The effect of hCG administration five days after insemination on the first service conception rate of anestrous dairy cows

The aim of this study was to investigate the effect of treating anovulatory anestrous (AA) dairy cows with 1500 IU of hCG IM, 5 d after insemination, on their first service conception rate. A clinical trial was conducted during the 2003/2004 breeding season involving 442 AA dairy cows in six herds. On Day -8, all cows were treated with a progesterone-containing intravaginal device (Cue-Mate). The devices were removed on Day -2, and on Day -1 all cows received an IM injection of 1mg of estradiol benzoate. Cows in the control group (n=220) received no further treatments. Cows in the treatment group (n=222) which had been inseminated on Days 0 or 1 were treated with 1500 IU of hCG IM 5 d after insemination. Blood was collected from 30 cows (15 in each group) on Days 5 and 12 after AI for analysis of plasma P4 concentration. There was no difference in first service conception rates between the control and treatment groups (46.3% versus 43.6%, respectively; P=0.68), despite the fact that plasma P4 concentrations were higher in the treatment group on Day 12 (4.9+/-1.3 ng/mL versus 6.2+/-2.7 ng/mL for control and treatment groups, respectively; P<0.01). In conclusion, 1500 IU of hCG 5 d after insemination did not improve first service conception rate in AA dairy cows.     

Resynchrony of postpartum dairy cows previously treated for anestrus

Cows that are diagnosed as anestrus (defined as >3 weeks postpartum, not detected in estrus by 1 week before commencement of seasonal breeding and without a palpable corpus luteum) have a lower probability of being inseminated, a lower conception rate to insemination and hence a lower probability of pregnancy within the breeding period. A current progesterone (P) and estradiol benzoate (EB) treatment for anestrus results in an 85% insemination rate, but only 57% of cows not conceiving to that first insemination are subsequently detected in estrus 14-28 days later. Resynchrony after an initial anestrous treatment has been used to increase the probability that cows not conceiving to first insemination will subsequently be detected in estrus. Anestrous cows (n=971) were initially treated with an intravaginal P-releasing device for 6 days and given 1 mg EB 1 day after device removal (day of EB treatment=Day 0). Cows detected in estrus between Days 0 and 3 were assigned randomly to be treated with reinsertion of a used P-releasing device for 8 days (commencing on Day 15), with 0.5 mg EB at reinsertion and again 1 day after removal (EB-RS), treatment as for EB-RS group but with substitution of 250 microg GnRH for EB at device reinsertion (GnRH-RS), or left as untreated controls (no-RS). Resynchrony treatments increased the proportion of non-pregnant cows detected in estrus from Days 14 to 28 compared to no-RS (79.1, 69.8, and 55.1% for EB-RS, GnRH-RS, and no-RS, respectively; P<0.05). Fewer cows were pregnant by Days 28 and 56 following GnRH-RS than EB-RS and no-RS. The final pregnancy rate was higher following EB-RS treatment than no-RS or GnRH-RS treatment (95.0, 88.3, and 88.6%; P<0.05). In conclusion, EB-RS enhanced reproductive performance of anestrous cows compared to no-RS.     

Reproductive performance in anestrous dairy cows following treatment with two protocols and two doses of progesterone

The objectives of this study were to evaluate the effect using two doses of progesterone (P4) releasing devices in two different programs on reproductive performance of anestrous dairy cows. Cows (n = 1555) not detected in estrus by 10 d before the planned start of the seasonal breeding program and in which no CL was palpable were treated with an intravaginal P4-releasing device ('Single'; approximately 1.56 g of P4) or a modified device with triple the normal P4 dose ('Triple'; approximately 4.7 g of P4). The devices were in place for either 6 d ('Short') or 8 d ('Long'), with 1mg estradiol benzoate (EB) given 24 h after device removal. The 'Long' program also included treatment with 2 mg EB at device insertion. The Long program resulted in a higher first service conception rate (RR = 1.18 (95% CI = 1.03-1.33); P = 0.02), but had no effect on the 28-d, 56-d or final pregnancy rate compared to the Short program. There were no effects of dose of P4 on any outcome. In conclusion, the Long compared to the Short program, but not the dose of P4, improved first service conception rates in anestrous cows.     

Improvement of first-service pregnancy rate in cows with gonadotropin-releasing hormone analog

The effect of an intramuscular injection of a new analog of gonadotropin-releasing hormone (GnRH), fertirelin, on the first-service pregnancy rate in cows was investigated by a double blind experiment. A total of 1,194 cows was injected intramuscularly either with 100 mug of GnRH or placebo (physiological saline solution) at the time of first insemination postpartum. Pregnancy rate (number of cows calved/ number of cows serviced) was 57.2 % in 605 cows treated with GnRH, while the performance was 49.7 % in 589 cows of the placebo group. The difference of pregnancy rates in both groups was significant (P<0.05). GnRH injected at insemination was effective, especially in cows at the first and third lactations, cows at 101 days postpartum or later, cows with daily milk yield of 26-30 kg, and also in cows from the area where a regional average fertility was relatively low.     

Induction of estrus in anestrous cows treated with 'Fertivet'

The effectiveness of 'Fertivet' (180 mg. Cisclomiphene Citrate and 120 mg. Transclomiphene citrate mixture) in the induction of estrus and fertility was tested on sixteen anestrous cows. Of all the animals treated 14 (97.5%) expressed estrus and 8 (57.1%) conceived. Among the three breeds of cows treated, 5 out of 5 Brown Swiss, 4 out of 5 Jersey cross breeds and 5 out of 6 Sahiwal (Zebu) cows expressed estrus. These results indicate the usefulness of 'Fertivet' in the induction of fertile estrus in cows.     

Effect of gonadotrophin releasing hormone on conception rate in repeat-breeder dairy cows

The effect of gonadotrophin releasing hormone (GnRH) on conception rate was studied in 961 repeat-breeder cows in five California dairy farms. Cows were injected with GnRH or saline at the time of the fourth insemination. All breeding was by artificial insemination (AI) using frozen semen from two bulls. Conception rate for GnRH-treated cows was higher than for controls (47.0 vs 37.7%, P < 0.01). There was no significant difference in conception rate between the two bulls or among five AI technicians. How GnRH acts upon fertility in repeat-breeder cattle has not been determined.     

Effect on pregnancy rate of nonestrus insemination in previously inseminated dairy cows

Lactating dairy cows (n = 157) designated for slaughter under the 1986 U.S. Dairy Termination Program were utilized in two trials to determine the effects of a second insemination on fertility. Cows that had been previously inseminated at estrus were inseminated again at 12-24 d later, when not in estrus. Cows that were reinseminated into the uterine body had significantly lower pregnancy rates (4% vs 40.6% for controls). Differences in pregnancy rates when the reinsemination was performed in the mid-cervical region were not significant (34.8% vs 50% for controls). Based upon these studies, intrauterine insemination of cows displaying questionable signs of estrus should be avoided in previously inseminated cows.     

Seasonal variations in post-partum plasma progesterone levels and conception in primiparous and multiparous dairy cows

Progesterone concentrations in peripheral plasma of 18 primiparous and 34 multiparous dairy cows were determined by radioimmunoassay every 4 days, from 10 days post partum until the next conception. The interval from parturition to the first progesterone peak (greater than 4 ng/ml plasma) was significantly longer (P less than 4 ng/ml plasma) was significantly longer (P less than 0.01) in primiparous than in multiparous cows. The progesterone concentrations on Days 4-15 of the oestrous cycle were significantly lower (P less than 0.025) during the summer than in cycles occurring in winter. The conception rate during the summer was lower than in winter. In cows inseminated in summer, conception was related to the shape of the progesterone curve in the cycle preceding insemination.     

Climate factors affecting conception rate of high producing dairy cows in northeastern Spain

Summer heat stress is a main factor related to low conception rate in high producing dairy herds in warm areas worldwide. We assessed the impact of several climate variables on conception rate in high producing dairy cows in northeastern Spain by examining 10,964 inseminations. The temperature-humidity index (THI) was compared with maximum temperature in terms of its efficiency at predicting conception rate. The following data were recorded for each animal: herd, lactation number, insemination number, insemination date, inseminating bull, and AI technician along with climate variables such as mean and maximum temperatures, rainfall, mean and maximum THI for individual time points Days 7 to 1 before insemination, the day of insemination and 1, 2 and 3 days after insemination. Averages were also established for the following periods: from 7 days before insemination to the insemination day, from 3 days before insemination to the insemination day and from the insemination day to 3 days postinsemination. Based on the odds ratios, the likelihood of conception rate increased significantly by factors of 1.48, 1.47, 1.5, and 1.1 for the respective maximum THI classes <70, 71-75, 76-80, and 81-85 only on Day 3 before AI, while on the insemination day, it increased by factors of 1.73, 1.53, 1.11, and 1.3 for the respective maximum THI classes <70, 71-75, 76-80, and 81-85. In a subsequent logistic regression excluding mean and maximum THI, the effectiveness of temperature at predicting conception rate was evaluated. Although high, the fit of the second logistic regression model was slightly lower than that of the full model (P=0.88 versus P=0.98, respectively) and the information provided by the THI model. The likelihood of conception rate increased significantly by factors of 1.5, 1.2, 1.0, 1.0 for the respective maximum temperature classes <20, 21-25, 26-30, 31-35 degrees C on Day 1 after AI. The choice of the THI or temperature to monitor the farm environment would have to depend on the particular farm and situation. In our study conditions, the use of maximum temperature alone gives a new point of view regarding the information provided by the THI variables.     

Factors affecting conception and expression of oestrus in anoestrous cows treated with progesterone and oestradiol benzoate

Nutrient balance affects the resumption of ovarian cyclic activity following calving in dairy cattle. However, few data are available about the relationships between nutrient balance and expression of oestrus or conception. It was hypothesised that previously anoestrous cows that conceived to first insemination and cows that expressed oestrus at the subsequent expected return to oestrus would be less likely to be in negative energy balance (i.e. would have higher body condition score, higher glucose, insulin-like growth factor (IGF), leptin or insulin concentrations, and lower non-esterified fatty acids (NEFA), blood urea nitrogen (BUN), beta hydroxy butyrate (BOH) concentrations 12-15 days after insemination) than herd mates not conceiving or expressing oestrus. Anoestrous cows were treated with progesterone and oestradiol benzoate (Day 0 = end of treatment period) and retrospectively categorised as conceiving (n = 108) or not conceiving (n = 108) to insemination. A subset of cows not conceiving to insemination were categorised as expressing (n = 44) or not expressing (n = 44) oestrus between Days 14 and 28 after initial insemination. Cows conceiving had a lower NEFA concentration (P = 0.014) than non-conceiving cows. Cows subsequently detected in oestrus had higher body condition scores (P = 0.016), IGF concentrations (P = 0.008) and milk protein percentages (P = 0.038), and lower BOH concentrations (P = 0.018) than cows not expressing oestrus. No difference in concentrations of leptin, insulin, glucose, blood urea nitrogen or milk yield were found between cows conceiving or not conceiving and those detected in oestrus or not detected in oestrus (P > 0.1). It is concluded that some measures of metabolic status at the time of pregnancy recognition affects the probability of pregnancy and of subsequent expression of oestrus in those treated, anoestrous cows not conceiving.     

Estrus synchronization and conception rate in dairy cattle treated with progestin-impregnated vaginal sponges.

Twelve Israeli-Friesian heifers and cows were treated for estrus synchronization with intravaginal sponges impregnated with 250 mg. medroxyprogesterone acetate (MAP) for a period of 14 days. All sponges were retained throughout the treatment period and 100% of the animals came into estrus 3–7 days after sponge removal. Conception rate from insemination at the synchronized estrus was 60%. It is concluded that further investigation is justified regarding the value of progestin-impregnated sponges as a practical means of estrus synchronization in cattle.     

Factors affecting conception rate after artificial insemination and pregnancy loss in lactating dairy cows

Objectives were to determine factors associated with conception rate (CR) and pregnancy loss (PL) in high producing lactating Holstein cows. In Study 1, CR was evaluated in 7633 artificial inseminations (AI) of 3161 dairy cows in two dairy farms. Pregnancy diagnosis was performed by palpation per rectum 39+/-3 days after AI. Environmental temperature was recorded at different intervals prior to and after AI. In Study 2, 1465 pregnancies from 1393 cows diagnosed at 31+/-3 days after AI by ultrasonography on three dairy farms were re-examined 14 days later to determine PL. Temperature > or =29 degrees C was considered to be heat stress (HS). Exposure to HS was defined as following: NH, no heat stress; HS1, exposure to at least 1 day of maximum temperature > or =29 degrees C and average daily maximum temperature (ADMT) <29 degrees C; and HS2, exposure to ADMT > or =29 degrees C. In Study 1, exposure of cows to HS1 and HS2 from 50 to 20 prior to AI was associated with reduced CR compared to cows not exposed to HS (28.8, 23.0, and 31.3%, respectively). Post-AI HS was not associated with CR. Cows inseminated following estrus detection or timed AI had similar CR. As the number of AI increased, CR decreased. Multiparous cows had lower CR than primiparous cows, and occurrence of milk fever and retained placenta was associated with decreased CR. In Study 2, PL was not associated with exposure to HS either prior to or after AI. Cows diagnosed with clinical mastitis experienced increased PL, but parity, number of AI, AI protocol, milk production, and days postpartum at AI were not associated with PL. In conclusion, CR was affected by HS prior to AI, parity, number of AI, and postparturient diseases, whereas PL was affected by clinical mastitis.     

Trucking stress at breeding does not lower conception rate of beef heifers

The goal of this study was to determine whether 1 h of trucking stress before or after artificial insemination (AI) altered the conception rate of beef heifers. Estrus was synchronized in heifers with prostaglandin F(2alpha).The 3 treatment groups consisted of 1) AI (control heifers, n = 93); 2) Truck + AI (trucked for 1 h immediately before AI, n = 81); and 3) AI + Truck (trucked for 1 h immediately after AI, n = 82). All heifers were artificially inseminated by a single technician with semen from a single ejaculate. Blood samples were collected for cortisol measurement 1 h before AI, immediately before and after AI, and 1 h after AI in the AI (n = 6), Truck + AI (n = 9), and AI + Truck (n = 8) groups Pregnancy in heifers was confirmed either at slaughter or by palpation per rectum. Trucking before AI elevated (P < 0.01) serum cortisol concentrations. Artificial insemination alone increased (P < 0.01) serum cortisol concentrations in AI heifers. The increase in serum cortisol concentrations caused by trucking after AI was not significant (P > 0.05). Areas under the cortisol curves in Truck + AI heifers are greater (P < 0.05) than in AI heifers. The conception rates of AI heifers (50.5%), Truck + AI heifers (51.9%) and AI + Truck heifers (58.5%) are not different (P > 0.05). This study demonstrates that 1 h of trucking stress either before or after AI did not lower the conception rate of heifers.     

Moderate summer heat stress does not modify immunological parameters of Holstein dairy cows

The study was undertaken during spring and summer months in a territory representative of the Mediterranean climate to assess the effects of season on some immunological parameters of dairy cows. Twenty Holstein cows were used. Eleven of those cows gave birth during spring; the remaining nine cows gave birth in summer. The two groups of cows were homogeneous for parity. Values of air temperatures and relative humidity were recorded both during spring and summer, and were utilized to calculate the temperature humidity index (THI). One week before the expected calving, rectal temperatures and respiratory rates of the cows were recorded (1500 hours), and cell-mediated immunity was assessed by measuring the proliferation of mitogen-stimulated peripheral blood mononuclear cells (PBMC). Within 3 h of calving, one colostrum sample was taken from each cow and analysed to determine content of immunoglobulin (Ig) G1, IgG2, IgM and IgA. At 48 h after birth, passive immunization of the calves was assessed by measuring total serum IgG. During summer, daytime (0900-2000 hours) THI values were above the upper critical value of 72 [75.2, (SD 2.6)] indicating conditions that could represent moderate heat stress. That THI values were able to predict heat stress was confirmed by the values of rectal temperatures and respiratory rates, which were higher (P < 0.05 and P < 0.001 respectively) during summer. Proliferation of PBMC, the colostral concentration of Ig fractions and serum levels of IgG in their respective off-spring did not differ between spring and summer cows. Results indicated that moderate heat stress due to the hot Mediterranean summer does not modify cell-mediated immunity, the protective value of colostrum and passive immunization of the offspring in dairy cows.     

Pregnancy patterns during the early fetal period in high producing dairy cows treated with GnRH or progesterone

In order to explore pregnancy patterns in high producing dairy cows treated with GnRH or progesterone at pregnancy diagnosis (Days 28-34), two consecutive experiments were designed. In Experiment 1, cows bearing a single embryo were randomly assigned to a PRID (n = 40; cows fitted with a progesterone releasing intra-vaginal device for 28 days), GnRH (n = 40; cows receiving GnRH) or Control (n = 26; untreated cows) group. PRID treatment led to a rise in plasma progesterone concentrations in the 7 days following the onset of treatment compared to the other two groups. In Experiment 2, in which we also examined twin pregnancies, animals were randomly assigned to PRID (n = 312) or GnRH (n = 294) treatment groups. Treatments were the same as described for Experiment 1. Logistic regression procedures revealed that in cows with a single corpus luteum, the probability of pregnancy loss between the first (Days 28-34) and second (Days 65-62) pregnancy diagnosis decreased by a factor of 0.51 in the PRID group compared to the GnRH group. However, in cows with two or more corpora lutea, PRID treatment increased the likelihood of pregnancy loss by a factor of three, compared to GnRH treatment. In cows carrying twins, the conceptus reduction rate was higher (P = 0.02) for the GnRH (36%) than for the PRID (16.4%) group. Formation of a new corpus luteum was recorded in 17.7% of cows in the GnRH group. Our results indicate that compared to GnRH treatment, progesterone treatment given at pregnancy diagnosis in high producing dairy cows, reduced by a factor of 0.51 and increased by a factor of 3 the probability of pregnancy loss in cows with a single or with two or more corpora lutea, respectively, and reduced the conceptus reduction rate in cows carrying twins. The practical implications of our findings are that in herds with a high incidence of early fetal loss of a non-infectious nature, treatment at the time of pregnancy diagnosis with PRID in cows with one corpus luteum and with GnRH in cows with two or more corpora lutea should offer considerable benefits.     

The effect of level of feed intake on progesterone clearance rate by measuring faecal progesterone metabolites in grazing dairy cows

The objective of the present study was to determine the effect of level of feed intake of pasture on P4 clearance rates in dairy cows. Twelve non-lactating Holstein-Friesian cows aged 4-9 years were randomly allocated to a restricted or ad libitum group. The ad libitum group had unrestricted access to irrigated pasture, whereas the restricted group had access for only 2h per day. Each animal was drenched orally twice daily with a chromic oxide capsule to allow daily feed intake to be estimated from faecal output (FO). Endogenous progesterone (P4) production was eliminated by subcutanously implanting a capsule containing 6 mg of a potent GnRH-agonist (deslorelin) into the ear of each animal 3 weeks before inserting a CIDR device containing 1.9 g P4 into the vagina. Two luteolytic PGF2alpha were given 10 days later. Each device was removed after 11 days and residual P4 measured. Daily plasma samples were assayed for P4. Faecal samples were also taken daily and assayed for pregnanes (FP4M) containing a 20-oxo-, a 20alpha- or a 20beta-OH group with EIAs. The average daily dry matter (DM) intake of pasture was higher for cows in the ad libitum group (15.9 versus 6.3 kg DM, P=0.001). Their plasma P4 concentrations were lower (1.08 versus 1.71 ng/ml, P=0.05), even though the average residual P4 content of the used CIDR devices was not affected by feed intake (1.20 versus 1.25 g, P>0.05). The concentrations of FP4M were not affected by level of feed intake (20-oxo-: 3.3 versus 1.7, 20alpha-: 3.5 versus 3.7, 20beta-: 2.1 versus 3.2 microg/g DM). Daily excretion rates of 20-oxo- and 20alpha- were higher in ad libitum cows (20-oxo-: 17.8 versus 4.3mg per day, P=0.05; 20alpha-: 18.2 versus 8.9 mg per day, P=0.001), but daily yield of faecal 20beta- was not affected by feed intake (11.9 versus 8.6 mg per day, P=0.5). These results show that there was a negative relationship between feed intake and plasma P4 concentrations in these CIDR-treated GnRH-downregulated Holstein cows. Concentrations of FP4M were not affected by level of feed intake or FO, but daily excretion rate of FP4M was associated with the volume of faeces.     

Autoclaved, previously used intravaginal progesterone devices induces estrus and ovulation in anestrous Toggenburg goats

Intravaginal progesterone devices are used worldwide for estrus induction in goats. Reused devices are able to induce estrus; however, this can be a health risk within a flock. The objective was to compare new and previously used (and autoclaved) progesterone-releasing intravaginal devices for induction of estrus and ovulation in seasonally anestrous Toggenburg goats. Anestrous goats (n=42) received new intravaginal devices containing 0.3g progesterone (CONTROL), or similar devices previously used for either 6 (USED6) or 12d (USED12) and subsequently autoclaved. All goats received 5mg dinoprost at device insertion and 200 IU eCG 5d later, and all devices were removed after 6d. After device removal, estrus was monitored and females displaying signs of estrus were mated by fertile bucks. Transrectal ovarian ultrasonography was performed after device removal until detection of ovulation. Blood samples were collected for determination of plasma progesterone concentration at different times. There was no difference (P>0.05) among groups CONTROL, USED6 or USED12 for: estrus response (87, 100 or 100%, respectively); duration of estrus (32.3±2.3, 25.2±3.4 or 27.3±4.1h); ovulation rate (100, 88 or 100%); number of ovulations (1.5±0.2, 1.9±0.3 or 1.7±0.3); and pregnancy rate (60, 58 or 67%). Plasma progesterone (P4) concentrations were greater (P<0.05) in CONTROL than in USED6-treated and USED12-treated goats (7.2±1.2, 4.7±0.7 and 4.3±0.6 ng/mL, respectively) at 6h after device insertion; these differences were maintained until 4d after device insertion (3.4±0.4, 2.3±0.2, and 2.5±0.2 ng/mL). Overall, plasma progesterone concentrations were greater (P<0.05) in nulliparous than in lactating goats (3.1±0.8 compared to 2.4±0.6 ng/mL, respectively). In conclusion, autoclaved, previously used intravaginal progesterone-releasing devices resulted in significant lesser plasma progesterone concentrations than new devices, but were similarly effective in inducing estrus and ovulation in anestrous goats.     

Effect of short-term treatment with bovine somatotropin at estrus on conception rate and luteal function of repeat-breeding dairy cows.

We studied the effect of recombinant bovine somatotropin (rbST) at the time of estrus on progesterone concentrations and conception rates of repeat-breeding Holstein cows. We used repeat-breeding cows of varied parity (n = 510). All the animals were clinically healthy and had had at least three unsuccessful services before entering the study. After detection of estrus, the cows were randomly assigned to either a treated (n = 201) or a control (n = 309) group. The animals in the treated group were given rbST (500 mg s.c.) at the time of estrus and again 10 d later. Artificial insemination was performed 12 h after the first detection of estrus. In order to evaluate the effect of rbST on luteal function, blood samples were taken from 10 cows in each group every 3 d for 18 d, starting on the day of insemination (Day 0) to determine progesterone concentrations. Conception rates were significantly higher (P < 0.05) in the cows treated with rbST (29.3%) than in the control cows (16.9%). The effects of rbST were maximal in cows with 8 or more previous unsuccessful services and in cows with 2 to 4 calvings. Progesterone concentrations tended to be higher in nonpregnant cows that were treated with rbST than in those that were not treated. The difference between groups was significant (p < 0.05) on Day 18 after insemination. In pregnant cows there were no significant differences in progesterone concentrations between treated and nontreated animals at any time. Treatment with rbST at estrus improved the conception rate of repeat-breeding Holstein cows. This effect was associated with an increase in circulating progesterone concentrations on Day 18.     

Supplementing lactating dairy cows fed high-quality pasture with black wattle (Acacia mearnsii) tannin

A reduction in urinary nitrogen (N) excretion from dairy cows fed pasture containing a high N concentration in the dry matter (DM) will have environmental benefits, because losses to soil water and air by leachate and nitrous oxides (N₂O) will be reduced. Condensed tannins (CT) reduce digestion of N, and provision as a dietary additive could have nutritional benefits for production, but the amount required and the responses to different sources of CT on milk production have not been defined. Two experiments were conducted to evaluate effects of supplementation with CT extracted from black wattle (Acacia mearnsii De Wild.) on milk production and faecal N concentration by lactating dairy cows grazing a vegetative Perennial ryegrass (Lolium perenne L.)-based pasture. In one experiment, CT was administered as a drench, twice daily, to 38 multiparous Holstein–Friesian cows assigned to four treatments; control (CONT, 0 g/day), low CT (LCT, 111 g/day), medium CT (MCT, 222 g/day) and high CT (HCT, 444 g/day), grazing as a single group. The CT supplementation affected milk yield (P < 0.001) with a trend of declining milk yield as CT concentration increased from about 0.6 to about 2.9% of dietary DM. Milk urea nitrogen (MUN) decreased at MCT and HCT levels of supplementation (P < 0.01) but milk fat, CP and lactose percentage were not affected by CT supplementation. The CT supplementation increased N concentration in faeces for LCT and MCT treatments (P < 0.05), suggesting partitioning of dietary N away from urine. When CT was pelleted with grain, in a second experiment and fed twice daily as a supplement at milking, it reduced the acceptability relative to pellets without CT, and tended to lower milk production from 25.4 to 24.5 kg/day, although the decline was not significant (P > 0.05). The diet of cows fed pellets with CT contained about 1.2% CT in the DM but neither milk constituents nor MUN were affected by CT-supplemented grain (P > 0.05). These findings demonstrate beneficial effects for production of low concentrations (c. 0.6% DM) of CT from black wattle when given to cows grazing pasture with an N concentration of 3.8%, and suggest a diversion of N from urine, but when CT exceeded about 1.4% of dietary DM, milk production was depressed. The value of supplementing a pasture diet for lactating dairy cows with black wattle tannin extract will depend on costs of supplementation, returns from milk production and liabilities associated with N losses to urine.