High Genetic Merit Dairy Cows Research Articles

The Autumn Low Milk Yield Syndrome in High Genetic Merit Dairy Cattle: The Possible Role of a Dysregulated Innate Immune Response.

Simple SummaryMilk yield worldwide is dominated by few cosmopolitan dairy cattle breeds producing high production levels in the framework of hygiene standards that have dramatically improved over the years. Yet, there is evidence that such achievements have gone along with substantial animal health and welfare problems for many years, exemplified by reduced life expectancy and high herd replacement rates. Also, these animals are very susceptible to diverse environmental stressors, among which hot summer climate plays a central role in the occurrence of diverse disease cases underlying early cull from the herd. Milk production is also affected by heat stress, both directly and indirectly, as shown by low milk yield in the following autumn period. This article highlights the low milk yield syndrome and sets it into a conceptual framework, based on the crucial role of the innate immune system in the response to non-infectious stressors and in adaptation physiology at large.The analysis of milk yield data shows that high genetic merit dairy cows do not express their full production potential in autumn. Therefore, we focused on metabolic stress and inflammatory response in the dry and peripartum periods as possible causes thereof. It was our understanding that some cows could not cope with the stress imposed by their physiological and productive status by means of adequate adaptation strategies. Accordingly, this study highlights the noxious factors with a potential to affect cows in the above transition period: hot summer climate, adverse genetic traits, poor coping with unfavorable environmental conditions, outright production diseases and consequences thereof. In particular, the detrimental effects in the dry period of overcrowding, photoperiod change and heat stress on mammary gland development and milk production are highlighted in the context of the autumn low milk yield syndrome. The latter could be largely accounted for by a “memory” effect on the innate immune system induced in summer by diverse stressors after dry-off, according to strong circumstantial and indirect experimental evidence. The “memory” effect is based on distinct epigenetic changes of innate immunity genes, as already shown in cases of bovine mastitis. Following a primary stimulation, the innate immune system would be able to achieve a state known as “trained immunity”, a sort of “education” which modifies the response to the same or similar stressors upon a subsequent exposure. In our scenario, the “education” of the innate immune system would induce a major shift in the metabolism of inflammatory cells following their reprogramming. This would entail a higher basal consumption of glucose, in competition with the need for the synthesis of milk. Also, there is strong evidence that the inflammatory response generated in the dry period leads to a notable reduction of dry matter intake after calving, and to a reduced efficiency of oxidative phosphorylation in mitochondria. On the whole, an effective control of the stressors in the dry period is badly needed for better disease control and optimal production levels in dairy cattle.

Effects of dry period length and concentrate protein content in late lactation on body condition score change and subsequent lactation performance of thin high genetic merit dairy cows

Improving body condition score of thin cows in late lactation is necessary, because cows that are thin at drying off exhibit decreased fertility postpartum and are at increased risk of disease and of being culled in the subsequent lactation. Offering a diet low in crude protein (CP) content in late lactation may help to improve body condition score (BCS) at drying off, whereas imposing an extended dry period (EDP) has been advocated as another way to increase BCS at calving. To test these hypotheses, 65 thin cows (mean BCS 2.25 at 14 wk precalving) were managed on 1 of 3 treatments between 13 and 9 wk prepartum: normal protein control {NP; grass silage + 5 kg/d of a normal protein concentrate [228 g of CP/kg of dry matter (DM)]}, low protein [LP; grass silage + 5 kg/d of a low-protein concentrate (153 g of CP/kg of DM)], or EDP (cows dried off at 13 wk precalving and offered a grass silage-only diet). Both NP and LP cows were dried off at wk 8 prepartum, after which all cows were offered a grass silage-only diet until calving. After calving, all cows were offered a common diet (supplying 11.1 kg of concentrate DM/cow per day) for 19 wk. Between 13 and 9 wk prepartum, LP cows had lower DM intake, milk yield, and body weight than NP cows. Whereas EDP cows had lower serum β-hydroxybutyrate and fatty acid concentrations than those of NP cows, BCS at wk 9 prepartum did not differ between treatments. Cows on the LP treatment continued to have lower DMI and BW than those of NP and EDP cows between 8 wk prepartum and calving, but only EDP cows had a higher BCS at calving. Treatment did not affect calving difficulty score or calf birth weight. Although all cows were offered a common diet postpartum, cows on the LP treatment had lower DM intake and milk fat + plus protein yield than cows on any other treatment during the 19-wk period postpartum, but we found no differences in any postpartum indicator of body tissue reserves. The treatments imposed from wk 13 to 9 prepartum had no effect on any fertility or health parameters examined postpartum. Extending the dry period for thin cows improved their BCS at calving but did not allow these cows to achieve the target BCS of 2.75, and we found no beneficial effects of this treatment on cow performance postpartum. Offering a lower-protein diet to thin cows in late lactation did not improve BCS at calving above that of cows on a normal protein diet, but had unexplained long-term negative effects on cow performance.

Use of empirical models that describe the response of lactating dairy cattle to varying ratios of silage to concentrate as described in terms of metabolizable energy intake: the prediction of milk yield and its constituents from a combination of two empirical models

AbstractGrass silage constitutes a major proportion of the food offered to high producing dairy cows in the western regions of the European Union. In order to achieve optimal milk yield and quality from high genetic merit dairy cows, it is common practice to offer concentrates in addition to grass silage at feeding. The aim of the current study was to combine two existing empirical models in order to allow prediction of the response in milk production to offering varying ratios of grass silage and concentrates to high genetic merit dairy cows, allowing for the fact that grass silage quality can vary. Results have demonstrated that it is possible to predict, with reasonable precision, the milk production response to various combinations of grass silage and concentrates in the diet. This will facilitate investigation of production and economic responses of offering increased levels of concentrates in the diet on milk yield, milk quality and milk value.

284THE EFFECT OF FEEDING PROPYLENE GLYCOL TO DAIRY COWS DURING THE EARLY POSTPARTUM PERIOD ON SERUM INSULIN CONCENTRATION AND THE RELATIONSHIP WITH OOCYTE DEVELOPMENTAL COMPETENCE

High yielding dairy cows are typically in negative energy balance postpartum (pp). It has been shown that initiation of the first pp ovulation and, therefore, the resumption of normal oestrous cycles is delayed in high genetic merit dairy cows and is associated with lower circulating insulin concentration (Gutierrez et al., 1999 J. Reprod. Fertil. 24, 32 abst). Evidence shows that propylene glycol (PG) rapidly elevates systemic concentrations of insulin (Bremmer et al., 2000 J. Dairy Sci. 83, 2239–2251). The aim of this study was to determine the effects of PG feeding to dairy cows in the early pp period, on serum insulin and ovarian function, and on oocyte developmental competence after in vitro maturation, fertilization, and culture. Thirteen Holstein-Friesian cows were assigned to PG (n=6) or control (n=7) groups. Each treated cow received 500mL of PG and each control was given 500mL of water daily from Day 5pp until day of AI. Blood samples for insulin were collected at 0, 30, 60 and 90min post-drenching on Days 5, 15 and 25 pp. All cows were fed 3kg concentrates at milking (twice daily) and had ad libitum access to a 50:50 maize silage : grass silage forage from the time of last blood collection. Oocytes were collected by ovum pick-up (OPU) in four sessions (following treatment with pFSH) beginning on Day 25–35 pp. The recovered oocytes were graded (Grade 1 to 4) in terms of their surrounding cumulus cells and the appearance of the cytoplasm. Grade 1–2 oocytes were matured in vitro, then fertilized using frozen-thawed bull semen, and subsequently cultured up to Day 8 in synthetic oviduct fluid. All data were analyzed using SAS version 6.12 and split-plot designs, following square root or arc sine transformation, if appropriate. PG significantly increased (P<0.001) serum insulin concentration (0min: 1.55±0.19; 30min: 4.48±0.82; 60min: 4.74±0.72; 90min: 4.10±0.56) compared to the control group (0min: 1.91±0.28; 30min 1.96±0.27; 60min: 2.37±0.44; 90min: 2.04±0.26). The follicle size distribution was similar between treated and control cows for categories 2–4mm (4.0±0.47; 4.3±0.70), 8–10mm (3.2±0.47; 2.5±0.39), and >10mm (0.42±0.12; 0.67±0.17). However, there were significantly more follicles in the 5–7mm category (6.2±0.82 v. 3.3±0.43; P<0.05) for treated cows. The number of follicles punctured (13.8±1.02; 10.7±1.04), the number of oocytes recovered (4.5±0.53; 3.5±0.61), and the number of Grade 1–2 oocytes (2.8±0.35; 1.8±0.35) were not different between treated and control cows. Although cleavage rate (68.3 v. 58.9%) and blastocyst yield (25.3 v. 14.4%) were higher for treated cows, the differences were not significant. In conclusion, these results indicate that feeding cows with PG during the early pp period increased the circulating insulin concentration. However, the developmental competence of the recovered oocytes did not differ between the groups.

The effect of level and type of supplement offered to grazing dairy cows on herbage intake, animal performance and rumen fermentation characteristics

AbstractA study was undertaken to examine the effect of level and type of supplement and changes in the chemical composition of herbage through the grazing season on herbage intake, animal performance and rumen characteristics with high-yielding dairy cows. Thirty-two high genetic merit dairy cows were allocated to one of four treatments in a two-phase change-over design experiment. The four treatments involved offering either high-fibre (HF) or high-starch (HS) supplements at either 5 or 10 kg dry matter (DM) per day. Animals rotationally grazed perennial ryegrass swards offered a daily herbage allowance of 23 kg DM. High levels of animal performance were maintained throughout the study with mean milk yields over the 21-week experimental period of 35·5 and 37·2 kg per cow per day on the 5- and 10-kg treatments respectively. Supplement type had no significant effect on herbage intake. Increasing the level of supplementation reduced herbage intake by 0·49 and 0·46 kg herbage DM per kg supplement DM and increased milk yield, with a milk yield response of 0·55 and 0·65 kg/kg supplement DM offered in phases 1 and 2. Supplement type had no significant effect on milk yield and milk butterfat content. However, in both phases 1 and 2, milk protein content was significantly higher with cows offered the HS supplement compared with the HF supplement, this being particularly evident at the higher level of supplementation. Supplement type also had marked effects on milk protein composition. These results indicate that high levels of performance can be achieved with dairy cows on grazed pasture with moderate levels of supplementation.

Effect of concentrate supplementation level on production, health and efficiency in an organic dairy herd

The proportions of organic feed and roughage in the feed ration for organic dairy cows have to reach 100 and 60%, respectively, in 2005. The aim of this study was to elucidate the long-term effects of reducing or omitting concentrate supplementation to high genetic merit dairy cows on a basal ad libitum diet of clover-grass (silage and grazing). Three concentrate levels, N, L and L+ (38, 0 and 19% of dry matter (DM) intake, respectively) were investigated in a herd of 60 cows during 3 years. The production in group N was 6723 kg energy corrected milk (ECM) per cow year, based on an intake of 6226 kg DM of which 38% was concentrates. In group L the omission of concentrates reduced intake to 4770 kg DM, and milk production to 5090 kg ECM per cow year. Milk protein content was reduced and milk free fatty acid content was increased, and the first calving interval was significantly increased, as compared to group N. The intake in group L+ was 5226 kg DM per cow year of which 19% was concentrates. Milk production in group L+ was reduced by only 493 kg ECM per cow year as compared to group N, primarily due to a significantly improved feed conversion ratio (12%). There were no indications of health problems associated with the reduced feeding levels.

Effects of four contrasting grassland-based milk production systems on dairy cow fertility

High genetic merit dairy cows were managed on four contrasting grassland based systems of milk production over a three-year period (Ferris et al., 2000). Each of the four systems were designed to incorporate a range of different approaches to increasing nutrient intakes. Key performance data from this study are presented in Table 1. The aim of this paper is to examine the effects of the four milk production systems on dairy cow fertility.

An in vitro model to assess the impact of lipid on the rate and extent of fibre degradation

Although high genetic merit dairy cows are capable of peak yields in excess of 50 kg per day these are seldom maintained primarily due to an inability to satisfy energy requirements. Fats and oils are often incorporated into rations as a means of increasing dietary energy content. However, unless included at relatively low levels or in a protected form, lipid supplements may adversely affect fibre degradation in the rumen. The current data are derived from a study conducted to evaluate the potential of an in vitro model to assess the effects of oil supplements on forage organic matter degradation (iOMD).

The effects of two contrasting grassland-based milk production systems, on the performance and fertility of high genetic merit, autumn calving, dairy cows

AbstractThe development of production systems, which allow increased nutrient intakes to be achieved, is a key issue in the management of high genetic merit dairy cows. Consequently, forty high genetic merit autumn calving dairy cows (PTA95fat + protein = 38.2 kg) were managed on either a ‘high forage (HF)’ or ‘high concentrate (HC)’ based system of milk production for the first 305 days of lactation, with the study encompassing both the indoor winter and outdoor summer grazing periods. System HF involved a high feed value silage, a lax grazing regime, and a low concentrate input (842 kg DM), while system HC involved a medium feed value silage, a tighter grazing regime and a higher concentrate input (2456 kg DM). Total milk outputs with each of systems HF and HC were 7854 and 8640 kg respectively (P<0.01), illustrating that high genetic merit cows can perform satisfactorily on very different inputs over a single lactation. However animals on system HF experienced a more extreme and prolonged period of negative energy balance post partum than those on system HC, and completed the winter with a significantly lower condition score. Detailed fertility records were maintained for all animals on the study. Days to first observed heat were 51.2 and 59.3 with systems HF and HC respectively, while the respective conception rates to first service were 26 and 21%. The number of services/conception were 2.22 and 2.50, while the calving interval was 390 and 404 days for systems HF and HC respectively. Despite the greater degree of negative energy balance associated with system HF, none of the fertility measures was significantly affected by system of milk production (P>0.05), although fertility with both systems was poor. There were no obvious reasons for the poor fertility noted in this trial.

Reproductive function in average and high yielding dairy cows

AbstractThe resumption of reproductive activity in the early post partum period and the subsequent fertility of dairy cows are of considerable practical and economic importance to the producer. Regular milk progesterone analysis was used in this study to monitor the re-establishment of reproductive cyclicity in animals of average (AGM, n=20) and high genetic merit (HGM, n=28) kept under standard management conditions. The profiles obtained were then characterised into normal and four different types of abnormal activity. Resumption of cycles occurred significantly later in the higher yielding cows than in those of average genetic merit (23 ±2.3 days in HGM cows and 15 ± 1.6 days in AGM cows; P=0.0089). There was a significant increase in the amount of abnormal cycles encountered in the high genetic merit animals compared with average yielding animals (61% and 30% respectively; χ2 test, P=0.045); in particular, the delayed occurrence of ovulation post partum (11% vs 0%) and an increased incidence of persistent corpora lutea (32% vs 20%). For high genetic merit animals, the incidence of abnormal cycles was associated with increased peak milkyields (abnormal cycles: peak milk yield 54.7 ± 1.11 kg/day; normal cycles: peak milk yield 46.4 ± 1.35 kg/day, P=0.002). This relationship was not apparent in average genetic merit animals. First service conception rates were 50% (9/18) for AGM cows and 38% (10/26) for HGM cows (χ2 test, n.s.). The calving to conception interval was 98 ± 9.7 days for AGM animals (n=l7) and 105 ± 9.1 days for HGMs (n=24) (t-test, n.s.). In conclusion, this study confirms that high genetic merit dairy cows have compromised reproductive function in terms of a delayed resumption of cyclicity and an increased incidence of abnormal progesterone cycles, which was associated with higher peak milk yields. There was also a reduced first service conception rate in high yielders although the difference was not significant with this group size.

High genetic merit and high-producing dairy cows in commercial Australian herds don't have substantially worse reproductive performance

AbstractThe National Dairy Herd Fertility Project (renamed the InCalf Project) consists of two large, prospective observational studies conducted in commercial dairy herds from 4 Australian states. The project aims to identify factors associated with variation in reproductive performance between cows and herds. The larger of the two studies included over 33,000 cows and 168 herds. The database has undergone extensive error checking and correction resulting in high quality data. Large variations in reproductive performance were observed between cows and herds, indicating that there are important risk factors for reproductive performance and that cows and herds are exposed to varying combinations of these risk factors. Models for several reproductive outcomes were constructed using multivariable stepwise logistic regression. No significant associations were detected between any measures of cow genetic merit and 6 week in-calf rate. Neither milk volume nor protein yield were significantly associated with 6 week in-calf rate. A weak negative association was detected between fat yield in the first 120 days of lactation and 6 week in-calf rate. However, differences in estimated 6-week in-calf rate between moderate and high producing groups (2 to 5 percentage points) were small when contrasted against the 63 percentage point range in performance observed between herds (23% to 86%).

Effect of genetic merit, milk yield, body condition and lactation number on in vitro oocyte development in dairy cows

The effects of milk yield, body condition score (BCS) and lactation number on the number of oocytes recovered and blastocysts formed were studied following in vitro maturation, fertilization and culture of bovine oocytes collected from 48 high and 46 medium genetic merit dairy cows in their first and third lactation. The cows were slaughtered between 125 and 229 d post partum. Ovaries were recovered, and 2- to 10-mm follicles were aspirated. Cleavage rate and number of blastocysts were determined at 44 h and 7 d after insemination, respectively. Oocytes from high genetic merit cows formed fewer blastocysts and had lower cleavage and blastocyst formation rates than those from medium genetic merit cows (0.36 ± 0.19, 70.4 and 6.8% vs 0.85 ± 0.22, 77.4 and 11.4%, respectively). The effect of milk production was tested by grouping cows in their third lactation into high and low groups. There was no difference in number of oocytes recovered and subsequent development into blastocysts between the cows in the high milk production group (4559 to 5114 kg, n = 20) and cows in the low yield (3162 to 3972 kg, n = 20) group (6.9 ± 1.34 vs 8.9 ± 1.32, respectively). The effect of BCS was tested by grouping cows in their first or third lactation into high and low groups. Cleavage and blastocyst formation rates were greater for oocytes from cows with a high BCS (3.3 to 4.0, n = 20) than a low BCS (1.5 to 2.5, n = 20) (75.7 vs 61.9% and 9.9 vs 3.0%, respectively). Cows in the first lactation yielded fewer oocytes (5.7 ± 1.24) than cows in the third lactation (7.8 ± 0.79). Thus, the quality of oocytes probably contributes to reduced fertility, often evident in high genetic merit dairy cows.

A comparison of four contrasting milk production systems for high genetic merit winter calving dairy cows in a grassland based production environment

Meeting the greater nutrient requirements of high genetic merit dairy cows in grassland based systems provides a very real challenge. This study examines the performance of high genetic merit animals, managed on four contrasting grassland based systems of milk production, including both the winter and summer periods.

Gene expression in different tissues of lactating cows of differing metabolic type: 1. Comparison of mRNA patterns by the differential display method

Among other factors the nutrient utilization of a cow is influenced by its individual physiological and genetic predisposition. There are large differences within cows in the manner of partition of the nutrients that may affect the size and activity of the individual organs and the whole body composition. High genetic merit dairy cows have a higher potential rate of protein excretion than medium genetic merit dairy cows even when energy and protein intake are equal. Medium genetic merit dairy cows show higher potential rates of protein and fat deposition of the body. This phenomenon was partly explained by the regulatory functions of the hormone insulin (see Brockman and Laarveld 1986; Broucek et al. 1991; Wylie et al. 1998). Additionally, it is widely considered that the somatotropic axis [growth hormone, growth hormone releasing factor and insulin‐like growth factor I (IGF‐I)], could be involved, as one of the key control and balance mechanisms which regulate the partition of nutrients (Breier and Sauerwein 1995; Schams 1995; Knight 1997). Several studies have provided insights into the physiological action of these and further hormones, transport proteins and enzymes, e.g. the erythrocyte‐type glucose transporter, the insulin‐responsive glucose transporter or the glycerol‐3‐phosphate dehydrogenase, and indicate that these molecules may affect nutrient repartitioning and utilization as well as milk yield, growth and fat accumulation (Mathews et al. 1988; Bauman and Vernon 1993; Burton et al. 1994; Girard et al. 1994; Binelli et al. 1995; Zhao et al. 1996; Abe et al. 1997). However, the mechanisms of regulation of most of these processes are insufficiently known.This paper presents a comparison of mRNA patterns of tissues potentially associated with protein and energy turnover in lactating cows of different metabolic types. The technique of differential display of messenger RNA species originally described by Liang and Pardee (1992), has already been applied as a powerful tool for cloning genes that are differentially expressed in various tissues or under altered conditions in the same tissue (e.g. Liang et al. 1993; Aiello et al. 1994; Li et al. 1994; Nishio et al. 1994). This method was applied to characterize the expression patterns in 12 tissues of three cows with divergent metabolic types (milk type, meat/milk type, and meat type) by the display of differentially and identically expressed cDNA bands. The aims of this exploratory study were first to indicate which of the 12 tissues analysed are mainly involved in trait differentiation according to their differentially displayed mRNA patterns. Secondly, the results of this study represent the prerequisite for the identification of tissue‐specific expressed sequence tags in general, and individual or type‐dependent differentially expressed sequence tags in particular.

The production response by dairy cows to four levels of straw inclusion in grass silage based diets

High concentrate feed levels, with a consequent fall in the fibre content of the diet, are increasingly being offered to high genetic merit dairy cows. To improve fibre intake, small quantities of straw are often incorporated into the diet. This study was undertaken to examine the production response of dairy cows, offered a medium quality grass silage based diet together with relatively high levels of concentrate supplementation, to straw inclusion in the diet.Forty high genetic merit dairy cows, 20 multiparious and 20 first lactation animals (PTA95 fat + protein = 41.4 and 39.7 kg respectively) were used in this study. The study consisted of four treatments, arranged in a two period (4 week duration), partially balanced change-over design trial, with first lactation and multiparious animals being blocked separately. The experimental treatments consisted of grass silage mixed with one of four levels of straw, and offered ad libitum in the form of a mixed diet.

A comparison of two contrasting milk production systems for high genetic merit autumn calving dairy cows in a grassland based production environment

In a previous short term study, Ferris et al. (1997) demonstrated that similar levels of nutrient intake and animal performance could be obtained by either increasing silage feed value and reducing concentrate feed level, or by reducing silage feed value and increasing concentrate feed level. The principles established in this study were incorporated into this trial to examine two systems of milk production over a full lactation, including both the winter and grazing periods.Forty high genetic merit dairy cows (PTA95 fat + protein = 38.2 kg), in their second or subsequent lactation, were used in a continuous design full lactation study. Animals had a mean calving date of 1 November and were allocated to one of two systems of milk production, HF or HC, within 36 hours of calving. During the winter, animals on system HF were offered a silage with high feed value characteristics, supplemented with 5.5 kg of concentrate (crude protein concentration of 280 g/kg DM) through an out-of-parlour feeding system.

The production response by dairy cows to the inclusion of small quantities of straw in diets based on high quality grass silage and zero grazed grass

To satisfy the increased nutrient requirements of high genetic merit dairy cows, diets of increased nutrient density and intake potential are required. This can be achieved through increasing the feed value of the forage component of the diet (grass silage or grazed grass) and by increasing concentrate feed levels. However both approaches tend to reduce the fibre concentration of the diet, and the physical effectiveness of the fibre in terms of its ability to stimulate ruminal processes. The incorporation of small quantities of straw into the diet has been suggested as a means of overcoming this problem and hence improving animal performance. This study was undertaken to examine the production response by dairy cows to straw inclusion in high quality grass silage and zero grazed grass diets.

Physiological characteristics of high genetic merit and low genetic merit dairy cows: a comparison

Genetic selection has greatly improved individual cow productivity. A high genetic merit Holstein Friesian cow will produce 10,000 of litres of milk in a 305 day lactation, those of lower genetic merit half this amount. Despite major research effort in generating these differences, quantitative biological description of what has been achieved is lacking. The aim of this study was to compare biological variables of relevance to milk synthesis in well defined high genetic merit and lower genetic merit dairy cows.12 cows of high genetic merit (HGM: top 5% of UK national herd) and 12 of lower genetic merit (LGM: close to UK average) were purchased from the Blythbank and Langhill herds; the ontogeny of these selection lines has been described elsewhere (Woolliams et al, 1993). Mean parity was 3.7±0.3 and was balanced between HGM and LGM. Management of the two groups was identical. Measurements were made over a period of two weeks close to peak lactation. Milk yield was recorded at each milking. Plasma samples were collected and analysed for a variety of hormones by radioimmunoassay and metabolites by established spectrophotometric assays. Udder size was determined by a casting technique (Dewhurst et al., 1993) and mammary biopsies for key enzyme activities were obtained as described by Knight et. al. (1992). Differences between lines were analysed by analysis of variance (ANOVA).

Physiological characteristics of high genetic merit and low genetic merit dairy cows: a comparison

Genetic selection has greatly improved individual cow productivity. A high genetic merit Holstein Friesian cow will produce 10,000 of litres of milk in a 305 day lactation, those of lower genetic merit half this amount. Despite major research effort in generating these differences, quantitative biological description of what has been achieved is lacking. The aim of this study was to compare biological variables of relevance to milk synthesis in well defined high genetic merit and lower genetic merit dairy cows.12 cows of high genetic merit (HGM: top 5% of UK national herd) and 12 of lower genetic merit (LGM: close to UK average) were purchased from the Blythbank and Langhill herds; the ontogeny of these selection lines has been described elsewhere (Woolliams et al, 1993). Mean parity was 3.7±0.3 and was balanced between HGM and LGM. Management of the two groups was identical. Measurements were made over a period of two weeks close to peak lactation. Milk yield was recorded at each milking. Plasma samples were collected and analysed for a variety of hormones by radioimmunoassay and metabolites by established spectrophotometric assays. Udder size was determined by a casting technique (Dewhurst et al., 1993) and mammary biopsies for key enzyme activities were obtained as described by Knight et. al. (1992). Differences between lines were analysed by analysis of variance (ANOVA).

The responses of high genetic merit dairy cows to changes in silage quality and level of supplementary concentrate

Supporting the higher milk yields of high genetic merit cows requires an increased supply of nutrients to the animal. In grass silage based production systems this is achieved by either offering additional concentrates or by improving the quality of the silage. However, the relationship between silage quality and the response to concentrate supplementation has not been defined for high merit animals. It might be expected that as level of concentrate supplementation increases, the quality of the silage offered will become less important. This study was undertaken to define the responses of high merit dairy cows to a range of concentrate levels given in addition to two contrasting silage types.