Predicted transmitting abilities for somatic cell counts (SCC) are available in the United Kingdom and there is a direct economic benefit attached to reducing SCC as the milk payment schemes include a penalty for high SCC levels in bulk tank samples and sometimes a premium for low SCC. The aim of the present study was to establish the economic importance of bulls' breeding values for SCC in relation to this payment scheme for SCC. To do this, an empirical method was developed using 645071 individual cow SCC and milk yield test-day records from 358 herds. The economic value was calculated by (i) decreasing all individual cow records by 0·01 and comparing the average penalty with the current average penalty, and (ii) taking the derivative of a Gompertz function describing the within-herd penalty per cow as a function of the average within-herd 3-month rolling geometric mean SCC. Mean milk test-day yield and average test-day SCC were 20·4 kg and 262 kcount per ml respectively with, on average, 83 cows tested each day. In the current situation the average penalty paid was 0·54, 0·18 and 3·2 pence per litre depending on whether an England and Wales, Scottish or future payment scheme was used, respectively. Across the population, the economic values per 0·01 decrease in SCC were £1·04, £0·54 and £6·03 per cow per year for these three payments schemes respectively. However these economic values depend strongly on the mean SCC. Herds have different means and as the future population mean is difficult to predict, it is suggested that for herds with the majority of their bulk tank samples in penalty bands 1, 2 or 3+ (average SCC of <150, 150 to 250 and 250+) the economic values are £0, £0·50, and £15 per cow per year per 0·01 reduction in SCC, respectively, until additional benefits have been quantified.

The objective of this study was to investigate the effects of silage characteristics on water intake of lactating dairy cows and to examine the prediction of water intake. Sixteen grass silages, differing in fermentation and intake characteristics, were offered ad libitum to dairy cows in early lactation supplemented with 7 kg/day of concentrate (13·3 MJ metabolizable energy per kg dry matter (DM) and 216 g crude protein per kg DM). Four silages were offered in each of four incomplete change-over design experiments, consisting of three 3-week periods. Water intakes were recorded through individual Kent water meters and press water bowls over the final week of each period. Tree (drinking) water intake ranged from 20·1 to 89·9 (mean = 45·2; s.d. = 12·96) I/day whilst total water intake (also including food water) ranged from 48·4 to 123·8 (mean = 87·3; s.d. = 14·12) I/day. Water intake increased with increasing silage DM concentration, however free water replaced silage water at a rate less than 1. Milk yield and silage D value (digestible organic matter, g/kg DM) were strongly positively correlated with free water intake (r = 0·751 and 0·595 respectively), though fermentation indices were not good single predictors of water intake. Further analysis revealed problems owing to collinearity within the predictors of water intake: DM intake, silage D value and milk yields being significantly correlated, as were pH and volatile fatty acids as a proportion of total fermentation acids. The ridge regression technique was used to reduce collinearity problems and produce stable equations. The best prediction equations for water intake involved a combination of both animal and analytical information: diet DM concentration, milk yield and silage pH. The use of fermentation information, whether from titration or high-performance liquid chromatography did not describe real variation in water intake beyond that described by silage pH. Free water intake was higher with higher diet DM concentrations, higher milk yields and higher silage pH.

The power for detection of quantitative trait loci (QTL) using marker information was compared in several schemes differing in the mating type and the number of parents involved. An experiment based on an F2 population of fixed size obtained by crossing two lines differing phenotypically for a single trait was simulated, assuming that QTLs could be fixed or segregating in the lines crossed. Different additive and dominant QTL effect values and allele frequencies were considered covering a range of different favourable situations for the detection of the QTL. Comparison was done by regression using flanking marker information. Mating animals at the F1 generation level minimizing relationships was not worse than mating at random or maximizing relationships. The number of parents used affected the power of the experiment when the QTL was segregating in the original crossed lines. Differences in power were mainly related to the number of males from the original line. When the power of the experiment was high as a result of genetic hypothesis assumed, considering several males increased the power. However, when the genetic hypothesis assumed led there to be less power to detect a QTL, the power was higher when fewer males were used.

The effects of duration of fasting on the short-term feeding behaviour of 12 grazing and 12 silage-fed lactating Holstein-Friesian dairy cows were examined. Four groups of three cows were rotated around fasting treatments of 1, 3, 6 or 13 h following a balanced Latin-square design. Herbage intakes for each treatment group were assessed over al-h period.

As intended there were no significant differences in sward characteristics between the experimental plots grazed by cows from different treatment groups. However, total dry-matter (DM) intake, biting rate and DM intake per bite, measured over the 1-h grazing period, increased significantly when the duration offasting was extended from 1 to 6 or 13 h (P < 0·05). There were only minor differences in grazing behaviour following fasting durations ofl and 3, or 6 and 13 h.

In a parallel study, undertaken to assess the influence offasting duration on appetite independently of grazing, four groups of three cows were housed indoors and offered 30 kg of a high quality grass silage. Silage DM intakes, measured over a 1-h period, increased significantly with extended fasting periods (P < 0·01), though silage DM intake was considerably lower than that of grazing cows for each fasting treatment.

These results suggest that dairy cows grazing on good quality swards may be able to compensate for increased degree of hunger by increasing both biting rate and DM intake per bite to increase DM intake rate. Although the DM intakes of silage and grazed grass followed similar patterns of increasing intake with extended fasting duration, DM intake rates were considerably higher in grazing cows for each fasting treatment.

The effect of ensiling grass and supplementing the resulting silage with water-soluble carbohydrate in the form of sucrose and undegradable dietary protein (UDP) in the form offish meal on forage intake and milk production and composition were evaluated in an experiment involving 63 mid-lactation Holstein-Friesian cows. Herbage from the primary growth of a predominantly perennial ryegrass sward was zero-grazed (ZG) from 19 April to 16 May 1993. The herbage was mown and picked up with a precision-chop harvester and offered as the sole diet, twice daily, to 18 cows which were on average 186 days into lactation. On 13 May, herbage from the same sward was harvested identically to the ZG herbage and ensiled, treated with formic acid at a rate of 2·85 lit grass. At 53 days after ensiling the silage was offered either as the sole diet (SO) or supplemented with sucrose at 10·3 g/kg fresh silage (SS) or supplemented with sucrose and fish meal each at 10·3 g/kg fresh silage (SSF) to 15 cows per treatment which were on average 164 days into lactation. The feeding period was 28 days for ZG and 21 days for the SO, SS and SSF treatments, and the last 7 days were the main recording interval. Prior to going on to the experimental diets all animals were offered a common silage ad libitum and supplemented with 5 kg of a 180 g/kg crude protein concentrate. Dry-matter intakes (DMI), milk yields and milk composition were recorded during the pre-experimental period and subsequently used as covariates in the statistical analysis. For diets ZG, SO, SS and SSF, forage DMI (kg/day), total DMI (kg/day), milk yields (kg/day), fat concentration (g/kg) and protein concentration (g/kg) were 14·4,14·0,13·7 and 13·9 (average s.e.d. = 0·55); 14·4,14·0,14·5 and 15·4 (average s.e.d. = 0·59); 15·4, 14·4, 14·5 and 16·7 (average s.e.d. = 0·42); 36·6, 38·6, 38·6 and 37·5 (average s.e.d. = 2·67); 33·0, 30·7, 32·2 and 32·8 (average s.e.d. = 0·75). Nitrogen (N) digestibility was higher on SO and SSF (P < 0·05) relative to ZG with SS being intermediate, otherwise treatment did not alter (P > 0·05) diet apparent digestibility. Ensilage increased the immediately soluble N fraction (a value) and degradability of N (P < 0·001) relative to ZG. It is concluded that ensilage had no effect on forage DMI but decreased milk yield and protein concentration relative to the parent herbage. The positive response in milk yield due to supplementation of the silage-based diet with sucrose and fish meal and the lack of response to sucrose supplementation alone suggests that nutrients absorbed from the digestive tract of cows offered silage-based diets are more limiting in protein or specific amino acids supplied by fish meal than in energy. The decrease in animal performance due to ensiling may be overcome by supplementation of silage-based diets with UDP at sufficient levels to equate that of the parent herbage.

Dose-response experiments were designed to determine the dosage of somatostatin (SRIF) necessary to alter serum growth hormone (GH) response to administration of GH-releasing factor (1·44; GRF) in cattle. The objective of this experimentation was to develop a GRF/SRIF challenge model that might be useful in early identification of animals which are genetically superior for milk production traits. In experiment 1, calves received either 0, 5 or 10 /JLg GRF per 100 kg body weight (BW) or GRF 1·29 at 1 ug per 100 kg BW. Both the 5 and 10 ug GRF per 100 kg BW dosages increased GH concentrations (P < 0·05). In experiment 2, animals received 3 ug GRF per 100 kg BW concurrent with administration of 0, 0·5, 1 or 2 ug SRIF per 100 kg BW at 0 min, with second dosage of GRF at +120 min. Administration of SRIF in those dosages did not alter GH response to GRF. In experiment 3, animals received 5 ug SRIF per 100 kg BW at -2, -1, 0 or +1 min relative to 3 ug GRF per 100 kg BW, with a second GRF injection at +120 min. No differences (P < 0·05) were found in response due to timing of SRIF administration. In experiment 4, animals were administered 0, 5, 10 or 20 Xg SRIF per 100 kg BW concurrent with 3 ug GRF per 100 kg BW. SRIF delayed (P < 0·05) the occurrence of maximum GH concentrations but did not affect the maximum concentration nor the area under the GH response curve (AUC) following either the first or second GRF injection. Utilizing data from experiment 4, individual animal response to GRF was reasonably consistent, as repeatabilities of AUCs for 1 h following first and second GRF injections were 0·80 and 0·65, respectively. Furthermore, sufficient among-animal variation existed so that animals could be distinguished from each other. Thus we conclude the method described herein might be useful in identifying superior dairy calves but accuracy would potentially be enhanced by collecting blood samples more frequently during the 20 min immediately after GRF injection.

A partially balanced change-over design experiment was made to examine the effects of concentrate energy source on the voluntary food intake and animal performance of 50 lactating dairy cows offered a diverse range of grass silages. The silages were also offered as the sole diet to 10 dairy cows in a partially balanced change-over design experiment. A total of five silages were prepared. Silages A, B and D and silages C and E were harvested from primary regrowths and secondary regrowths respectively of predominantly perennial ryegrass swards. Herbage was ensiled either pre-wilted or unwilted and either untreated or treated with a bacterial inoculant or formic acid based additives. For silages A, B, C, D and E, dry matter (DM) concentrations were 473, 334, 170, 170 and 256 (s.e. 4·0) g/kg, pH values 4·42, 4·01, 4·88, 4·46 and 3·91 (s.e. 0·059), ammonia-nitrogen (N) concentrations 86, 88, 289, 182 and 135 (s.e. 10·6) glkg total N and in vitro DM apparent digestibilities 0·76, 0·76, 0·75, 0·73 and 0·75 (s.e. 0·009) respectively. When offered as the sole diet DM intakes were 14·1,14·7,10·5,10·1 and 11·5 (s.e. 0·50) kg/day. Five concentrates were formulated to contain similar concentrations of crude protein, effective rumen degradable protein (ERDP), metabolizable energy (ME) and fermentable ME (FME) but using different carbohydrate sources to achieve a wide range of starch concentrations. For the low and high starch concentrates, starch concentrations were 50 and 384 g/kg DM, and acid-detergent fibre concentrations were 128 and 75 g/kg DM respectively. The silages were offered ad libitum supplemented with 10 kg concentrate per head per day. For silages A, B, C, D and E silage DM intakes were 10·6, 10·5, 8·5, 8·6 and 9·0 (s.e. 0·37) kg/day and milk yields 23·9, 28·1, 26·2, 26·1 and 25·0 (s.e. 0·76) kg/day respectively. Concentrate energy source did not influence (P > 0·05) silage DM intake, diet apparent digestibility or the yields of milk or fat plus protein. For concentrates containing 50, 131, 209, 310 and 384 g starch per kg DM, milk protein concentrations were 32·0, 32·2, 32·5, 33·0 and 33·6 (s.e. 0·13) glkg, milk fat concentrations were 44·5, 43·9, 43·8, 43·3 and 43·1 (s.e. 0·35) glkg and urinary allantoin concentrations 15·2,15·4, 17·0, 1.7·6 and 18·0 mmolll respectively. Increasing starch intake resulted in positive and negative linear relationships for milk protein (P < 0·01, R2 = 0·96) and fat (P < 0·01, R2 = 0·96) concentrations respectively. There were no significant concentrate energy source × silage type interactions on silage intake or yields of milk or fat plus protein (P > 0·05). However there was a concentrate energy source × silage type interaction on milk fat yield (P > 0·05). It is concluded that, with silages of varying fermentation and intake characteristics but similar apparent digestibility, there were no concentrate energy source × silage type interactions on food intake, milk composition or milk yield. Also concentrate energy source had no effect on silage DM intake or milk yield. However increasing starch intake linearly increased milk protein concentration, probably due to increased microbial protein synthesis and decreased milk fat concentration.

The potential of the spot urine sampling technique as an alternative to performing a total urine collection was evaluated. Twelve multiparous Holstein-Friesian cows were given two experimental diets in a complete change-over design using two 14-day experimental periods. Experimental diets were either silage offered ad libitum with 7 kg fresh weight concentrate supplement as a single meal (SF), or a complete diet formulated from the same ingredients with a similar foragexoncentrate ratio (CD). Total urine collections were performed every 2 h on days 11 and 14 of each experimental period. Subsamples of urine were stored at 20°C and subsequently analysed by high-performace liquid chromatography. Daily allantoin and purine derivative (PD) excretion were highly correlated (r = 0·995, no. = 48, P < 0·001). PD and creatinine excretion during each 2-h interval depended on time of collection (PD, P < 0·001 and creatinine, P < 0·05) and on cow (P < 0·01) but were unaffected by sampling day or treatment. Diurnal variations in the molar ratio ofPD or allantoin to creatinine (PD/c and Ale, respectively) followed similar diurnal patterns as observed for PD and allantoin excretion. The data were used to assess the error of prediction of daily mean PD/c or Ale ratios. Three spot sampling regimens (based on the collection of four 4-h samples, three 8-h samples or two 12-h samples) and also on either single or 2-day urine collections were evaluated. Collection of multiple samples within a day was more reliable than collecting fewer samples over several days. Prediction errors were greater for SF compared with CD. Even the most intensive sampling regimen did not allow an acceptable prediction of daily mean PDIc or Ale ratio, minimum r values for PDIc and Ale ratios were 0·098, 0·136 and 0·547, 0·579 for SF and CD, respectively. Furthermore, daily mean PDIc and Ale ratios proved poor predictors of daily PD and allantoin excretion (r values of 0·69 and 0·72, respectively). Total urine collection appears necessary to assess accurately daily PD excretion in dairy cows.

Data from about 4000 Sahiwal cows from eight large herds in Pakistan were used to study the influence of genetic and environmental factors on some milk production traits. First-lactation mean values were 1363 kg, 1395 kg and 252 days for milk yield up to 305 days after calving, total lactation yield and lactation length, respectively. Second and third lactation yields were proportionately 0·12 and 0·18 higher, respectively, at 305 days. The effect of herd-year at calving was by far the most important source of variation for all traits. Heritabilities estimated in uni- and trivariate analyses, using restricted maximum likelihood (REML) with an expectation maximization algorithm for an animal model, ranged from 0·14 to 0·17 for first-lactation traits. The estimates were generally lower for second lactation and higher for third lactation traits. Genetic correlation between lactations for the same trait were close to unity, whilst the phenotypic were considerably lower. Repeatabilities for milk yield traits were 0·42 and for lactation length 0·31. Genetic and phenotypic correlations between first-lactation 305-day milk yield and lactation length were 0·83 and 0·71, respectively. Genetic trends for all traits were close to zero but a substantial deterioration in performance, caused by negative environmental factors, was observed. Although the heritabilities were low, the prospects for genetic improvement are good, as indicated by a rather large additive genetic variation. A multivariate animal model, including the first three lactations of 305-day milk yield, was recommended for the most accurate prediction of breeding values for milk production.

The effect of three protein (77, 97 and 117 g DVE (true protein digested in the small intestine) per kg dry matter (DM)) and two energy levels (7·38 and 8·03 MJ NEF (net energy for fattening) per kg DM) on the performance of Belgian White-blue double-muscled finishing bulls and on the quality of their carcasses and meat was investigated. The diet, offered ad libitum, consisted of 650 g/kg concentrates and 350 g/kg maize silage on DM basis.

No significant influence was found of the energy level on the growth rate. The low protein level reduced live-weight gain, from 370 to 501 kg (1·43 kg on average v. 2·60 and 1·66 kg daily; P < 0·02). The growth rate during the entire period (370 to 692 kg) and the live weight at slaughter were significantly influenced by the protein content.

The bulls given the high energy level lost less weight during the 20-h fasting period before slaughter. Their carcasses were classified with a higher fatness score and a better conformation. These carcasses had a higher fat content, while the proportion of bone in the carcass was lower than in the low energy groups. The cold carcass weight of the low protein groups (456 kg on average) was significantly smaller than that of the four other groups (470 kg on average). Although no differences between the six groups were found concerning the dressing proportion, this measurement was significantly influenced by the protein level (685 g/kg for low protein v. 692 for high protein). The SEUROP conformation was positively influenced by the protein and the energy level, while the fatness score was influenced only by the energy level (5·5 v. 6·0 for the low and the high energy level, respectively). Although the differences in conformation, fatness score and dressing proportion are significant, because of the small variation, the practical meaning is less important.

An analysis has been carried out of over 88000 calving survey records for bulls of eight beef breeds tested in commercial dairy herds in England and Wales. The data were collected from 1981 to 1993. The traits analysed were the degree of difficulty at calving, assessed from no difficulty to serious difficulty, calf mortality to 48 h, gestation length, and subjective assessments of calf size and conformation.

Major sire breeds in the survey were the Hereford (30070 records, 93 sires), Charolais (25816 records, 105 sires) and the Limousin (15420 records, 55 sires). Other sire breeds were Belgian Blue (5391 records, 24 sires), Simmental (3864 records, 19 sires), Aberdeen Angus (2886 records, 12 sires), Piedmontese (2423 records, 7 sires) and the Blonde d'Aquitaine (2271 records, 8 sires).

Information was routinely recorded on cow age (heifer or mature), sex of the calf, and the year and month of the calving, while cow size and the regional location of the herd were also recorded in later years.

Heifers had more difficult calvings, higher calf losses and shorter gestations than mature cows (all P < 0·05) Seriously difficult calvings and calf mortality were higher for male calves, which were also larger and had better conformation than heifer calves (P < 0·05).

Sire breed effects were significant for all traits, although sire breed ranking for calving ease was different for cow and heifer calvings and some interactions involving sire breed were also significant. For cow calvings, the easiest calving breeds were the Hereford (1·1%) and the Aberdeen Angus (1·4%), while the Charolais (4·3%), Blonde d'Aquitaine (3·7%) and Simmental (3·1%) had most difficult calvings.

Gestation length differed between sire breeds, from Aberdeen Angus (281·0 days) and Hereford (282·7 days) up to the Limousin (288·1 days) and the Blonde d'Aquitaine (289·0 days). For calf size and conformation, the Simmental, Charolais and Blonde d'Aquitaine were the best.

In general, summer months were associated with lower incidences of difficult calvings, lower losses and shorter gestations.

Large cows had calves which were larger and of better conformation than small or medium cows, but large cows also had longer gestations, more difficult calvings and poorer calf survival (all P < 0·05).

Statistical models are discussed that should be used for the genetic evaluation of calving survey traits, when beef bulls are to be used in dairy herds in the United Kingdom.

Somatic cell count (SCO data collected on individual cows since 1991 were obtained from National Milk Records. Following validation genetic and phenotypic parameters were estimated from the lactation average SCCfor 63 424 Holstein/Friesian (HOD, 7966 Ayrshire (AYR) and 14 509 Jersey (JER) animals. The first three lactations were included in the analyses. The heritabilities (h2) for first lactation log SCC (LSCC) were 0·11 (s.e. 0·01), 0·12 (s.e. 0·02) and 0·09 (s.e. 0·03) for the HOL, JER and AYR breeds respectively. Estimates for herd sire interaction (c2) effects ranged from 0·01 to 0·02. Analysis of the first three lactations with a repeatability model produced similar h2 and c2 estimates. Permanent environment estimates ranged from 0·21 to 0·25. Heritabilities of individual test day LSCC ranged from 0·04 (s.e. 0·02) to 0·10 (s.e. 0·03). Genetic correlations between SCC and milk, fat and protein yield for the HOL breed were 0·06 (s.e. 0·05), 0·14 (s.e. 0·06) and 0·09 (s.e. 0·06) respectively. Genetic evaluations were obtained for 666 595 and 9136 animals for Holsteins and Ayrshires, providing evaluations on 13 525 and 1713 bulls respectively in each breed. The range of LSCC predicted transmitting abilities (PTA) was ±25% although the proportion of bulls with reliability > 0·50 was low at 0·17 and 0-05 for HOL and AYR respectively. The correlations between the LSCC PTAs for HOL bulls with at least 50 daughters and their genetic evaluations for linear type were essentially zero for body and some udder traits. Significant negative correlations were obtained for a number of traits including foot angle (0·14), fore-udder attachment (0·19) and udder depth (0·19) and a positive correlation for teat length (0·15). SCC evaluations will be implemented by the Animal Data Centre. Reliabilities will be lower than production because of the lower h2 for SCC and the lower progeny group size since only 0·80 of recorded cows have SCC records.

The Dairy Information System (DAISY) was developed to record fertility and health information for use in research and to help farmers manage their farms. Data from 33 herds recording health and fertility over a 6-year period were used to study genetic relationships of several health, fertility and production traits. There were 10 569 records from 4642 cows of all parities. These were used to estimate genetic parameters for health: mastitis, lameness and somatic cell score (SCS), for fertility: calving interval, days to first service, conception to first service and for production: 305-day milk, butterfat and protein yields. Heritabilities for these traits were also estimated for the first three lactations. (Co)variances were estimated using linear, multitrait restricted maximum likelihood (REML) with an animal model. Mastitis and lameness were treated as all-or-none traits. The incidence of these diseases increased with lactation number, which may lead to variance component estimation problems, as the mean is linked to the variance in binomial distributions. Therefore, a method was used to fix the within-lactation variance to one in all lactations while maintaining the same mean. The heritability for SCS across lactations was 0·15. Heritabilities for other health and fertility traits were low and ranged between 0·013 and 0·047. All genetic correlations with the production traits were antagonistic implying that selection for yield may have led to a deterioration in health and fertility. The genetic correlation between SCS and mastitis was 0·65 indicating that indirect selection for improvements in mastitis may be achieved using somatic cell counts as a selection criterion. The potential use of linear type scores as predictors of the health traits was investigated by regressing health traits on sire predicted transmitting abilities for type. The results indicate that some type traits may be useful as future selection criteria.

Genetic parameters were estimated for traits recorded in a calving survey for beef sires used in commercial dairy herds in England and Wales. The five traits included in the survey were calving difficulty score, calf mortality to 48 h after birth, gestation length, and subjectively assessed calf size and conformation. The data file examined included over 88 000 records, on 323 sires of eight beef sire breeds, namely Aberdeen Angus, Belgian Blue, Blonde d'Aquitaine, Charolais, Hereford, Limousin, Piedmontese and Simmental.

Estimates of heritability and genetic and phenotypic correlations were obtained for all traits by REML procedures using a sire model. The effects of cow age, calf sex, year and season of the calving and sire breed were included in the model, together with interactions.

Heritability estimates were 0·09 (s.e. 0·01) for calving difficulty score, 0·32 (s.e. 0·03) for gestation length, 0·02 (s.e. 0·003) for mortality, 0·09 (s.e. 0·01) for calf size and 0·06 (s.e. 0·01) for calf conformation.

Heritability estimates for the scored categorical and binomial (mortality) traits were also obtained using threshold model analysis. As expected, these estimates were higher than the REML estimates; 0·16 (s.e. 0·01) for calving difficulty score, 0·08 (s.e. 0·01) for mortality, 0·14 (s.e. 0·01) for calf size and 0·09 (s.e. 0·01) for calf conformation. Calving difficulty score was strongly correlated genetically with calf size (-0·84), mortality (0·74) and calf conformation (-0·72), and moderately correlated with gestation length (0·26). Calf size and conformation were very highly correlated genetically (0·86), and both were also strongly correlated with mortality (-0·63 and -0·52 respectively). In all cases, the genetic correlations were stronger than the phenotypic correlations.

Sire genetic merit was predicted for all traits and the distribution of these predictions is described. Sire predictions of genetic merit for calving difficulty score on the underlying scale were backtransformed to predict the expected incidence of serious difficulties in future calvings and these predictions are illustrated for the major sire breeds.

An experiment was devised to examine the effect of animal age at slaughter, within genotype and finishing system, on bull beef eating quality characteristics to assess the 15 month age limit imposed for bulls at slaughter by the Meat and Livestock Commission specification for improved beef eating quality. The experiment was a factorial design using bulls of two crossbred genotypes (Aberdeen Angus × and Charolais ×), offered two diets (silage-based and barley-based) and slaughtered at two abattoirs. Bulls were slaughtered from approximately 10 months of age on seven dates (ages) at 6-week intervals. Bulls given barley were slaughtered on date numbers 1 to 5 (43 to 67 weeks of age) and bulls given silage on date numbers 2 to 7 (49 to 79 weeks of age). On each slaughter date and at each abattoir the first four commercial steers which followed the bulls on the slaughterline and met the United Kingdom (UK) modal carcass specification R 4L were selected as abattoir, slaughter date and industry representative controls. At 48 h post mortem samples of m. longissimus lumborum; m. biceps femoris; and m. semimembranosus were recovered from all bull and steer carcasses, vacuum packed and stored at 2°C for a fiirther 12 days before freezing and subsequent assessment by a 12 member taste panel and texture analysis using a Warner Bratzler shear jig.

Bulls achieved high growth rates during the finishing period averaging 1·22, 1·55, 1·41 and 1·98 kg/day for silage-fed Angus × bulls; barley-fed Angus × bulls; silage-fed Charolais × bulls; and barley-fed Charolais × bulls, respectively (s.e.d. = 0·08; P < 0·05). Age at slaughter had no significant effect on beef tenderness from young bulls in this study. Similarly, no significant differences in beef tenderness either between genotypes or diets were detected. Bulls, however, produced meat which had lower mechanical shear force values (10·2 v. 11·9 kg for m. biceps femoris; P < 0·001) and was considered to be more tender (P < 0·001), less flavoursome (F < 0·001) and more acceptable (P < 0·01) than that produced by steers.

It was concluded that beef aged for 14 days post mortem and recovered from young bulls which have achieved high growth rates can be at least as good as that representative of the UK standard product. The 15-month age limit for bulls ensures that commercially produced bulls achieve high lifetime growth rates and will produce beef of a quality comparable with that achieved in this study.

The size and composition of three herds belonging to Maasai pastoralists were monitored for more than 5 years (1990 to 1996). Animals were categorized as either suckling calves, weaned heifers, weaned males or breeding females. The dates of entries and exits were used to estimate the total number of observed animal days for each category. Fertility rates of breeding females and mortality and disposal rates for each category were estimated using animal days as the denominator. Herd differences were tested using contingency tables. Age to first calving and calving intervals were estimated and examined using analysis of variance tests.

The patterns of births and deaths were seasonally influenced. The mean annual calving rate of all breeding females was 65·6% and for females excluding first calvers was 46·9%. No herd effects were observed. The mean observed interval between calvings was 609 days and the calving interval, calculated from the parturition rate, was 649 days with no herd differences. Mean age at first calving was approximately 4 years with no herd differences. Mean annual mortality rates were 8·9% for breeding cows, 7·8% for weaned males, 6·7% for weaned heifers and 22·1% for calves. The mean annual culling rate of cows was 10·8% and the sale rate of heifers and weaned males was 12·9% and 41·5% respectively. The mean age at disposal of heifers was 655 days and of steers was 801 days. Herd sizes were shown to fluctuate annually with no obvious trend between herds. The main determinants of production identified were the effect of seasonally poor nutrition on the rate of reproduction and the effect of season, herd mobility and disease on mortality.

The study describes production levels that can be used to predict future changes to the system. The results are discussed in terms of the factors influencing and their relevance to pastoral development. The findings are compared with those observed in other, similar areas.

Two experiments were conducted to compare the response oflactating cows to supplementary light in their lying area during increasing and decreasing natural daylength. During decreasing daylength, supplementary light in the lying area increased the time cows spent lying down and considerably reduced calculated food intake, milk production, live weight and body condition, so that lights were installed in the feeding area for the last half of the experiment, which partially restored intake and live weight. Plasma cortisol concentrations and milk somatic cell counts were increased by supplementary light in decreasing daylength before, but not after, lights were installed in the feeding passage, suggesting that cows may have been stressed by the difficulties encountered during feeding in the dark. During increasing daylength supplementary light did not affect lying time, had less effect on food intake and no effect on milk production or live weight. There was a small reduction in plasma corticosteroid by the end of the experiment with supplementary light. It is concluded that providing supplementary light only in the lying area of dairy cows will have adverse effects on their production and welfare in decreasing, but not increasing daylength

Muscle fibre characteristics and biochemical properties of muscle recovered from young bulls of two genotypes (Aberdeen Angus × and Charolais ×), reared on two different diets (silage-based and barley-based) and slaughtered at varying ages between 10 and 19 months of age were established. These analyses were restricted to samples ofm. longissimus lumborum (LI) recovered at 48 h post mortem, vacuum packed and stored at 2°C for 14 days. Biochemical measurements included intramuscular fat content, intramuscular collagen content and its solubility, haem pigment concentration, sarcomere length and myofibril fragmentation. Muscle fibre type was classified according to the contractile nature of thefibres and their metabolic properties.

Intramuscular fat content increased (P < 0·01) with age at slaughter and at a fixed age was greater for Angus × than Charolais × bulls (211·5 v. 295·8 mg/g dry matter, P < 0·01). Total intramuscular collagen and its solubility tended to decrease with age (P < 0·01). Differences in haem pigment concentration in samples of LI were detected between genotype (3·99 v. 3·59 mg/g for Angus × and Charolais × bulls; P × 0·01) and diet (3·97 v. 3·62 mg/g for bulls given barley and silage; P < 0·01), and increased with age at slaughter (P < 0·01). There was a significant increase in eye muscle cross-sectional area with increasing slaughter date (P < 0·01) and this was paralleled by an increase in cross-sectional area of individual muscle fibres (P < 0·001). Differences in cross-sectional area of individual muscle fibres between genotype and diet were small and inconsistent. Charolais × bulls had a greater percentage area of fast twitch glycolytic fibres than Angus × bulls (54·3 v. 49·3%; P < 0·01) and a smaller percentage area of slow twitch oxidative fibres (15·8 v. 18·9%; P < 0·05). Beef tenderness was positively correlated (r = 0·48; P < 0·01) with the frequency of slow twitch oxidative fibres and negatively correlated (r = -0·38; P < 0·05) with the frequency offast twitch glycolytic fibres.

Estimates of (co)variance components and genetic parameters were obtained for birth (no. = 3909), weaning (no. = 3425), yearling (no. = 2763), and final weight (no. = 2142) for Gobra cattle at the Centre de Recherches Zootechniques de Dahra (Senegal), using single trait animal models. Data were analysed by restricted maximum likelihood. Four different animal models were fitted for each trait. Model 1 considered the animal as the only random effect. Model 2 included in addition to the additive direct effect of the animal, the environmental effect due to the dam. Model 3 added the maternal additive genetic effects and allowed a covariance between the direct and maternal genetic effects. Model 4 fitted both maternal genetic and permanent environmental effects. Inclusion of both types of maternal effects (genetic and environmental) provided a better fit for birth and weaning weights than models with one maternal effect only. For yearling and final weights, the improvement was not significant. Important maternal effects werefound for all traits. Estimates of direct heritabilities were substantially higher when maternal effects were ignored. Estimates of direct and maternal heritabilities with model 4 were 0·07 (s.e. 0·03) and 0·04 (s.e. 0·02), 0·20 (s.e. 0·05) and 0·21 (s.e. 0.05), 0·24 (s.e. 0·07) and 0·21 (s.e. 0·06), and 0·14 (s.e. 0·06) and 0.16 (s.e. 0·06) for birth, weaning, yearling and final weights, respectively. Correlations between direct and maternal genetic effects were negative for all traits, and large for weaning and yearling weights with estimates of -0·61 (s.e. 0·33) and -0·50 (s.e. 0·31), respectively. There was a significant positive linear phenotypic trend for weaning and yearling weights. Linear trends for additive direct and maternal breeding values were not significant for any trait except maternal breeding value for yearling weight.

Two studies investigated the effect of contaminated pasture on selective grazing, overall grazing behaviour and the process of defoliation of dung patches through experiments targeted at four major questions: (a) how does relative utilization of tall and short grass change as the sward is grazed down? (b) what effect does herbage mass and sward height have on the relative utilization of short and tall grass? (c) how are tall grass patches actually utilized by cattle? and (d) how is overall grazing behaviour influenced by contamination of the sward?

Experiments were conducted in mid to late season using Friesian dairy cattle. In experiment 1, two -pasture types (topped sward (T) v. grazed-only sward (G)) were used. The distribution of bites on tall grass from both pasture types indicated that the grazing animals tended initially to graze short grass when they met a new sward, and then select tall grass as the swards were progressively grazed down. This switch happened earlier in the defoliation process in the topped sward. In experiment 2 observations were conducted on previously grazed and previously ungrazed swards. The distribution of bites on tall grass showed a similar trend to that found in experiment 1 and as the sward was gradually grazed, biting rate significantly declined. There was also a significantly higher total grazing time on the previously ungrazed sward (no contamination by dung). Comparing data based on a consistent biting rate (calculated as the time for 20 consistent bites) and natural biting rate (calculated as the total time for 20 bites) suggested that the grazing animals had increased difficulty in handling tall grass which may explain the declining biting rate as the swards were being grazed down and more bites were directed at tall grass. The defoliation of tall grass dung patches appeared to be concentrated around the edges of the patch. The average area of sward affected by a single dung pat was 1·04 m2 measured at the pre-grazing stage and was markedly reduced to 0·51 m2 at the post-grazing stage. In conclusion, selective grazing is likely to exist due to the presence of dung and conditioned by dung distribution and sward type and this in turn modifies biting rate during grazing down of a sward.