An adverse reaction in pigs to the anaesthetic halothane at approximately 8 weeks of age appears to be controlled by a single recessive gene affecting stress susceptibility, lean content and reproductive performance. In a survey of British nucleus herds, a total of 2998 pigs between 5 weeks of age and 28 kg live weight were given 3-min halothane tests during visits to 17 breeders or companies. There were no positive halothane reactions among 764 Large Whites from 7 herds. The frequency of positive reaction among 1538 Landrace from 14 herds averaged 11%, with a range from 0 to 23 % in individual herds. Frequencies in other pure breeds or synthetic lines varied from 0 to 88%. British and foreign breeds are compared, and the possibility of eliminating the halothane gene is discussed.

Calculations on the relative efficiencies of cattle, sheep and pigs as meat producers indicate that the main factor limiting the energetic efficiency of sheep meat production is the low average fecundity of the ewe (Blaxter, 1968). Nevertheless with an estimated ‘biological ceiling’ of five lambs per ewe per pregnancy, and a potential mean lambing interval of 6 months (Wilson, 1968), the sheep industry is much further from achieving its biological ceiling than any other form of domestic livestock production. Because these estimates are based on the performance of specific individuals one must question their attainment on a flock basis. Furthermore, even if the necessary technical information were available, its adoption on a commercial scale may be prohibitive on economic grounds. These uncertainties should not deter us from seeking a greater understanding of the basic factors controlling ewe productivity, nor from attempting to integrate this knowledge into existing production systems.

The effect of undegradable protein supply was studied in overfat lambs given low-energy diets to reduce carcass fatness. Two trials involving sequential batches of 48 overfat Suffolk × lambs investigated the effects of supplementing an ad libitum straw diet with either 0 or 100 g fish-meal pellets daily for 14-, 28- and 42-day periods. Lambs supplemented with fish meal in trials 1 and 2 lost significantly less weight. Weight losses (g/day) were - 2 v. -129 (s.e.d. 26) and -5 8 v. -188 (s.e.d. 29) respectively. There were highly significant and favourable effects of fish-meal supplementation on carcass weight and composition (recorded in the second trial only). Carcass weight (kg) after 14, 28 and 42 days was 21·8, 20·4 and 18·3 for unsupplemented lambs and 22·4, 21·9 and 21·6 for supplemented lambs; saleable lean (kg) as determined by a commercial boning-out process was 12·1, 11·3 and 10·0 v. 12·7, 12·6 and 12·3 respectively. There were corresponding changes in conformation but fat trimmed off the carcass was not significantly reduced by the dietary treatments. There was, however, a reduction by one-fifth in excess carcass fat (about 0·5 kg) over the period 14 to 42 days on trial, and over the trial as a whole, fat trim fell from an estimated proportion of carcass weight of 0·17 to 012, equivalent to a fall in Meat and Livestock Commission fatness score from 4H to 3L. Results indicated very favourable financial rewards for supplementing diets of overfat lambs with fish meal.

1. All-concentrate diets given ad libitum and containing different levels of fish meal, and varying in crude protein content from 16 to 22% in dry matter were given to eight Friesian steers at 150 kg. and at 240 kg. live-weight.

2. At a mean live-weight of 150 kg., the retention of nitrogen increased significantly to a maximum of 41.5 g. per day on the diet having 22% crude protein in dry matter. At 240 kg. live-weight, the maximum retention of 37 g. per day was achieved at 20% crude protein in dry matter.

3. Faecal nitrogen excretion per unit of metabolic body weight was approximately constant at all protein levels and at both live-weights.

4. Urinary nitrogen excretion per unit of metabolic body weight increased with increasing crude protein level in the diet and for any crude protein level was higher at 240 kg. than at 150 kg. live-weight.

A dramatic fall in the colostral whey proteins of nine sows occurred within 24 hr following the birth of the first piglet. The mean levels fell from 19·6 g/100 ml to 4·1 g/100 ml, 50% of this fall occurring within 4 to 6 hr. It is suggested that as early-born piglets will be able to obtain colostrum of a much higher protein concentration this may give them some physiological and immunological advantages over later-born littermates.

Predicted rate of genetic response for multiple trait breeding objectives from multivariate analyses is compared with that from univariate analyses for a range of breeding schemes, A selection index that approximates a multipl trait best linear unbiased prediction (BLUP) animal model is utilized. Changes in genetic parameters due to linkage disequilibrium generated by selection are accounted for. This study shows that asymptotic response in the aggregate breeding value, and its component traits, to selection on multivariate predictions, can easily be calculated from first generation responses, which use ancestral information. For two-path breeding schemes with equal accuracy for males and females, proportional reductions in responses depend only on selection intensity. In general, benefit from multivariate over univariate analyses is slightly smaller at the asymptote than in the first generation. Multivariate analyses give substantially more response than univariate analyses on untransformed data, when correlations between traits are high and when genetic and phenotypic correlations have opposite signs. The advantage of multivariate analyses decreases with increasing family size. When three random factors (additive genetic, common and individual environmental effects) are included in the model, the benefit of multivariate analysis is negligible if univariate analyses are performed on canonical traits. In this case, the advantage of multivariate analyses is never higher than 1% for all cases studied. These results are of particular relevance to the design of genetic evaluation programmes.

The study was designed to provide quantifiable information on both within- and between-herd variation in pig growth rate from birth to slaughter and to examine how this was influenced by moving pigs at a common age to a common environment. Five litters were selected from each of eight pig herds in Northern Ireland with varying growth performance. All eight herds were offered the same nutritional regime. Five pigs (three boars and two gilts) were selected from each litter. In each herd, 22 pigs (12 boars and 10 gilts) were weighed individually, every 4 weeks, from 4 to 20 weeks of age. At 4 weeks of age (weaning) three non-sibling boars were taken from each herd and brought to a common environment where they received medication, were housed individually from 6 weeks of age and offered the same dietary regime. They were weighed and feed intakes were recorded twice weekly. A growth rate difference of 61 g/day (P < 0.001), 112 g/day (P < 0.01) and 170 g/day (P < 0.001) was observed on farm, between the top and bottom quartile of herds during 4 to 8, 8 to 12 and 12 to 20 weeks of age, respectively. This difference in growth rate equated to an average difference in cost of production of ¢13/kg carcass on a birth to bacon unit. When pigs from the different herds were housed in the common environment, large variation in growth performance (143 g/day (P < 0.01) and 243 g/day (P < 0.001) for 8 to 12 and 12 to 20 weeks, respectively) was also observed between the top and bottom quartile of herds. Although feed efficiency was similar, a significant feed intake difference of 329 g/day (P < 0.01) and 655 g/day (P < 0.001) between 8 to 12 and 12 to 20 weeks of age was observed. The variation in growth rate between pigs whether managed on farm or in the common environment was similar (variation in days to 100 kg on farm and in the common environment was 18 and 19 days, respectively). When housed in the common environment, although the top and bottom quartile of pigs converted feed equally efficiently, pigs in the top quartile had significantly higher feed intakes suggesting greater appetites. It is difficult to assess the extent to which these differences can be attributed to genetic effects or pre-weaning environment, and how much the effects of management, disease or genetics contributed to the variation between and within herds.

As vitamin B12 is only synthesized by bacteria, ruminant products, especially dairy products, are excellent sources of this vitamin. This study aims to identify if diet and cow characteristics could affect vitamin B12 concentration in milk of dairy cows. Information on 1484 first, 1093 second and 1763 third and greater parity Holstein cows in 100 herds was collected during three consecutive milkings. During the first morning milking, all dietary ingredients given to cows were sampled and quantities offered were recorded throughout the day. Nutrient composition of ingredients was obtained by wet chemistry to reconstitute nutrient composition of the ration. Milk samples were taken with in-line milk meters during the evening milking of the 1st day and the morning milking of the 2nd day and were analyzed for vitamin B12 concentration. Milk yields were recorded and milk components were separately analyzed for each milking. Daily vitamin B12 concentration in milk was obtained using morning and evening vitamin B12 concentrations weighted with respective milk yield, then divided by daily yield. To decrease the number of interdependent variables to include in the multivariable model, a principal component analysis was carried out. Daily milk concentration of vitamin B12 averaged 3809±80 pg/ml, 4178±79 pg/ml and 4399±77 pg/ml for first, second and third, and greater lactation cows. Out of 11 principal components, six were significantly related to daily milk concentration of vitamin B12 when entered in the multivariable model. Results suggested that vitamin B12 concentration in milk was positively related to percentage of fiber and negatively related to starch as well as energy of the diet. Negative relationships were noted between vitamin B12 concentration in milk and milk yield as well as milk lactose concentration and positive relationships were observed between vitamin B12 concentration in milk and milk fat as well as protein concentrations. The percentages of chopped mixed silage and commercial energy supplement in the diet as well as cow BW were positively related to vitamin B12 in milk and percentages of baled mixed silage, corn and commercial protein supplement in the ration were negatively related to vitamin B12 concentration in milk. The pseudo-R2 of the model was low (52%) suggesting that diet and cow characteristics have moderate impact on vitamin B12 concentration in milk. Moreover, when entering solely the principal component related to milk production in the model, the pseudo-R2 was 46%. In conclusion, it suggests that studied diet characteristics have a marginal impact on vitamin B12 concentration in milk variation.

A comparison was made of sow lying behaviour, piglet aggregation behaviour and performance in crates (no. = 10) and oval pens (no. = 8). Twenty-four hour time-lapse video tapes were made and a farrowing day defined for each sow by noting the 24-h period during which the sow gave birth (09:00 to 09:00 h). Each sow and litter, balanced for parity and time of year, was analysed from 12:00 to 20:00 h during the 24 h immediately following this day. The following analyses were conducted: (1) the number and type of lying behaviour; (2) each litter was scanned every 10 min and at each lying event the number of piglets within 0·3 m of the sow noted; two indices were then calculated, based on the mean of the 10-min scans and the mean for the lying events, for each sow expressed as a proportion of the total litter size. Any dead piglets were removed and cause of mortality established by post-mortem examination. Production data showed that there was no significant difference between litter size at birth and at weaning but overall level of mortality was higher in the pen compared with the crate due to crushing. The majority of crushing events occurred in the first 3 days after farrowing (crate 75%; oval pen 64%). The total number of lying events and related posture changes did not differ between systems; only ‘roll-over’ events (movement from lateral on one side to the other within 10 s) were higher in the oval pen. There was no difference in the proportion of aggregating piglets at the 10-min scans or the lying events. Increased crushing mortality in the pen does not appear to be due to the aggregation behaviour of piglets but to the increased number of sow roll-over behaviours.

The effects of in ovo feeding (IOF) of creatine pyruvate (CrPyr) on the hatchability, growth performance and energy status of embryos and broilers (Arbor Acres) were investigated. Five treatments were arranged as non-injected treatment (Control), 0.6 ml physiological saline (0.75%) injected treatment (Saline), and IOF treatments injected with 0.6 ml physiological saline (0.75%) containing 3, 6 or 12 mg CrPyr (CrPyr3, CrPyr6 or CrPyr12) into the amnion per fertile egg on day 17.5 of incubation. After hatching, 80 male chicks from each treatment with similar weight close to the average BW of their pooled group were selected and randomly assigned into eight replicates of 10 chicks each. The results showed that the hatchability was not affected among groups, whereas the hatching weight of broilers in CrPyr12 was significantly higher than the control and saline groups (P<0.05). At 21 day post-hatch, the BWs of broilers in CrPyr6 and CrPyr12 were increased relative to the control and saline groups (P<0.05). Chickens in CrPyr6 and CrPyr12 exhibited higher BW gain and feed intake than the control and saline groups during 8 to 21 days post-hatch and the entire experiment period (P<0.05). Compared with the control and saline groups, the total and relative weight of pectoral muscle of embryos or chickens were greater in CrPyr6 and CrPyr12 at 19th day of incubation (19 E), hatch, 3 and 21 days post-hatch (P<0.05). The concentrations of glucose and glycogen in liver were increased in CrPyr6 and CrPyr12 at 19 E and hatch (P<0.05). Neither glycogen nor glucose concentration in pectoral muscle was altered among treatments (P>0.05). Irrespective of dosage, the concentrations of creatine and phosphocreatine, and activities of creatine kinase in embryos were enhanced in CrPyr treatments at 19 E when compared with the control and saline groups (P<0.05). The activities of glucose-6-phosphatase in liver in CrPyr6 and CrPyr12 treatments were higher than the control and saline groups at 19 E (P<0.05). In conclusion, these results indicated that IOF of CrPyr, especially at the level of 12 mg/egg, could improve energy status of embryos and hatchlings, which was useful for enhancing hatching weight, BW and pectoral muscle weight until the end of the experiments at 21 days post-hatch in broilers.

A meta-analysis on the effects of management and animal-based factors on the performance and feed efficiency of growing pigs can provide information on single factor and interaction effects absent in individual studies. This study analysed the effects of such factors on average daily gain (ADG), feed intake (FI) and feed conversion ratio (FCR) of grower and finisher pigs. The multivariate models identified significant effects of: (1) bedding (P<0.01), stage of growth (P<0.001) and the interaction bedding×lysine (P<0.001) on ADG. ADG was higher on straw compared with no bedding (710 v. 605 g/day). (2) FI was significantly affected by stage of growth (P<0.01), bedding (P<0.01), group composition (P<0.05), group size (P<0.01), feed CP content (P<0.01), ambient temperature (P<0.01) and the interaction between floor space and feed energy content (P<0.001). Pigs housed on straw had a lower FI in comparison with those without (1.44 v. 2.04 kg/day); a higher FI was seen for pigs separated by gender in comparison with mixed groups (2.05 v. 1.65 kg/day); FI had a negative linear relationship with group size, the CP content of the feed and ambient temperature. (3) Stage of growth (P<0.001), feed CP (P<0.001) and lysine content (P<0.001), ambient temperature (P<0.001) and feed crude fibre (CF) content (P<0.01) significantly affected FCR; there were no significant interactions between any factors on this trait. There was an improvement in FCR at higher ambient temperatures, increased feed CP and lysine content, but a deterioration of FCR at higher CF contents. For ADG, the interaction of bedding×lysine was caused by pigs housed without bedding (straw) having higher ADG when on a feed lower in lysine, whereas those with bedding had a higher ADG when on a feed higher in lysine. Interaction effects on FI were caused by animals with the least amount of floor space having a higher FI when given a feed with a low metabolisable energy (ME) content, in contrast to all other pigs, which showed a higher FI with increased ME content. The meta-analysis confirmed the significant effect of several well-known factors on the performance and efficiency of grower and finisher pigs, the effects of some less established ones and, importantly, the interactions between such factors.

The whole-animal model described in this paper is intended to be a research model with an intermediary structure between sophisticated and simple cow models. The mechanistic model structure integrates the main metabolic pathways of the lactating dairy cow. Milk yield and related feed intake for varying production potentials were considered to be the driving forces and were empirically defined. The model was designed to explain the main metabolic flows and variations in body reserves associated with the push of nutrients from dry matter intake and their pull by the mammary gland to synthesise milk components throughout lactation. The digestive part of the model uses either known feed unit systems (e.g. PDI system for protein) or published empirical equations for the prediction of digestive flows of organic matter, starch and fatty acids and ruminal volatile fatty acid production. The metabolic sub-system is made up of four tissue compartments (body protein, body lipid, protein in the uterus and triglycerides in the liver) and five circulating metabolites (glucose, amino acids, acetate + butyrate, propionate, and triglycerides and non-esterified fatty acids). A major original aspect of the model is its homeorhetic control system. It was assumed that flows relating to adipose and protein tissue anabolism and catabolism were driven by a couple of linked theoretical hormones controlling mobilisation and anabolism. The evolution in body composition and body weight were outcomes of this control. Another originality of the model is its suitability for milk yields varying from 10 to 50 kg at peak production. This was achieved by homeorhetic control of milk potential acting on several key metabolic flows. Homeostatic regulation was also introduced in order to confine the behaviour of the model within realistic physiological values. For some basic aspects (e.g. Uterine involution, standard kinetics of body protein and fat), it was necessary to build specific databases from the literature and interpret these by meta-analysis. Fairly realistic simulated kinetics were obtained for body composition, liver triglycerides, blood plasma metabolite concentrations, milk protein and fat contents, and also for major groups of milk fatty acids.

Tail lesions caused by tail biting are a widespread welfare issue in pig husbandry. Determining their prevalence currently involves labour intensive, subjective scoring methods. Increased societal interest in tail lesions requires fast, reliable and cheap systems for assessing tail status. In the present study, we aimed to test the reliability of neural networks for assessing tail pictures from carcasses against trained human observers. Three trained observers scored tail lesions from automatically recorded pictures of 13 124 pigs. Nearly all pigs had been tail docked. Tail lesions were classified using a 4-point score (0=no lesion, to 3=severe lesion). In addition, total tail loss was recorded. Agreement between observers was tested prior and during the assessment in a total of seven inter-observer tests with 80 pictures each. We calculated agreement between observer pairs as exact agreement (%) and prevalence-adjusted bias-adjusted κ (PABAK; value 1=optimal agreement). Out of the 13 124 scored pictures, we used 80% for training and 20% for validating our neural networks. As the position of the tail in the pictures varied (high, low, left, right), we first trained a part detection network to find the tail in the picture and select a rectangular part of the picture which includes the tail. We then trained a classification network to categorise tail lesion severity using pictures scored by human observers whereby the classification network only analysed the selected picture parts. Median exact agreement between the three observers was 80% for tail lesions and 94% for tail loss. Median PABAK for tail lesions and loss were 0.75 and 0.87, respectively. The agreement between classification by the neural network and human observers was 74% for tail lesions and 95% for tail loss. In other words, the agreement between the networks and human observers were very similar to the agreement between human observers. The main reason for disagreement between observers and thereby higher variation in network training material were picture quality issues. Therefore, we expect even better results for neural network application to tail lesions if training is based on high quality pictures. Very reliable and repeatable tail lesion assessment from pictures would allow automated tail classification of all pigs slaughtered, which is something that some animal welfare labels would like to do.

A total of 32 lactating Holstein cows with mean body weight of 622 kg (s.e. = 24) were allotted, at week 25 of lactation, to eight groups of four cows blocked for similar days in milk. The objective of the experiment was to determine the effect of feeding four dietary concentrations (0, 50, 100 or 150 g/kg of dry matter) of whole flaxseed, which contains the plant lignan precursor secoisolariciresinol diglucoside (SDG), on concentrations of two mammalian lignans (enterodiol and enterolactone) in milk. The effects of the four diets on feed intake, milk production, milk composition and digestion were also studied. Cows within each block were assigned to one of the four isonitrogenous and isoenergetic total mixed diets and the experiment was carried out from week 25 to 29 of lactation. Diets were fed for ad libitum intake. Enterolactone was the mammalian lignan, of the two metabolites studied, detected in the milk of cows and its concentration in milk tended (P = 0.08) to increase linearly with higher intake of SDG in the diet. Feed intake, milk yield and milk composition were similar among diets. Milk fatty acid profile was slightly improved by feeding flaxseed, as shown by higher concentrations of fatty acids (e.g. n-3) recognized as being beneficial for human health. Those results suggest that feeding of whole flaxseed may result in changes in milk fatty acid composition and enterolactone content, which offer benefits for consumers.

An experiment was conducted with 60 male Friesian calves between 5 and 11 weeks of age. The experiment was a 5×2×2 factorial design. The five major treatments were the inclusion of 20 or 40% ground wheat straw or 20, 40 or 60% ground lucerne in a pelleted diet. A methane inhibitor was included in the diet given to 30 of the calves. The diets were given alone or with a chaffed wheat straw supplement ad libitum. All diets contained 2% sodium bicarbonate.

The weight gain of the calves was greater with the diets containing ground lucerne than with those containing ground wheat straw, but in both cases weight gains were reduced with increasing roughage content of the diets. The efficiency of feed conversion showed a similar trend. The supplementation of the diets with chaffed wheat straw did not affect the performance of the calves. The inclusion of the methane inhibitor in the diets improved the weight gains and efficiency of feed conversion. The daily retention of nitrogen in the calves showed a similar trend to the growth rates.

The molar percentage of acetate in the rumen increased as the roughage content of the diets increased while the proportions of propionate, butyrate and valerate decreased. The inclusion of the methane inhibitor in the diets decreased the concentration of total volatile fatty acids (VFA) in the rumen and increased the molar percentage of propionate, butyrate and valerate.

A series of animal feeding trials was designed to determine differences between grasses in dry-matter intake and animal live-weight gain, with a view to indicating better selection criteria for improving the feeding value of herbage.

Wide differences were demonstrated in feed intake and live-weight gain between single-variety grass feeds at similar levels of digestibility.

S.37 cocksfoot gave consistently good and S.51 timothy, consistently poor responses.

Supplementation of grass feeds with red clover frequently resulted in marked improvement in feed intake and animal performance.

1. A comparison was made of the output per hectare and per cow of 3 systems of grazing management each based on a cycle of 5 days grazing followed by a 20-day rest period. The treatments were: C, strip grazing within a paddock, varying the area allotted so that a similar quantity of herbage was available each day, the pasture being frequently trimmed; treatment R, restricted grazing; the cows being allotted a fixed area of the paddock each day (67–5 m2 per cow per day) which was not trimmed or cut after grazing; and treatment RS, as treatment R but with each cow receiving cereal concentrates. Eight cows were on each treatment. Nitrogenous fertilizer was applied at the rate of 86 kg N per ha [1 kg per ha = 0.891b per acre] four weeks before the first grazing and at the same rate after each grazing; a total of 516 kg/ha was applied. The feed intake of each cow was measured on six occasions during the experiment.

2. The experiment continued for 150 days and the mean stocking rate on treatment C was 4–09 cows per ha [1 ha = 2·47 acre] and on the other two treatments 5·92 cows per ha. The number of cow grazing days per hectare were 610, 890 and 890 for treatments C, R and RS respectively and the outputs of milk were C 8450, R 12 420 and RS 13 380 kg/ha. Allowing for the concentrates given the milk outputs were C 7440, R 10 350 and RS 6660 kg/ha.

3. There were only slight differences between the treatments in output per animal.

4. The average intakes per kg W0·73 were similar in treatment C and R but the average intake of cows on treatment C was more uniform over the season. Cows on treatment RS ate consistently more digestible organic matter than those on treatment C and R but the differences became smaller as the season progressed.

The effects of fat source on lipogenesis and lipid traits of longissimus muscle, liver and perirenal adipose tissue, were studied in 48 New Zealand White rabbits, slaughtered at 11 or 15 weeks of age. Rabbits were offered diets with 20 g added fat per kg, containing either medium-chain (COC: coconut oil), saturated and monounsaturated (PAL: palm oil) or polyunsaturated (SUN: sunflower oil) fatty acids as major components. Diets did not affect growth performance, dressing proportion and tissue weights. Intramuscular lipid content was lower for COC than for SUN and PAL (e.g. 10 v. 13 and 12 glkg, at 11 weeks, respectively, P < 0·05), whereas lipid content was unaffected by diet in liver and perirenal fat. In muscle, the fat source did not influence the activities of acetyl-CoA-carboxylase (CBX), malic enzyme (ME) and glucose-6-phosphodehydrogenase (G6PDH). In liver, activities ofGSPDH and ME were depressed from the SUN diet, as compared with the COC or PAL diets. The diet-induced variations in enzyme activities in perirenal fat were lower than in the liver and were not significant. Medium-chain fatty acids were found only in tissue lipids of animals given COC. The ratio of polyunsaturated to saturated fatty acids decreased in the order SUN > PAL > COC in muscle and perirenal fat. Thus, polyunsaturated fatty acids exert an inhibition of G6PDH and ME activities specifically in liver. Compared with COC, the addition of SUN to the basal diet increased total lipids and polyunsaturated fatty acids contents in the longissimus lumborum muscle, which might improve the organoleptic and dietetic qualities of rabbit meat.

Selection indices to improve the efficiency of lean meat production in cattle were derived with an aggregate breeding value which comprised growth rate, food conversion efficiency, killing-out proportion and carcass lean proportion. Index measurements were growth rate, food conversion efficiency and ultrasonic fat area. Relative economic values of traits in the aggregate breeding value were calculated for an 18/20-month beef system, assuming a fixed national output of lean meat. Literature estimates of phenotypic and genetic parameters were used. Two indices were derived, one with a complete restriction on genetic changes in birth weight, and the other without restriction. Correlations between the index and the aggregate breeding value were 0·53 for the restricted index, and 0·57 for the unrestricted index. The maximum proportional reduction in expected economic response, due to complete restriction of birth weight was about 0·08. Selection on either index would actually lead to a slight decrease in carcass lean proportion, but this was less than the decrease expected from selection solely on growth rate. Correlations between the indices and the aggregate breeding value (measuring the accuracy of selection) fell by only about 0·01 when ultrasonic measurements were omitted from the index, but fell by about 0·09 when food conversion efficiency was omitted. Sensitivity of the indices to changes in parameters was also examined. With proportional changes of ±0·5 in individual economic weights, or absolute changes of ±0·2 in genetic correlations or −0·2 in heritabilities, the efficiency of selection ranged from 0·93 to 1·00.

Mastitis is one of the most prevalent and costly diseases in dairy cattle. Key components for adequate mastitis control are the detection of early stages of infection, as well as the selection of appropriate management interventions and therapies based on the causal pathogens associated with the infection. The objective was to characterize the pattern of electrical conductivity (EC) in milk during intramammary infection, considering specific mastitis-causing pathogen groups involvement. Cows (n = 200) identified by an in-line mastitis detection system with a positive deviation ≥15% in the manufacturer’s proprietary algorithm for EC (high electrical conductivity (HEC)) were considered cases and enrolled in the study at the subsequent milking. One control (CON) cow, within normal ranges for EC, was matched to each case. A composite milk sample was collected aseptically from each cow for bacteriological culture. Milk yield (MY) and EC were recorded for each milking during ±7 days relative to enrollment. Milk cultures were categorized into gram positive (GP), gram negative (GN), other (OTH) and no growth (NOG). Data were submitted for repeated-measures analysis with EC as the dependent variable and EC status at day −1, bacteriological culture category, parity number, stage of lactation and days relative to sampling as main independent variables. Average (± standard error (SE)) EC was greater in HEC than in CON cows (12.5 ± 0.5 v. 10.8 ± 0.5 mS/cm) on the day of identification (day −1). Milk yield on day −1 was greater in CON than in HEC (37.6 ± 5.1 v. 33.5 ± 5.2 kg). For practical management purposes, average EC on day −1 was similar for the different bacteriological culture categories: 11.4 ± 0.6, 11.7 ± 0.5, 12.3 ± 0.8 and 11.7 ± 0.5 mS/cm in GN, GP, OTH and NOG, respectively. Parity number was only associated with day −1 EC in HEC group, with the greatest EC values in parity 3 (12.3 ± 0.3 mS/cm), followed by parity 2 (11.9 ± 0.2 mS/cm), parity >3 (11.6 ± 0.5 mS/cm) and primiparous cows (11.2 ± 0.2 mS/cm). An effect on EC for the interaction of day relative to identification by pathogen gram category was observed. The same interaction effect was observed on daily MY. Overall, the level of variation for MY and EC between- and within-cows was substantial, and as indicated by the model diagnostic procedures, the magnitude of the variance in the cows in the CON group resulted in deviations from normality in the residuals. We concluded that characteristic temporal patterns in EC and MY in particular pathogen groups may provide indications for differentiation of groups of mastitis-causing pathogens. Further research to build detection models including EC, MY and cow-level factors is required for accurate differentiation.