Effect of weaning system on milk composition and distribution of milk fat within the udder of East Friesian dairy ewes

We investigated whether the inhibition of milk ejection during and/or between machine milkings is responsible for the low milk fat observed in commercial milk obtained from dairy ewes managed with a mixed system (MIX) of partial daily suckling (10 h) and once daily machine milking (after 14 h of udder filling). East Friesian crossbred dairy ewes were randomly allocated postpartum to the MIX system (n = 9) or to exclusive twice-daily machine milking (DY1, n = 8). Following wk 4, MIX ewes were permanently weaned from their lambs and milked twice daily. All ewes were injected with saline, oxytocin, or an oxytocin-receptor antagonist prior to three morning milkings during wk 2,4, and 6 of lactation to study cisternal and alveolar milk distribution. Overall milk yield (cisternal + alveolar) for MIX ewes was 42% greater than for DY1 ewes during wk 2 and 4, which demonstrates the beneficial effect of lamb suckling on milk production of dairy ewes. However, during normal machine milking, only the cisternal fraction was obtained from MIX ewes, confirming that milk ejection did not occur for as long as these ewes remained in partial daily contact with their lambs. Although the volume of milk stored within the cistern, and its concentration of milk protein was similar for the two weaning systems, milk of MIX ewes was significantly inferior in cisternal milk fat concentration and yield compared to DY1 ewes. This provides evidence that not only is there inhibition of milk ejection during machine milking of MIX ewes, there is additional inhibition of transfer of milk fat, but not milk protein, from the alveoli to the cistern during the evening when MIX ewes a reseparated from their lambs. Following weaning of MIX ewes, the majority of lactation traits studied were similar compared to DY1 ewes.     

Effect of milking interval on alveolar versus cisternal milk accumulation and milk production and composition in dairy ewes

Cisternal and alveolar milk fractions were measured in East Friesian crossbred dairy ewes (n = 32) after 4, 8, 12, 16, 20, or 24 h of milk accumulation in a 6 x 6 Latin square design by administration of an oxytocin receptor antagonist for recuperation of cisternal milk followed by injection of oxytocin to remove the alveolar fraction. Less than half (38 to 47%) of the total milk yield was stored within the cistern for the first 12 h of udder filling compared with up to 57% after 24 h of udder filling. Subsequent milk yield was significantly reduced following the 16-, 20-, and 24-h treatments. Cisternal milk fat percentage, but not milk protein percentage, was lower than in alveolar milk (4.49 vs. 7.92% milk fat, respectively), indicating that casein micelles pass more freely from the alveoli to the cistern between milkings compared with fat globules. Alveolar compared to cisternal somatic cell count was higher for the 16-, 20-, and 24-h treatments. Significant increases in cisternal milk yield and milk composition observed for the 24-h compared with the 20-h treatment demonstrated the importance of the cistern as a storage space when the alveoli and small intramammary ducts became full. The main difference between cisternal and alveolar milk fractions is the poor fat content of cisternal milk, which is an important reason for the milk ejection reflex to be present during machine milking of dairy ewes. In a second experiment, milking every 16 h compared with every 12 h during mid- to late-lactation did not effect milk yield, milk composition, and quality, or lactation length; however, a 25% savings in labor was achieved with the longer milking interval.     

Losses of precision associated with simplified designs of milk recording for dairy ewes

Simplified designs of milk yield recording based on the yield of a single monthly milking, adjusted or not for interval between milkings and for production level, were simulated and evaluated for 3173 ewe-test-day records belonging to 155 lactations of Churra dairy ewes. Losses of precision associated with simplified methods were evaluated by comparing estimated lactation yields with those observed both in a reference plan, where the two daily milkings were recorded at weekly intervals, and in the official A4 milk recording (monthly records of the two daily milkings). Estimates of lactation yields were less precise when the usual monthly designs were compared with a weekly sampling of both a.m. and p.m. milkings. The losses of precision were high at 9.4-36.2% including the A4 plan. The yield from only the milking period was more predictable than milk yield from the whole lactation (suckling and milking periods) and should consequently be adopted in dairy ewes. All options with one daily milking every month were more accurate when the corresponding plan was based on, or began with, the a.m. milking (loss of precision 14.9-15.8%). There was no evidence of improvement in sampling accuracy by adjusting for the preceding interval between milkings or production level. For practical and economic reasons, the design alternating a.m. and p.m. milkings every month, without adjustment, is suggested for ovine milk recording.     

Individual differences in behavioral and physiological responsiveness of primiparous dairy cows to machine milking

An experiment was performed in primiparous dairy cows (n = 23) to examine consistency of individual differences in reactivity to milking, and correlations between measures of behavior, physiology, and milk ejection. Responsiveness to milking was monitored during the first machine milking, on d 2 of lactation, and during milkings on d 4 and 130 of lactation. Measurements included kicking and stepping behavior, plasma cortisol and plasma oxytocin, heart rate, milk yield, milking time, milk flow rate, and residual milk obtained after administration of exogenous oxytocin. With repeated early lactation milkings, residual milk and the incidence of abnormal milk flow curves decreased. On d 130 of lactation all heifers exhibited normal milk ejection. Except for higher plasma cortisol concentrations on d 2, all measures were consistent over time between d 2 and 4 of lactation as indicated by significant rank correlations. Individual differences in the behavioral response to udder preparation were consistent over time between early lactation milkings and d 130 of lactation. Residual milk, milk yield, maximum milk flow rate, plasma oxytocin and heart rate during udder preparation were similarly interrelated on d 2 and 4 of lactation. High heart rate responses on d 2 and 4 were associated with enhanced inhibition of milk ejection. In contrast, behavior recorded during the milking process was unrelated to ease of milk removal. Our results indicate that milking at the beginning of lactation may be stressful to some heifers, to the extent that milk ejection is inhibited, but less disturbing to others. The existence of consistent behavioral and physiological responses in the present study suggests that responsiveness of dairy heifers to milking is mediated by stable animal characteristics.     

Long- and short-term effects of omitting two weekend milkings on the lactational performance and mammary tight junction permeability of dairy ewes

The long- and the short-term effects of omitting 2 milkings weekly in early (wk 8 to 14) and mid lactation (wk 15 to 22) were investigated in an experiment conducted with a total of 58 dairy ewes (40 Manchega and 18 Lacaune). Ewes submitted to 2 milking omissions were milked twice daily from Monday to Friday (0800 and 1800 h), and once daily on Saturday and Sunday (1600 and 1400 h, respectively). Individual data were collected for milk yield (weekly), milk composition (biweekly), and somatic cell count (SCC; monthly). Omitting 2 milkings per week in early lactation tended to decrease milk yield in Manchega ewes (−15%), whereas no effects were observed in Lacaune ewes. Averaged milk composition was not modified by milking omissions in either breed. Milking omissions in late lactation did not affect milk yield and milk composition in either breed. The SCC were unaffected by milking omissions in both breeds and in both stages of lactation. A sample of 22 Manchega and 11 Lacaune ewes were used to evaluate the short-term (daily) effects of the 2 milking omissions per week on milk yield and composition, udder health, and tight junction permeability, both in early lactation (wk 12) and in mid lactation (wk 20). Milking omission decreased milk yield, milk fat, and milk lactose contents on the first omission day in both breeds, with losses being more noticeable in early lactation than in mid lactation. Milk protein content and SCC did not vary by effect of the weekend milking omissions. After restoring the twice-daily milking routine on Monday, milk yield showed a compensatory increase that was greater in the large-cisterned than in the small-cisterned ewes, which allowed milk yield to return to Friday values in both breeds. Milk fat content increased during Sunday and Monday, reestablishing Friday values thereafter in both breeds. Weekend milking omissions in early lactation caused tight junction leakiness in both breeds, but mammary epithelia adapted to extended milking intervals when applied successively, recovering their tight state after milking. In mid lactation, the mammary tight junction showed leakiness only in Manchega ewes. In conclusion, 2 milkings per week could be omitted with no negative effects on milk yield, milk composition, and milk SCC values in large-cisterned dairy ewes, as observed in Lacaune and large-cisterned Manchega ewes. Losses in milk yield could be reduced if milking omissions were done from mid lactation in small-cisterned ewes.     

Mammary cisternal size, cisternal milk and milk ejection in Murrah buffaloes

The internal arrangement of the mammary gland cavity system, cisternal and alveolar milk fractions and the characteristics of milk ejection were investigated in buffaloes. Twenty-four Murrah buffaloes in three different stages of lactation and of two age groups were used. Continuous ultrasound cross-sections during milk ejection induced by exogenous oxytocin were performed to record the latency period of milk ejection. Buffaloes had small cisterns and the cavity area in the teat and gland regions were not significantly different (P > 0.05). The animals had long teat canals (3.1 +/- 0.1 cm), longer in the hind than fore quarters. Cisternal milk yield was low (0.17 +/- 0.01 kg) and cisternal fraction was only 4.9 +/- 0.1% of the total milk. The cisternal area (cm2) was 69.6 +/- 4.6, 51.61 +/- 4.8 and 26.01 +/- 4.8 while the cisternal yield (kg) was 0.32 +/- 0.05, 0.18 +/- 0.05 and 0.05 +/- 0.05 in early, mid and late lactation, respectively. A close correlation (r = 0.87, P < 0.05) existed between the ultrasound cisternal area and cisternal milk yield. The latency period of induced milk ejection was similar to that reported for cows (25 +/- 1 s) and was negatively correlated with milk yield (r = -0.75, P < 0.05). Milk ejection occurred shortly after elevated oxytocin concentrations were present. Delayed milk ejection reported earlier in this species must therefore be due to the absence of cisternal milk and delayed oxytocin release. An increase in teat length and circumference at milk ejection was also evident in the ultrasound cross sections.     

Genetic parameters of several measures of milk flow rate and milking time.

Heritabilities and genetic correlations of milking rate, time, and yield were estimated by intrasire regression of daughter on dam for 252 Holstein pairs. Heritabilities were: lactation yield, .51 +/- .11; peak rate, .47 +/- .11; bucket milk yield, .38 +/- .10; average rate, .37 +/- .12; milk yield to .45 kg/min, .19 +/- .12; total time, .17 +/- .12; duration of peak rate, .10 +/- .10; strip time, .08 +/- .15; yield during peak rate, .07 +/- .11; and strip yield, .01 +/- .17. The genetic correlation of peak rate with lactation milk yield was .69 +/- .08, indicating substantial genetic improvement in milking rate by selecting for milk production. The genetic correlation between total milking time and lactation milk yield was .50 +/- .20, indicating an increase in total milking time due to selection for milk. Direct selection for peak rate would provide an opportunity to reduce total milking time.     

Accuracy of simplified sampling procedures for estimating milk composition in dairy ewes

Simplified designs of milk-composition recording, based on information from a single monthly milking, adjusted or not for interval between milkings and for milk yield, were simulated and evaluated for 2553 ewe-test-day records belonging to 155 lactations of Churra dairy ewes. Accuracy of simplified methods was evaluated by comparing estimated trait values (fat, protein, casein and total solid yields) with those observed both in a reference plan, where the two daily milkings were recorded at weekly intervals (A1), and in the official A4 milk recording (monthly records of the two daily milkings). Trait yields per lactation were estimated and adjusted to the only milking period (days in milk 30-120). Estimates of milk component traits were less precise when monthly designs, including the A4 design, were compared with a weekly sampling of both a.m. and p.m. milkings, with fat yield being the most difficult trait to estimate. All options with one daily milking every month were more accurate when the corresponding plan was based on, or began with, the a.m. milking. Adjustment for the preceding interval between milkings or milk yield did not improve sampling accuracy. The design alternating a.m. and p.m. milkings every month, without adjustment, is suggested for recoding milk component traits in dairy ewes.     

Effect of weaning system on commercial milk production and lamb growth of East Friesian dairy sheep

East Friesian crossbred ewes (n = 99) and their lambs (n = 232) were used to study the effects of three weaning systems on milk production and lamb growth. Prior to parturition, a ewe and her lambs were assigned to one of the following three treatments for the first 28 +/- 3 d of lactation: 1) ewes weaned from their lambs at 24 h postpartum, ewes machine milked twice daily, and their lambs raised artificially (DY1); or 2) beginning 24 h postpartum, ewes separated from their lambs for 15 h during the evening, ewes machine milked once daily in the morning, and their lambs allowed to suckle for 9 h during the day (MIX); or 3) ewes not machine milked and exclusively suckled by their lambs (DY30). After the treatment period, lambs were weaned from MIX and DY30 ewes, and all three groups were machine milked twice daily. Daily commercial milk yield and milk composition were recorded weekly or twice monthly, and lambs were weighed at weaning or at 28 d and at approximately 120 d of age. Average lactation length (suckling + milking period) was 183 +/- 5 d and was similar among weaning systems. Differences among weaning systems for milk yield, milk fat and protein percentages, and somatic cell count were highly significant prior to and around weaning, and became nonsignificant by 6 wk in lactation. Total commercial milk production was greatest for DY1 and MIX ewes (261 +/- 10 and 236 +/- 10 kg/ewe, respectively) and least for DY30 ewes (172 +/- 10 kg/ewe). Daily gain of lambs to 30 d and weight at 30 d were similar regardless of weaning system; however, by 120 d, DY30 lambs tended to be heaviest, MIX lambs intermediate, and DY1 lambs lightest (47.3 +/- 1.6, 45.9 +/- 1.8, and 43.7 +/- 1.2 kg, respectively). Overall financial returns for milk and lamb sales were greatest for the MIX system because of the increase in marketable milk during the first 30 d of lactation compared with the DY30 system and because of acceptable 120-d lamb weights without the expenses of artificial rearing compared with the DY1 system. A mixed system of suckling and milking during early lactation appears to be a valuable management tool for dairy sheep production.     

Predicting milk yield in sheep used for dairying in Australia

It is necessary to identify traits that are simple to measure and correlated with milk yield to select ewes for dairying from existing populations of sheep in Australia. We studied 217 primiparous and 113 multiparous (second parity, n = 51; third parity, n = 40; and fourth parity, n = 22) East Friesian crossbred ewes, for 2 consecutive lactations, that were milked by machine following a period of suckling (24 to 28 d). We measured lamb growth, milk production, milk yield, and residual milk during early lactation (<d 60 of lactation) to test whether milk production during the suckling period or the growth rate of the lamb predicts milk yield. Milk production at weaning, or the amount of residual milk, or both, predict milk yield within lactations. These measures also predict milk yield between lactations. Lambs were weighed at birth and weaning and milk production in ewes was measured using a 4-h milk production test at d 5 of lactation and at weaning. Following weaning, ewes were milked twice daily and milk yield was recorded weekly for 8 wk and once a month thereafter. Milk production (using a 16-h milk production test) and residual milk were measured at weaning, and again 1 wk and 4 wk later. Milk yield to 120 d was correlated (r² = 0.39) between lactations, and 120-d milk yield (primiparous 82.7 ± 2.0 L; multiparous 107.1 ± 4.2 L; second lactation 146 ± 3.7 L) can be predicted after 4 wk of machine milking using a single measurement of either daily milk yield (primiparous 770 ± 25 mL/d; multiparous 940 ± 44 mL/d; second lactation 1,372 ± 46 mL/d, r² = 0.60 to 0.65) or daily milk production (primiparous 1,197 ± 27 mL/d; multiparous 1,396 ± 62 mL/d; second lactation 1,707 ± 45 mL/d, r² = 0.50 to 0.53). Residual milk in primiparous ewes (38%) and multiparous ewes (34%) was high (292 ± 11 and 321 ± 20 mL, respectively) in the first lactation, but lower (17%) in the second lactation (238 ± 17 mL). Residual milk and 120-d milk yield were not correlated in either lactation and we suggest that the transfer of milk from the alveoli to the cistern between each milking may be an important mechanism that maintains milk yield in these ewes.     

Effect of omission of machine stripping on milk production and parlor throughput in East Friesian dairy ewes

Due to the large cisternal storage capacity and non-vertical teat placement in most dairy ewes, machine stripping is commonly performed to remove milk not obtained by the machine. However, stripping requires individual manual intervention, lengthens the milking routine, and could inadvertently lead to overmilking of other ewes in the parlor. The objective of the present experiment was to estimate the effect of omission of machine stripping on milk production and parlor throughput. East Friesian crossbred dairy ewes that had been machine milked and stripped twice daily from d 0 to 79 postpartum, were randomly assigned to two stripping treatments for the remainder of lactation: normal stripping (S, n = 24), or no stripping (NS, n = 24). The NS ewes yielded 14% less commercial milk during the experiment, but had similar lactation length, milk composition, and somatic cell count compared to S ewes. Average machine milk yield (amount of milk obtained without manual intervention) tended to be greater for NS compared to S ewes. Average machine-on time for S ewes was longer than for NS ewes because of stripping, which may have resulted in over-milking of many ewes in the S group. Results from a milking simulation indicated that parlor throughput would increase by 33%, and overmilking would not occur when stripping was omitted from the milking routine. These results collectively suggest that residual milk left in the udder as a result of omission of machine stripping does not negatively influence milk quality and the loss in commercial milk yield could be compensated for by improved parlor throughput.     

Lactational changes in oxytocin release, intramammary pressure and milking characteristics in dairy cows.

Two experiments were conducted to investigate possible changes of milking-related oxytocin release (Expt 1) and of intramammary pressure and milking characteristics (Expt 2) throughout entire lactations in German Braunvieh dairy cows. Mean oxytocin concentrations after stimulation at onset of milking increased from 18.3 +/- 15.9 to 30.7 +/- 24.1 pg/ml in Expt 1 and decreased from 23.9 +/- 17.6 to 15.4 +/- 9.1 pg/ml in Expt 2, respectively, but remained above the level necessary to elicit complete milk ejection in both trials. Premilking baseline intramammary pressure had its maximum in early lactation until about month 4 and then decreased to approximately 50% of its initial level. Ejection pressure followed a similar pattern, but dropped only to approximately 75% of its maximum. This was due to the constant elevation of pressure increase, reaching its highest level in late lactation. Time from commencement of stimulation until maximum pressure exceeded 1 min in almost all instances even in early lactation and increased throughout lactation. Despite the normal decrease of milk yield average milk flow fell only slightly while maximum flow rate remained almost constant. Pressure increase, milk yield and milk flow were not different after 1 min and after extended stimulation. Thus there were no indications of a decreasing sensitivity of the milk ejection reflex during lactation, and milking characteristics were positively affected by intense teat stimulation. Suggestions for practical dairying are made.     

Analysis of milking characteristics in New Zealand dairy cows

The objective of this study was to describe the variation in milking characteristics, and factors associated with these traits, in grazing dairy cows milked without premilking stimulation. Milk yield, duration, and average and maximum milk flow rate data were collected from 38 farms in New Zealand at 2 time points (spring and autumn) during the 2010 to 2011 season. Subsequently, a second data set, allowing the generation of daily milk flow profiles, was collected from 2 farms in the 2011 to 2012 season. Corresponding animal data, such as breed, date of birth, and ancestry information, were extracted from the New Zealand Dairy Industry Good Animal Database (New Zealand Animal Evaluation Ltd., Hamilton, New Zealand). Residual milking duration (deviation from the regression line of milk yield on milking duration) was calculated, allowing the identification of fast-milking cows independent of milk yield. Variance components for the milking characteristics traits were estimated using an animal linear mixed model. The average milk yield was 10 kg/milking and the average milking duration was 360 s. The average milk flow rate was 1.8 kg/min and maximum milk flow 3.3 kg/min, with 44% of milk flow curves being classified as bimodal. Primiparous animals exhibited different milk flow profiles, with a lower maximum flow, than multiparous animals, possibly due to differences in cisternal capacity. Residual milking duration was shortest (−10 s) in mid-lactation (121–180 d) and was 13 s longer for Jersey compared with Friesian cows; however, it was 19 s shorter when adjusted for energy content. Residual milking duration had a negligible genetic correlation (−0.07) with milk yield, indicating that selection for cows with shorter residual milking duration should have a negligible effect on milk yield. A heritability of 0.27 indicated that residual milking duration could be valuable as part of a breeding program. Knowledge of the distribution of milking durations for a given milk yield allows farmers to choose appropriate cluster-on time when using a maximum milking time strategy to improve milking efficiency.     

Effect of omitting one milking weekly on lactational performances and morphological udder changes in dairy cows

The response of dairy cows to omitting one milking weekly was investigated in two successive experiments conducted with Holstein cows milked twice daily. Experiment 1 compared the lactational performances and udder changes in eight cows (31.2 L/d, 201 d in milk) in the 5 wk before and the 5 wk after introducing the suppression of one milking weekly. Milk yield was recorded daily and milk composition twice weekly. Milk partitioning in the udder (alveolar and cisternal milk) and cisternal size (ultrasonography), 8 h after milking, were also measured at the start and the end of the experiment. Although daily milk yield decreased 32% during the experiment (10 wk), linear regression analysis revealed a loss of milk yield of 1.1 L/d (3.7%) as a consequence of the omission of one milking weekly. Milk composition, lactation persistency, and somatic cell count (SCC) were unaffected by milking omission. Milk partitioning in the udder decreased by 38% in alveolar milk volume and showed a tendency to decrease in cisternal milk volume (15%) and cisternal size (7%), as a result of milking omission and advancing lactation. Loss in total milk yield was negatively related with cisternal milk volume (r = -0.77) and cisternal size (r = -0.70) indicating smaller losses in the udders with large cisterns. In Experiment 2, five cows (21.0 L/d, 227 d in milk) previously adapted to the milking omission schedule were used to study the daily effects of milking omission on milk yield, milk composition and udder health during 10 wk. Milk yield and milk composition were approximately constant but SCC increased with lactation stage. The omission of one milking caused an important decrease in milk yield, fat content and SCC on the omission day and a compensatory increase over the following 2 d, but milk protein and lactose did not vary. All variables reached the average weekly value three days after the milking omission (six milkings). In conclusion, under the conditions used, omitting one milking weekly slightly reduced milk yield and did not affect milk composition when healthy cows were used. Milk losses by milking omission depend on udder cistern characteristics; evaluating cistern size by ultrasonography may be a useful tool for choosing cows that are better adapted to a reduced milking frequency.     

Oxytocin release and lactation performance in Syrian Shami cattle milked with and without suckling

Oxytocin (OT) release and lactation performance in primiparous Syrian Shami cows were evaluated in response to two different machine milking regimes. Six cows were milked in the presence of the calves (PC) and subsequently suckled, whereas six cows were exclusively machine milked without the presence of their calves (WC) until day 91 post partum. Milk yield and milk constituents were determined weekly. The degree of udder evacuation was determined by the succeeding removal of residual milk. PC released OT during the milking process, whereas in WC no OT release was detected throughout the milking process. Consequently, the residual milk fraction was much lower in PC than in WC (11% v. 58%, P<0.05) and daily milk yield until day 91 post partum was higher in PC than in WC (12.6+/-0.3 v. 7.1+/-0.4 kg, P<0.05). In conclusion, Syrian Shami cattle are not suitable to be exclusively machine milked without the presence of their calves.     

Improving parlor efficiency in block calving pasture-based dairy systems through the application of a fixed milking time determined by daily milk yield and milking frequency

Adjusting end-of-milking criteria, in particular applying a maximum milking time determined by expected milk yield at an individual milking session, is one strategy to optimize parlor efficiency. However, this strategy can be difficult to apply practically on farm due to large differences in session milk yield, driven by milking interval, which affects milking routines and can be limited by in-parlor technology. The objective of this study was to test the hypothesis that a single fixed milking time (duration) could be applied at all milking sessions without compromising milk production or udder health for a range of milking intervals. To test the hypothesis, 4 experimental herds were established: (1) herd milked twice a day (TAD) using a 10- and 14-h interval, (2) herd milked TAD using an 8- and 16-h interval, (3) herd milked 3 times in 2 d using a 10–19–19-h interval, and (4) herd milked once a day (OAD). Herds consisted of 40 cows each, and were established for two 6-wk experimental periods, one in peak lactation and the other in mid-late lactation. Within each herd, half the cows had an end-of-milking criterion of 0.35 kg/min (Flow), and the other half had milking ended after a fixed period of time (FixedT) based on the average milking session yield, the daily milk yield divided by average number of milkings per day, irrespective of milking interval. We found no differences in daily milk yield between end-of-milking criteria due to residual milk from one milking likely increasing the proportion of milk in the udder cistern at the next milking session for the FixedT treatment. However, fat yield was compromised when the percentage of the herd with a truncated milking exceeded an estimated 33% at a milking session, which occurred in the TAD 8–16 herd due to the divergence from the average milking interval (in the case of TAD, 12–12 h). Applying a fixed milking time had no detrimental effects on udder health, except in the OAD herd in mid-late lactation, which had both a higher cell count and new intramammary infection rate. This warrants further investigation, although the majority of cultured bacteria were coagulase-negative staphylococci (CNS). Consequently, we conclude that, in general, with appropriate monitoring (e.g., weekly inspection) to ensure the proportion of the herd with truncated milkings does not exceed 33%, farmers in pasture-based dairy systems can use a fixed milking time to improve parlor efficiency.     

Effect of presence or absence of corpora lutea on milk production in East Friesian dairy ewes

The potential luteal effects on milk production were examined in dairy ewes that were not superovulated in contrast to studies using superovulated ewes. Lactating East Friesian crossbred ewes (n = 24) were synchronized using intravaginal progesterone (controlled intravaginal drug-releasing device), PGF2alpha, and gonadotropins. After ovulation, corpora lutea (CL) were counted via laparoscopy on d 4 and 11. On d 5, ewes received either saline (CLYES, n = 12) or PGF2alpha (CLNO, n = 12) to allow CL persistence (2.4 +/- 0.3 CL on d 11) or regression (0 CL on d 11), respectively. Each ewe received two CIDR d 5 to 18 to maintain high concentrations of plasma progesterone (P4) and to suppress estradiol (E2). Each ewe received PGF2alpha on d 18. Data were collected during three periods (pretreatment: d 0 to 5; treatment: d 6 to 18; posttreatment: d 19 to 25). Milk yield and milking time were recorded daily, milk samples were obtained for analyses of fat and protein, and blood samples were collected for P4 and E2 immunoassay. During treatment, CLYES ewes had higher milk yield (1.56 vs. 1.44 +/- 0.01 kg/d), milk fat (92.2 vs. 81.1 +/- 1.3 g/d), and milk protein (83.7 vs. 77.5 +/- 0.8 g/d) compared with CLNO ewes, respectively. Differences were maintained posttreatment, despite luteolysis in CLYES ewes. Estradiol concentrations did not differ between treatments and were low after d 5. Milk production was increased in East Friesian ewes in the presence of an average of 2.4 corpora lutea, an effect independent of estradiol.     

Covariance among milking frequency, milk yield, and milk composition from automatically milked cows

Automatic milking systems allow cows voluntary access to milking and concentrates within set limits. This leads to large variation in milking intervals, both within and between cows, which further affects yield per milking and composition of milk. This study aimed to describe the degree to which differences in milking interval were attributable to individual cows, and how this correlated to individual differences in yield and composition of milk throughout lactation. Data from 288,366 milkings from 664 cow-lactations were used, of which 229,020 milkings had milk composition results. Cows were Holsteins, Red Danes, and Jerseys in parities 1, 2, and 3. Data were analyzed using a linear mixed model, with cow-lactation as a random effect and assuming heterogeneous residual variance over the lactation. Cow-lactation variance was fitted using linear spline functions with 5 knot-points. Residual variance was generally greatest in early lactation and declined thereafter. Accordingly, animal-related variance tended to increase with progression of lactation. Milking frequency (the reverse of milking interval) was found to be moderately repeatable throughout lactation. Daily milk yield expressed per milking was found to be highly repeatable in all breeds, with the highest values occurring by the end of lactation. Fat percentage had only moderate repeatability in early to mid lactation but increased toward the end of lactation. Individual level correlations showed that cows with higher milking frequency also had greater yields, but had lower fat percentage. Correlations were slightly weaker in very early lactation than in the remaining parts of lactation. We concluded that individual differences exist among cows milked automatically. Cows with higher yields are milked more often and have lower fat content in their milk.     

Changes in cisternal udder compartment induced by milking interval in dairy goats milked once or twice daily

Fourteen Murciano-Granadina dairy goats were used to evaluate udder compartments (cisternal and alveolar) and cisternal recoil after an oxytocin (OT) challenge at different milking intervals (8, 16, and 24 h) during wk 7 of lactation. Goats were milked once (1x; n = 7) or twice (2x; n = 7) daily from wk 2 of lactation. Average milk yields for wk 4 and 8 were 1.76 and 2.24 L/d, for goats milked 1x and 2x, respectively. For each half udder, cisternal area was measured by ultrasonography and cisternal milk was measured by machine milking after i.v. injection of an OT receptor blocking agent. Alveolar milk was then obtained after i.v. injection of OT. Regardless of milking frequency, alveolar milk increased from 8 to 16 h after milking, but did not change thereafter. Cisternal area and cisternal milk increased linearly (R2 = 0.96 to 0.99) up to 24 h, indicating continuous milk storage in the cistern at any alveoli filling degree. Cisternal to alveolar ratio increased with milking interval (from 57:43 to 75:25), but differences between milking intervals were significant at 8 h only, at which time goats milked 2x showed a greater ratio (1x = 51:49; 2x = 62:38). Despite extended milking intervals, cisterns of goats milked 1x did not become larger than cisterns of goats milked 2x after 5 wk of treatment. The highest correlation between cisternal area and cisternal milk was detected at 8 h after milking (r = 0.74). Primiparous goats had smaller cisternal areas and less cisternal milk than multiparous goats at all milking intervals. Cisternal recoil was studied in a sample of multiparous goats milked 1x (n = 4) and 2x (n = 4) by scanning cisterns by ultrasonography at 0, 5, 15, and 30 min after an OT challenge for each milking interval. Cisternal area increased after OT injection for the 8- and 16-h milking intervals, but no differences were observed for the 24-h interval. Unlike cows, no changes in cisternal area were observed after OT injection, indicating the absence of cisternal recoil in goats. We conclude that goats show a large cisternal compartment that increases linearly after milking. Nevertheless, cisternal size did not increased after 1x milking, probably because of lesser milk yield. Multiparous goats had larger cisterns than primiparous goats and were able to store more milk in their cisterns at all milking intervals. Because of the high capacity of goat cisterns, no milk return from cistern to alveoli is expected if milking is delayed after milk letdown.     

Reproductive performance and milk production of Assaf sheep in an intensive management system

The Assaf breed of dairy sheep, a stabilized cross of the Awassi and East Friesian breeds, has replaced the Awassi as the breed of choice in its country of origin, Israel, and has spread to other Mediterranean countries. In Israel the Assaf breed is managed under an intensive production system involving weaning lambs at birth, rearing them artificially, and milking ewes after parturition. There are several breeding periods in the year when ewes are mated following hormonally synchronized estrus. Records of 18,976 lactations from 5 farms were analyzed to investigate factors that influenced Assaf milk and reproductive performance. Lactation curves were fitted to each lactation, and a range of parameters and calculated values were analyzed. Daily milk yield records also were analyzed to describe a typical Assaf lactation and compared with those of the Awassi breed. Factors affecting age at first lambing also were studied. An average Assaf ewe kept under this intensive management regimen was found to produce 334 L of milk during a 173-d lactation. Mean litter size was 1.57 lambs/ewe lambing, and lambing interval was 272 d. Milk production was affected by litter size, with twin- and triplet-bearing ewes producing approximately 20 L more milk per lactation than single-bearing ewes. Day length was the major environmental variable influencing milk yield. The difference between midsummer and midwinter day lengths accounted for a difference in daily milk yield of 0.44 L in favor of summer. Ewe lambs that were mated for the first time at later ages produced more lambs and more milk due to greater early lactation characteristics. Milk production was found to be negatively associated with subsequent reproductive performance. Comparing these results with those from an earlier study in the Awassi breed, the Assaf was found to produce less milk during a shorter lactation than the Awassi, but its greater litter size made it a more profitable breed.