Effects of calving age, breed fraction and month of calving on calving interval and survival across parities in Irish spring-calving dairy cows

Association of age at first calving with longevity, milk yield, and fertility up to the third lactation in a herd of Holstein dairy cows in Japan

Accelerating age at first calving (AFC) is a strategy for sustainable dairy farming, whereas the impact of a reduction in AFC on long-term performance remains unclear. In this study, longevity and milk productivity until the end of the third lactation period were investigated retrospectively according to AFC. A total of 169 cows were categorized according to AFC as young, moderate, old, and very old (< 22.5, 22.5 -< 24.0, 24.0 -< 25.5, and > 25.5 months). The young AFC group had approximately 70 kg lower body weight before first calving (620 vs. 695 kg, P < 0.05) and experienced their first calving approximately 4.2 months earlier than the very old AFC group (21.9 vs. 26.1 months, P < 0.05). The survival rate at the third calving stage was 61% in the young AFC group, which was higher than those in the moderate (42%), old (35%), and very old (33%) AFC groups. In the young AFC group, no cows were culled because of low productivity and hoof disease, compared to 5.0-8.1% of older AFC cows. The young AFC group had a higher overall lifetime milk yield (cumulative milk yield/days from birth to the end of final lactation) than the old AFC group (14.3 vs. 8.7 kg/d, P = 0.11). The cows that survived the third calving had better reproductive performance than non-surviving cows; however, no statistical difference was detected among the AFC groups. In conclusion, AFC as early as 22.5 months could be associated with better survivability and higher overall lifetime milk yield than older AFC without impairing reproductive performance. Our results suggest that accelerating AFC may lead to higher profitability.

Impact of age at first calving on performance traits in Irish beef herds

Reducing age at first calving (AFC) has been a challenge in beef herds. There is anecdotal evidence that herd owners choose to calve heifers older because of the perceived consequences of calving heifers at 24 mo of age compared to 36 mo on performance traits in beef herds. The objective of this study was to estimate the association of calving heifers at younger ages on subsequent performance traits, calving interval, longevity, cow weight, dystocia, and progeny weaning weight for parities 1 to 5. Available to the study after data edits were 219,818 calving interval records, 219,818 longevity records, 118,504 cow live-weight records, 230,998 dystocia records, and 230,998 weaning weight records. Linear mixed models were used to quantify performance of each trait in AFC groups for each parity. As parity increased, there was a favorable reduction in calving interval and dystocia (P < 0.001), while the likelihood of cows surviving reduced (P < 0.001). Both cow live weight and progeny weaning weight increased as parity increased. Age at first calving only had a significant association with dystocia within parity 1 (P < 0.001), where older heifers at first calving subsequently had lower risk of calving. Calving interval for parity 1 cows was observed to be longer by 6 d in cows that calved for the first time at 33 to 36 mo compared to cows calved for the first time at 22 to 24 mo (P < 0.001). No statistical difference was observed for longevity between cows with an AFC of 22 to 24 mo compared to cows with an AFC of 33 to 36 mo (P > 0.05). Cows that calved at a younger age did wean lighter calves for their first three lactations (P < 0.01) but had no association with weaning weight for parity 4 and 5 cows (P > 0.05). Cows with a lower AFC were lighter for parity 1 to 4 (P < 0.001); at parity 5, AFC had no association with cow live weight (P > 0.05). The performance of mature cows for calving interval, longevity, calving difficulty, cow live weight, and weaning weight was not impacted by AFC. In conclusion, calving cows for the first time at younger ages do pose risks and associated performance loss but this risk and loss should be minimized by good management.

An Appropriate Genetic Approach for Improving Reproductive Traits in Crossbred Thai-Holstein Cattle under Heat Stress Conditions

Thailand is a tropical country affected by global climate change and has high temperatures and humidity that cause heat stress in livestock. A temperature−humidity index (THI) is required to assess and evaluate heat stress levels in livestock. One of the livestock types in Thailand experiencing heat stress due to extreme climate change is crossbred dairy cattle. Genetic evaluations of heat tolerance in dairy cattle have been carried out for reproductive traits. Heritability values for reproductive traits are generally low (<0.10) because environmental factors heavily influence them. Consequently, genetic improvement for these traits would be slow compared to production traits. Positive and negative genetic correlations were found between reproductive traits and reproductive traits and yield traits. Several selection methods for reproductive traits have been introduced, i.e., the traditional method, marker-assisted selection (MAS), and genomic selection (GS). GS is the most promising technique and provides accurate results with a high genetic gain. Single-step genomic BLUP (ssGBLUP) has higher accuracy than the multi-step equivalent for fertility traits or low-heritability traits.

A comparison of the age at first calving of contract-reared versus home-reared replacement dairy heifers

Recent rapid expansion and increased specialisation of dairy industries internationally has prompted growing interest in collaborative farming enterprises such as contract-rearing of replacement heifers. While contract-rearing provides obvious benefits to dairy farmers intending to expand their herd, it may adversely impact heifer reproductive performance as the farmer no longer manages the heifers on a day-to-day basis. Hence, the objective of this study was to determine if contract-rearing impacted heifer reproductive performance and ultimately achievement of the industry target age at first calving (AFC) of 24 months. Live weight, body condition score (BCS) and morbidity data were collected from 3908 heifers on 99 farms (54 contract-rearing dairy farms; 45 home-rearing dairy farms) on three occasions between one month of age and prior to commencement of their first breeding season; average daily gain (ADG) between farm visits was calculated. Heifer birth dates, breeds, economic breeding index (EBI), mating start dates (MSD) and calving dates were extracted from the national cattle breeding database. To assess the reproductive performance of study heifers, the outcome of interest was age at first calving (AFC). In addition to the effect of heifer rearing system, the association between pre-breeding ADG, body weight and BCS and AFC was also investigated. Descriptive and survival analysis was performed to calculate the distribution of AFC across study farms and to assess the association of rearing strategy and other variables on AFC. Results indicate that there was no difference in median AFC between home and contract-reared heifers (24.4 months). At univariate level, variables significantly associated with AFC were occurrence of pyrexia before MSD, pre-breeding ADG, body weight and BCS. However, when the frailty effect of farm was incorporated into the multivariable model, these variables were no longer significant, indicating wide variation in individual farm management practices not measured in this study. It was concluded that rearing strategy (home versus contract-rearing) did not significantly affect heifer AFC.

Business analysis of IRT, Visual observation, and Ovsynch as breeding strategies in Alberta dairies

The dairy industry is searching for new technologies to address low (<50%) estrus detection. However, the lack of information on the potential economic benefits regarding new technology implementation has led some dairy producers to continue using conventional estrus detection methods (e.g. visual observation of standing to be mounted). The objective of this study was to compare the costs of infrared thermography (IRT), visual observation (VO) and ovulation synchronization (Ovsynch: OVS) as breeding strategies at different accuracy levels (Sensitivity [Se], Specificity [Sp]) and pregnancy rates (PR). The costs associated with Breeding, Feeding, Operation Costs, Return to Equity and Culling Risk per estrus detection rate (ER; 30-100%, conception rate for OVS; 30-100%), PR [PR per Parity group; 1-2 (50%), 3-4 (43%), and >4 (41%)], and ER accuracy determined the potential financial benefit of each breeding method for a representative farm. Breeding Cost results (Canadian dollars per cow; CAD/cow) showed a higher cost of OVS (138.99) as compared to VO (115.78) and IRT (127.69). Pregnancy Costs were affected by Breeding Cost; however, ER had a significant effect on PR expense for each method, IRT (ER; 30%: 210.38; 100%: 132.19), VO (ER; 30%: 205.93; 100%: 129.39), and OVS (ER; 30%: 247.21; 100%: 155.33). The minimum Se level with a positive Financial Effect for IRT and VO was 60% with a Sp of 100%, and for the OVS was Se 65% and Sp 100%. However, when the Se was 100% a positive Financial Effect was observed with a minimum Sp of 85% for IRT and 75% for VO. Culling Risk was reduced if ER increases differently depending on the parity group. Implementing of IRT as an estrus detection method yields a competitive breeding cost compared to VO and OVS. Further, breeding methods must accomplish at least ∼60% accuracy to have a positive net return.

Association between age at first calving and lactation performance, lactation curve, calving interval, calf birth weight, and dystocia in Holstein dairy cows

In the present study, records on 115,291 heifers distributed in 113 herds were used to investigate the association between age at the first calving (AFC) and lactation performance, lactation curve, the length of the first calving interval (CI), calf birth weight (CBW), and the incidence of dystocia in Holstein heifers in Iran. Based on the AFC, the heifers were classified into eight classes: AFC of 541 to 690 d, 691 to 720 d, 721 to 750 d, 751 to 780 d, 781 to 810 d, 811 to 840 d, 841 to 900 d, and 901 to 1200 d (AFC1 to AFC8, respectively). Multiple regression mixed models were used to investigate the association between AFC and lactation curve parameters, partial and 305-d lactation performance, 100- and 305-d SCS, and the length of the first calving (CI) interval. The mean (SD) and median AFC across all heifers was 760.2 (74.01) and 750 d, respectively. Of 115,291 heifers included, 28,192 and 7,602 heifers were, respectively, ≤ 720 and > 900 d when calving for the first time. More than 44% of the heifers were at 691 to 750 d (23 to 25 months) of age when calving for the first time. An increased AFC was associated with increased partial and 305-d lactation performance, 100- and 305-d SCS, initial milk yield, milk production at the peak of lactation, upward and downward slopes of the lactation curve. The 305-d fat percentage was associated with AFC; however, there was no association between AFC and 305-d protein percentage. An increased AFC was also associated with decreased milk production persistency, delayed peak time, longer CI, and higher calf birth weight. Compared to heifers calving for the first time between 691 to 780 d (23 to 26 months) of age, both increasing and decreasing AFC were associated with increased risk of dystocia. Controlling AFC is an important management factor in achieving a lower risk of dystocia, higher lactation performance, lower SCS, and shorter length of the calving interval.

Age at first calving in UK Jersey cattle and its associations with first lactation 305-day milk yield, lifetime daily milk yield, calving interval and survival to second lactation

BACKGROUND

The association between age at fist calving (AFC) and production, fertility and survivability in Jersey heifers requires evaluation and comparison with published literature on Holstein heifers.

METHOD

Lactation records from 7256 pedigree Jersey heifers were grouped by AFC. Analysis comprised mixed effects multivariable regression modelling and binary logistic regression modelling.

RESULTS

Increased lifetime daily yield (LDY) was significantly associated with a reduced AFC; calving UK Jersey heifers at 24 months or below is optimal. Heifers calving at 25-26 months had a significantly lower mean calving interval (391 days; 95 per cent CI 386 to 397) than those calving over 30 months (399; 95 per cent CI 394 to 405) (P=0.03). Heifers with an AFC over 30 months were less likely to survive to second lactation than those with an AFC 18-24 months. Spring calving heifers produced significantly less milk per day (9.87 kg; 95 per cent CI 9.64 to 10.1) than autumn (10.18; 95 per cent CI 9.94 to 10.41) or winter calvers (10.07; 95 per cent CI 9.84 to 10.3) (P≤0.01). Heifers that calved in the winter had a significantly shorter calving interval (389 days; 95 per cent CI 394 to 383) (P≤0.01) than heifers calved in other seasons.

CONCLUSION

The impact of AFC on Jersey production, fertility and longevity traits shows a similar trend to that seen in the Holstein, potentially on a lesser scale.

Effect of Age at First Calving on First Lactation Milk Yield, Lifetime Milk Production And Longevity of Cows

The objective of the study was to analyse the effect of age at first calving (AFC) on first lactation milk yield, lifetime milk production and longevity of dairy cows. The study used SYMLEK data on the milk yield and culling of 111 857 Polish Holstein-Friesian cows from the active population in Pomerania and Kujawy, Poland. The cows first calved during 2000–2009 and were used or removed from the herds until the end of 2015. For each cow, calculations were made of first lactation milk yield, lifetime milk production, first lactation milk yield per milking day, lifetime milk production per day, as well as the lifespan (length of productive life, duration of rearing) and number of calvings. Based on the age at first calving (duration of the rearing period), the cows were grouped as follows: ≤22, 22.1–24.0, 24.1–26.0, 26.1–28.0, 28.1–30.0, 30.1–32.0 and >32 months. The GLM, CORR PEARSON and FREQ procedures from the SAS package were used in the statistical calculations. Considering first lactation milk yield, lifetime milk production and longevity, it is recommended that the cows should be first milked between 22.1 and 26.0 months of age. This is supported by the approximately 24% higher lifetime milk production in relation to the latest calving cows (P<0.01). Late commencement of the first milking (especially after 28 months) causes a considerable decrease in the first lactation milk yield and lifetime milk production, shortens the productive period, reduces the number of calvings, and increases culling rate due to low milk yield and udder diseases.

Associations between age at first calving and subsequent lactation performance in UK Holstein and Holstein-Friesian dairy cows

Lactation records from 396,534 pedigree Holstein and Holstein-Friesian primiparous cows from 6,985 UK milk recorded herds, calving for the first time during the period between the 1^st of January 2006 and the 31^st of December 2008, were examined in order to determine the associations between age at first calving (AFC) and subsequent production, udder health, fertility and survivability parameters. Heifers were grouped by AFC into single month classes ranging from 21 to 42 months. Mixed effects multivariable regression modelling was used for data analysis. Mean and median AFC were 29.1 and 28 months respectively. Within the study, only 48,567 heifers (12.3% of the studied population) calved for the first time at 24 months of age or younger. 162,157 heifers (40.9%) were 30 months or older at their first calving. An increased AFC was associated with increased first lactation milk, fat and protein yields. The lowest predicted mean 305-day yield (6,617kgs; 95% confidence interval (CI): 6,546–6,687 kgs) was recorded for the 21 month AFC class, significantly lower than any other class. The 36 month AFC class had the highest predicted mean (7,774 kgs; 95% CI: 7,737–7,811 kgs). However, an increased AFC was also associated with increased calving interval and increased first lactation somatic cell count (SCC). Animals calving at 21 months had a predicted mean lactation SCC of 72,765 (95% CI: 68427–77378). Animals calving at 36 months of age had a predicted mean lactation SCC of 86,648 (95% CI: 84,499–88,853). Importantly, an increased AFC was also associated with decreased lifetime daily milk yield and decreased likelihood of calving for a second successive time. Animals calving at 22 months of age had a predicted mean daily lifetime milk yield of 15.24 kgs (95% CI: 15.06–15.35); animals calving at 36 months of age had a predicted mean daily lifetime milk yield of 12.83 kgs (95% CI: 12.76–12.91). Our results highlight the importance of achieving a lower age at first calving which was here associated with improved udder health, increased lifetime daily milk yield, improved reproductive performance and increased likelihood of calving for a second time.

Relationship between in vitro sperm functional assessments, seminal plasma composition, and field fertility after AI with either non-sorted or sex-sorted bull semen

The hypothesis of this study was that different in vitro parameters are required to predict the in vivo fertility of non-sorted (NS) and sex-sorted (SS) semen. Thus, the aim was to correlate in vitro bull sperm functional parameters (experiment 1) and seminal plasma composition (experiment 2) with pregnancy rates using 2 cohorts of bulls (NS and SS). Experiment 1: ejaculates from each bull (n = 3 ejaculates per bull; n = 6 bulls for both NS and SS) were assessed for motility, thermal stress tolerance and morphology using microscopy, and viability, osmotic resistance, mitochondrial membrane potential, and acrosome integrity using flow cytometry. Fertilizing ability was assessed using IVF. Experiment 2: ejaculates (n = 3 per bull; n = 8 and 6 bulls for NS and SS, respectively) were collected, seminal plasma harvested and frozen and later analyzed for amino acid and fatty acid composition using gas chromatography mass spectrometry. In the NS cohort of bulls, there was no correlation between pregnancy rate and any of the sperm functional parameters assessed. However, within the SS cohort, motility and viability were correlated with pregnancy rate (r = 0.84 and 0.80, respectively; P < 0.05). There was no correlation between IVF outcome and pregnancy rate in either the SS or NS cohort of bulls. In the NS cohort of bulls, concentrations of the amino acid isoleucine and the fatty acid tricosylic acid (C23:0) were correlated with pregnancy rate (r = 0.80 and 0.74, respectively; P < 0.05). Within the SS cohort of bulls, the amino acid glutamic acid and the fatty acid arachidic acid (C20:0) were correlated with pregnancy rate (r = 0.84 and 0.82, respectively; P < 0.05). In conclusion, this study suggests that different in vitro markers of fertility are required to predict the fertility of NS and SS sperm.

Prevalence of BoHV-1 seropositive and BVD virus positive bulls on Irish dairy farms and associations between bull purchase and herd status

Background

BVD and IBR are contagious viral diseases highly prevalent in Irish cattle. Despite their significant reproductive and economic impact very little is known about the BVD and IBR status of stock bulls (a bull used for breeding purposes). There are still a high proportion of dairy farms in Ireland that rely on the use of a bull for breeding cattle and ensuring the fertility of the bulls is of paramount importance for the efficiency of the farms. The prevalence of BoHV-1 and BVD in stock bulls in Irish dairy herds has never been investigated. The objectives of this study therefore were: (i) to provide descriptive, observational data on the use of stock bulls on Irish dairy farms; (ii) to investigate the BVD and BoHV1 status of a sub-set of stock bulls; (iii) to investigate factors associated with BVD and BoHV1 status of stock bulls and (iv) to investigate factors associated with dairy herd status for BVD and BoHV1, including any associations with the use of stock bull.

A total of 529 blood samples from bulls involved in the dairy breeding process were analysed for BVD virus using RT-PCR, and BoHV-1 antibodies by ELISA test. A total of 305 different dairy herds took part in the study and the overall BVD and BoHV-1 herd status was determined by ELISA using four bulk tank milk samples over the 2009 lactation. Logistic regression was used to investigate the associations between the stock bulls and BVD and BoHV-1 herd and individual status.

Results

Of the 305 total participating farms, 235 farms (77 %) had at least one bull and 167 farms had purchased bulls. Two bulls (0.4 %) out of 529 tested were found positive for BVD virus and 87 (16.7 %) tested seropositive for BoHV-1. Some significant associations were identified between the purchase of bulls and both viral diseases. Purchased bulls were three times more likely to be seropositive for BoHV-1 than homebred bulls. In the same way, herds with purchased bulls were three times more likely to be classified as seropositive for BVD and four times more likely to have evidence of recent BoHV-1 circulation than farms where all the bulls were homebred.

Conclusions

The prevalence of BoHV-1 and BVD in stock bulls in Irish dairy herds has never been investigated. This study highlights the widespread use of stock bulls in Irish dairy herds, as well as the high rate of exchange of bulls between farms. Significant associations were found between the origin of the bull and their serological BoHV-1 status. In keeping with these results, bulls with higher number movements between farms were more likely to be seropositive for BoHV-1.

Effect of exogenous progesterone supplementation in the early luteal phase post-insemination on pregnancy per artificial insemination in Holstein-Friesian cows

One of the main determining factors of pregnancy per artificial insemination (P/AI) is an optimum concentration of progesterone (P4) in the early luteal phase. This study examined the effects of P4 supplementation on P/AI in lactating Holstein-Friesian cows. A total of 453 cows in 8 spring-calving herds were used in the study. Following AI, cows were randomly assigned to 1 of 2 treatment groups: (1) no subsequent treatment (control; n=221); (2) insertion of a Controlled Internal Drug Release device (CIDR) from day 4 to day 9 post-estrus (supplemented; n=232). Pregnancy per AI was determined by transrectal ultrasonography at day 30 following AI. Insertion of a CIDR increased concentrations of milk P4 in supplemented cows by 4.78ng/mL between day 4 and 4.5 in comparison with a 0.55ng/mL increase in control cows. Progesterone supplementation from day 4 to 9 after AI decreased P/AI by 12 percentage points (56 vs 44%). There was a positive linear and quadratic relationship between P/AI and milk concentration of P4 on day 4 post-estrus in control cows. An optimum concentration of 2.5ng/mL on day 4 was calculated from the logistic regression curve to achieve a probability of P/AI of 65%. When both treatments groups were included in the analysis, there was no association between P/AI and concentrations of P4 on day 4. The results of the study indicate that supplementation with P4 initiated in the early luteal phase had a negative effect on P/AI in dairy cows.

Simultaneous estimation of genotype by environment interaction accounting for discrete and continuous environmental descriptors in Irish dairy cattle

Genotype by environment interaction can be analyzed by using a multi-trait model in which a trait measured in different environments is considered as separate traits. Alternatively, it can be analyzed by using a reaction norm model, in which the trait is considered a function of an environmental descriptor. Here, a model is developed where the 2 approaches are combined such that the effect of a continuous environmental descriptor can be analyzed in 2 or more discrete environments. The model is applied to somatic cell score (SCS) in relation to average herd milk production in 2 production environments: spring calving and year-round calving in Ireland. Heritabilities and additive genetic variances for SCS increased somewhat with increasing milk production and were higher in year-round calving. Under the combined model, the genetic correlation between spring and year-round calving was estimated at 0.82 to 0.84, clearly lower than obtained in a bivariate analysis ignoring effects of herd milk production. Thus, when estimating the genetic correlation between environments, effects of one environmental descriptor may be obscured by another, but can be disentangled in an analysis combining the reaction norm and the multi-trait approach. Such models will be especially useful for analyzing questions such as whether the effect of increasing production or temperature is more severe in different production systems or geographic regions.

Factors influencing reproductive performance of cows from different Nguni ecotypes in southern Mozambique

The objective of this study was to assess the reproductive performance of two Nguni ecotypes (Nguni and Landim) raised in a subtropical environment to enhance strategies for livestock development and restocking programmes within the southern African region. Reproduction data collected between 1996 and 2009 from 365 cows of the Landim and Nguni ecotypes were analysed. From the results, ecotype, place of birth, year and season of birth/calving had significant effects on age at first calving (AFC) and calving interval (CI). Overall means for AFC and CI were 1,071 ± 166 days and 432 ± 85 days, respectively, while average calving rate was 88.0 ± 4.7%. Heifers born in the dry season had lower AFC than heifers born in the wet season. Heifers born at Impaputo Breeding Center were the youngest at first calving, followed by the South African born ones. Heifers of the Landim ecotype also calved younger than heifers of the Nguni ecotype. CI was shorter in wet seasons (main breeding seasons) than in dry seasons. Interaction between ecotype and year-season (p < 0.005) showed that, in wet and dry seasons, Nguni cows had shorter CI than the Landim. This study demonstrates for the first time a possible genotype-by-environment interaction between Nguni ecotypes. This might aid future cattle development and restocking programmes in southern Africa taking into consideration the adaptation of indigenous genotypes and climate change.

Crossbreeding effects on milk yield traits and calving interval in spring-calving dairy cows

The aim of the study was to assess crossbreeding effects for 305-day milk, fat, and protein yield and calving interval (CI) in Irish dairy cows (parities 1 to 5) calving in the spring from 2002 to 2006. Data included 188,927 records for production traits and 157,117 records for CI. The proportion of genes from North American Holstein Friesian (HO), Friesian (FR), Jersey (JE) and Montbéliarde (MO) breeds, and coefficients of expected heterosis for HOxFR, HOxJE and HOxMO crosses were calculated from the breed composition of cows' parents. The model used to assess crossbreeding effects accounted for contemporary group, age at calving within parity, linear regression on gene proportions for FR, JE and MO, and linear regression on coefficients of expected heterosis for HOxFR, HOxJE and HOxMO, as fixed effects, and additive genetic, permanent environmental and residual as random. Breed effects for production traits were in favour of HO, while for CI were in favour of breeds other than HO. The highest heterosis estimates for production were for HOxJE, with first-generation crosses yielding 477 kg more milk, 25.3 kg more fat, and 17.4 kg more protein than the average of the parental breeds. The highest estimate for CI was for HOxMO, with first-generation crosses showing 10.2 days less CI than the average of the parental breeds. Results from this study indicate breed differences and specific heterosis effects for milk yield traits and fertility exist in Irish dairy population.

Effects of Varying Lactation Length on Milk Production Capacity of Cows in Pasture-Based Dairying Systems

The aim of this experiment was to quantify the milk production capacity of cows undergoing extended lactations while fed a pasture-based diet typical of those used in the seasonal-calving dairying systems of Victoria, Australia. One hundred twenty-five Holstein cows were randomly assigned to 1 of 5 groups. Breeding was progressively delayed after calving to enable management of the groups for lactation lengths of 10, 13, 16, 19, and 22 mo (equivalent to calving intervals of 12 to 24 mo). Cows were provided with a daily energy intake of at least 180 MJ of metabolizable energy/cow. This was supplied primarily by grazed pasture with supplementary cereal grain, pasture silage, and hay. Cows were dried off when milk volume fell below 30 kg/wk or when they reached 56 d before their expected calving date. Most cows (>96%) could lactate above this threshold for 16 mo, >80% for 19 mo, and >40% for 22 mo. There were negative relationships between lactation length and annual production of milk and milk solids (milk fat + protein), but losses were small until 16 mo. Annualized yields of milk solids were 497, 498, 495, 474, and 463 kg/cow for the 10, 13, 16, 19, and 22 mo groups, respectively. This reduction in annual production of milk solids with increasing lactation length was relatively less than for milk volume because during extended lactation, cows produced milk with higher concentrations of protein. Cows undergoing extended lactations also finished their lactations having gained more body weight and body condition than cows lactating for only 10 mo. The data showed that many cows on pasture-based diets were capable of lactating longer than the 10 mo that is standard for Victorian herds with seasonally concentrated calving patterns. Further, such extended lactations could be achieved with little penalty in terms of annual milk solids production.