment. This heat stable fraction can be purified by adsorption onto highly polymerised deoxyribonucleic acid (DNA). It does not resemble lysozyme, polylysine, salmine or spermidine in antimicrobial spectrum, electrophoretic properties or amino acid composition. Data from dialysis, polyacrylamide gel electrophoresis, ultrafiltration and ultracentrifugation indicate that the active moiety occurs in varying molecular weight aggregates, caused by self association and interaction with other microsolutes present in its environment. At least two of the aggregates (cationic at pH8.6) are antibacterial by the overlay technique which detects biological activity in electrophoresed proteins. However, it migrates as a single band (molecular weight 25,000) in sodium dodecyl sulphate (SDS) polyacrylamide gel electrophoresis, and has a single N terminal amino acid. when compared with antibacterial compounds described by other workers, the anti-microbial proteins from bovine seminal plasma were found to have a unique chemical composition. Bovine seminal plasma proteins had a rapid lethal effect on both Gram-positive and Gram-negative bacteria as well as ureaplasmas and mycoplasmas. Bacterial resistance to bovine seminal plasma exhibited a degree of correlation with resistance to antibiotics affecting cell membrane integrity. Detection of cytoplasmic leakage, loss of accumulated labelled compounds and failure to accumulate a labelled amino acid by bacteria in contact with seminal plasma supports the hypothesis that membrane damage is induced by bovine seminal plasma antimicrobial proteins. Resistance of Pseudomonas aeruginosa to bovine seminal plasma appears to be due to the bacterial cell wall preventing access of the active moiety to the cell membrane. Bovine seminal plasma antimicrobial cationic proteins may well contribute to an efficient primary nonspecific defence mechanism of the genital tissues. Few microorganisms colonise the upper reproductive tract; ascending infections from the lower urethra and preputial cavity, where bacterial populations are large, are uncommon. Only bacteria shown to be resistant to antimicrobial factors present in bovine seminal plasma have been implicated in bovine male reproductive tract infections. During coitus the antimicrobial activity may also protect the female genital tract from infection.Item Studies of Holstein-Friesian cattle bred for heavy or light mature live weight : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy, Institute of Veterinary, Animal and Biomedical Sciences, Massey University(Massey University, 1998) Garćia-Muñiz, José G.; Garćia-Muñiz, JoséThe new Animal Evaluation Model predicts that heavier live weight (LW) of the lactating cow reduces the profitability of the pasture-based dairying farm in New Zealand, because its effects on increased maintenance requirements are not fully compensated by the extra income generated from selling heavier culled cattle and surplus progeny. The work outlined in this thesis was intended to validate the expected effects of selection for differences in LW on actual LW from birth to maturity and on herbage intake and feed conversion efficiency (FCE) of growing cattle and lactating cows. It also investigated the existence of any associated effects on calving difficulty, calf mortality, onset of puberty and reproductive performance of the two lines of Holstein-Friesian (HF) cattle bred for heavy or light mature LW. These two lines have been developed at the Dairy Cattle Research Unit, Massey University, New Zealand, since 1989. The high genetic merit HF cows from the base herd have been mated to high genetic merit HF sires with either high or low breeding value (BV) for LW but with similar breeding worth (BW) in order to generate the heavy (H) and the light (L) mature LW selection lines. During the period 1994 to 1997, a series of experiments with growing heifers and lactating cows from the H and L lines, and analysis of data collected from the cows were undertaken to compare the two genetic lines. The BV's for live weight of the sires were 86 kg for the H and 31 kg for the L cows and the actual H animals were heavier at birth (41 vs. 35 kg) and at maturity (510 vs. 460 kg). In addition the BV's for milk (1037 vs. 737 l), milkfat (33.0 vs. 27.5 kg) and milk protein (31 kg vs. 22 kg) of H sires were also higher and the H cows produced significantly more milk (4708 vs. 4323 l/lactation), more milkfat (207 vs. 198 kg/lactation) and more milk protein (157 vs. 150 kg/lactation) than the L cows. However, the L sires had slightly higher breeding worth ($ 46 vs. $ 37) than the H sires and theoretically calculated and experimentally measured feed intakes and the resultant feed conversion efficiencies, confirmed that the L cows had slightly higher values for FCE than the H cows in three short-term grazing experiments and when FCE was calculated over multiple lactations. Therefore the basic assumptions in the Animal Evaluation Model seem to be correct. Sires of H cows had a higher proportion of USA Holstein genes in their pedigrees than the L sires. Consequently cows from the H line had a higher (≈27%) proportion of USA Holstein genes compared to cows from the L line (≈7%), whose sires were mainly of New Zealand ancestry. New Zealand bulls are progeny tested under grazing conditions and a very tight seasonal system of reproduction, whereas North American bulls are progeny tested under dairying systems of all year round milk production and feedlot feeding. There were significant differences in the pattern of grazing behaviour of H and L cows. The L cows displayed a more 'aggressive' pattern of grazing behaviour than H cows given by significantly longer grazing times (520 vs. 499 min/d), faster rates of biting (58 vs. 52 bites/min), higher number of total bites per day (31053 vs. 25046 bites/d), lower rumination times (471 vs. 572 min/d), and the selection of herbage of higher digestibility (72.0% vs. 69.3%). These results may reflect not only a difference in mature LW between the H and L cows, but may also reflect a strain of Holstein (i.e. NZ vs. USA Holstein) difference due to the sires' ancestry referred to above. There were no differences between H and L cows in the incidence of calving difficulty. However, offspring of bulls with high BV for rump width (i.e. wider pelvises) were more likely to face a difficult calving, and so were daughters of bulls with low BV for rump angle (i.e. less sloping pelvises). There were no differences between H and L cows for calf mortality. However, induced calves were more likely to die or undertake an emergency slaughter, and the H cows had significantly higher induction rates than the L cows (10.5 vs. 4.2%). The H heifers grew faster, ate more feed (4.3 vs. 3.8 kg/hd/d) and were heavier (241 vs. 221 kg) and older (325 vs. 300 d) at puberty than L heifers, and there were no differences between H and L heifers in pregnancy rate, age at first calving and first lactation yield of milk and milk components. There were only small differences in the reproductive performance of H and L cows after adjusting by differences in induction, calving date and percentage of USA Holstein genes in the cows. The L cows had slightly shorter intervals from first service to conception (13 vs. 17 d) and from the start of mating to conception (24 vs. 29 d), and slightly higher first service conception rate (65 vs. 54%), which translated in a more concentrated calving pattern and lower induction rate (4.2 vs. 10.5%) for the L cows. The results of this thesis indicate that selecting for heavier mature live weight produced the expected results of heavier animals with higher yields of milk and milk components, higher feed requirements and higher herbage intakes and slightly lower feed conversion efficiency than lighter mature live weight cows. However, there were also differences in grazing behaviour in which the L cows displayed a more competitive pattern of grazing behaviour than the H cows. The results of this thesis suggest that for the New Zealand seasonal system of milk production based almost completely on grazed pasture, lighter mature LW HF cows may have an advantage over heavier mature LW cows. Under the conditions of this experiments L cows were slightly more efficient, younger at puberty, had a more concentrated calving pattern, and were less prone to be induced to calve than heavier mature live weight HF cows.