Much evidence has accumulated showing that female reproductive functions are disrupted when changes in nutritional status take place in both over- and undernutrition. The peptide hormone leptin is considered a possible link between nutrition and reproduction. One objective with the present study was to investigate how different levels of feed intake during the dry period, thereby creating differences in body condition at parturition, affected the plasma leptin concentration and the reproductive function after parturition.

In modern dairy cows, poor fertility is an ever-increasing problem. Milk progesterone analysis has revealed that this poor fertility is associated with a high incidence of reproductive cycle problems during the post partum period and with reduced progesterone secretion following mating. One of the likely causes of these problems is the increasing metabolic demand placed on these cows by increasing milk yield. In recent years, the search for an endocrine link between nutritional status and reproductive function has focussed on leptin. The aim of the present study was to determine whether plasma concentrations of leptin differ between cows with and without post partum reproductive problems and to determine if any relationship exists between plasma leptin and post mating plasma progesterone concentration.

In mammals leptin plays a key role in regulating the whole-body energy homeostasis and is important for for normal reproduction Much evidence indicates that leptin mediates the undernutrition-induced alterations of the reproductive axis (Ahima and Flier, 2000). In birds leptin gene was cloned in chickens (Taouis et al., 1998) and recombinant chicken leptin (chLep) was prepared in our laboratory (Raver et al., 1998). Unlike mammals, leptin is expressed not only in the adipose tissue but also in liver and leptin plasma level is lower in fasted than in fed hens (Dridi et al., 2000). However, the importance of leptin in avian reproduction is not known. Therefore, the aim of the present study was to examine whether in chickens leptin affects events occuring in the ovary during fasting.

Mice lacking a functional leptin gene (ob/ob) are obese and sterile. Treatment with exogenous leptin will restore fertility and allow full-term pregnancy, but when leptin is withdrawn at parturition the young die, apparently as a result of total lactation failure (Chehab et al., 1996). From this one could hypothesise that leptin is an essential requirement for mammary development and/or initiation of milk secretion. Since a few of the mice used in this work were subsequently able to rear pups following a second pregnancy and parturition, we decided to re-examine this hypothesis.

In the adult, circulating leptin concentrations are dependent on body fat content and on current nutrient intake. Whilst leptin concentrations in umbilical cord blood correlate with fetal adiposity in the human neonate, it is unknown whether leptin acts as a signal of fat mass before birth or whether changes in maternal nutrient intake alter plasma leptin concentrations in the fetus. We have therefore investigated the relationship between fetal plasma leptin concentrations and fetal fat mass in pregnant ewes fed at or above maintenance energy requirements.

Leptin, a 16kDa polypeptide hormone, is synthesised and secreted by adipocytes and acts to regulate energy stores and energy expenditure in the adult. In the newborn infant, there is a positive correlation between cord blood leptin concentrations and neonatal fat mass but it is unknown whether leptin can act to regulate the endocrine or metabolic characteristics of adipose tissue before birth. We have therefore tested the hypothesis that an intrafetal infusion of leptin will act to alter on leptin mRNA expression and the morphological characteristics of perirenal fat, the major fat depot in the sheep fetus during late gestation.

The long-chain polyunsaturated fatty acids (PUFA) docosahexaenoic acid (DHA) and arachidonic acid (AA) are the most abundant fatty acids in the brain and are vital for its correct development and for that of the nervous system (Huang and Craig-Schmidt, 1996). Ruminant diets are low in DHA and its precursor alpha-linolenic acid. In addition, dietary PUFAs are substantially hydrogenated in the rumen. Consequently, it may be argued that the diets of pregnant and lactating ewes may be deficient in DHA and that a response to supplementation may be observed. Studies involving the supplementation of pregnant ewes with supraoptimal levels of vitamin E have shown that lambs born to supplemented dams are more vigorous immediately after birth and have higher liveweight gains (Merrell, 1998). The objective of this experiment was to investigate the effects of dietary long-chain PUFA in combination with vitamin E supplementation of ewes on ewe and lamb performance.

Dietary fatty acids have been shown to affect the activity of the immune system in a variety of species through eicosanoid dependent or independent mechanisms (Miles 1998, Calder 2001). MLN and spleen are lymphoid tissues which play a key role in the immune function. Changing the fatty acid composition of these tissues could change the profile of eicosanoid produced by immune cells in these tissues and this could alter the immune response. This study was carried out to establish the extent to which different oil supplements could change the fatty acid composition of MLN and spleen in milk fed calves.

Increasing the ratio polyunsaturated:saturated fatty acids (P:S) in beef muscle by nutrition is hampered by the high levels of ruminal biohydrogenation of dietary polyunsaturated fatty acids (PUFA). Effective ruminal protection of dietary fatty acids, such as that provided by encapsulation of PUFA in formaldehyde-treated protein may ameliorate this situation. This study evaluated the effects of including in the diet a ruminally protected lipid supplement (PLS), containing linoleic (C18:2) and α -linolenic (C18:3) acids, on the fatty acid composition of the m. longissimus.

Forages are a rich source of beneficial n-3 PUFAs (?-linolenic acid; C18:3n- 3) and may be a useful means of modifying the fatty acid composition of ruminant products. Altering the fatty acid composition of ruminant products is difficult due to the biohydrogenation action of rumen micro-organisms, which hydrogenate the PUFAs into monounsaturated and saturated fatty acids. Little information is available on biohydrogenation of PUFAs in forages hence this study investigated this in steers fed on perennial ryegrass, red or white clover silages or combinations thereof.

The health benefits of n -3 polyunsaturated fatty acids (PUFA) and conjugated linoleic acid (CLA) to humans are now widely recognised. Polyunsaturated fatty acids of the n -3 series such as a-linolenic acid (C18:3n -3), eicosapentaenoic acid (C20:5n -3; EPA) and docosahexaenoic acid (C22:6n -3; DHA) reduce the risk of coronary heart diseases (Department of Health, 1994). CLA has a diverse array of potential beneficial health effects which include anticarcinogenesis, antiatherogenesis, immune system modulation, antidiabetic effects and reduction of body fat accretion (Bessa et al., 2000). However, the consumption of both n -3 PUFA and CLA by humans is currently less than optimal. The current study investigates the extent to which n -3 PUFA and CLA in milk fat of lactating ewes may be enhanced by feeding diets rich in EPA and DHA.

Breeding goals differ for different breeds of dairy cattle. For example the breeding programmes for Holstein-Friesian (HF) animals have been based primarily on improved milk production with little emphasis on functional traits such as fertility. In contrast, Norwegian (NC) dairy cattle have been bred using a multi-trait selection procedure for 25 years. These differences in selection procedures for the two breeds may have major effects on overall herd output and profitability with the forage-based systems of milk production employed in Northern Ireland. The objective of the present paper is to present the effects of two levels of nutrient input on food intake and utilisation, and animal performance of the two breeds, with diets based on grass silage during the first and second lactation.

It has previously been demonstrated that feeding urea treated whole crop wheat to dairy cows results in a significant increase in dry matter intake but has little effect on milk yield (Sutton et al, 1997). Part of the reason behind this lack of response has been attributed to a decrease in digestibility, particularly that of starch. A forage mill has recently been developed which allow the grains to be ground prior to ensiling and potentially increase their digestibility. An alternative way to increase the energy value of whole crop wheat is to increase cutting/stubble height. Work by Weller et al, (1995) demonstrated an increase in calculated ME from 10.6 to 11.2 MJ/kg DM by increasing stubble height from 10cm to 40cm. The objective of the current experiment was therefore to determine the effects of forage processing (grinding) and cutting height at harvest of urea-treated whole crop wheat on the intake, milk production and diet digestibility in dairy cows.

The objective was to estimate the influence of genetic merit on milk production at different levels of concentrate feeding. The presence of an interaction between genetic merit and level of feeding would mean that cows with a high potential for milk production are unable to express their potential under all conditions. Oldham and Simm, (1998) showed there was a significant interaction between level of concentrate feeding and PTAf+p under experimental conditions, and that the milk yield response to genetic merit increased with increased concentrate feeding. The current work estimated the value of genetic merit (PIN95 and PTAf+p) on 305-day milk production traits of dairy cattle on commercial farms, where farms were categorised by the level of concentrate (Cg) fed.

The increased use of Holstein genes in the British dairy herd led to the replacement of Friesian cows with the taller, thinner Holstein type. The objective of this study was to use measurements of cow height (Ht) at withers and average body condition score throughout lactation (ACS), to investigate their relationship with commercial herd performance.

Under Northern Ireland conditions, until recently, it was difficult to achieve high starch maize (greater than 200 g/kg DM) more often than one year in fifteen. However, recent developments in maize breeding and in agronomic practices, particularly the development of degradable full cover plastic mulch, have resulted in the possibility of consistently producing high starch maize. As there is considerable variability in the quality of grass silage produced on Northern Ireland dairy farms, the objective of the present study was to examine the effects of maturity of maize silage at harvest on the performance of lactating dairy cattle offered two contrasting feed value grass silages. The potential concentrate sparing effect of contrasting maize silages was also examined.

Lifespan (LS) evaluations in the United Kingdom (UK) are based on bivariate analyses of LS scores, a direct longevity measure calculated from the number of lactations a cow has completed or is expected to complete, and an index of 4 linear type traits as an indirect predictor (Brotherstone et al, 1998). However, the study did not consider composite type traits or somatic cell count (SCC) and the data was only from pedigree herds. This study examines the relationship between type traits and SCC with LS in pedigree and non-pedigree cows.

In recent years there has been an influx of Holstein genes into the UK dairy herd, largely achieved by a ‘grading up’ process using imported Holstein semen on Friesian cows. The research reported here investigates this process using performance records from UK dairy herds.

Currently, genetic evaluations in the United Kingdom for somatic cell count (SCC) are based on a single trait repeatability animal model (AM). A test day model (TDM), which allows for improved correction of environmental effects and the modelling of the individual lactation curve of each cow (Schaeffer and Dekkers, 1994), would be the preferred method of choice. In an attempt to reduce costs, farmers are opting for cheaper milk recording schemes, such as bimonthly recording. Bimonthly schemes might miss short frequency mastitis infection and it is not clear what effect this might have on evaluations for SCC. The objective of this paper is to report preliminary evaluations for SCC using a TDM and to evaluate the effect of bimonthly recording on such evaluations.