Daily Pasture Allowance Research Articles

Intake, milk production and grazing behaviour responses of strip-grazing dairy goats to daily access time to pasture and to dehydrated lucerne supplementation

The ability of grazing dairy goats to adapt to short daily access time to pasture is not well known, while, in practice, access time to pasture (AT) is often lower than 12 h/d. A 3-week trial was carried out in spring with 36 Alpine dairy goats in mid-lactation to study the effect of a restriction of access time to pasture, coupled or not with a dehydrated lucerne supplement. Three treatments were compared: (1) 7 h/d of access time to pasture, between milkings, without dehydrated lucerne (namely AT7); (2) 7 h/d of access time to pasture, between milkings, with 371 g DM/d of dehydrated lucerne, distributed individually after evening milking (namely AT7D); and (3) 11 h/d of access time to pasture, 7 h/d between milkings plus 4 h/d after evening milking, without dehydrated lucerne (namely AT11). The daily pasture allowance was 2.3 kg DM/goat at 4 cm above ground level in each treatment. Each goat received 602 g DM/d of a commercial concentrate, given twice daily at milking. Pasture intake decreased by 433 g DM/d (i.e. 18%) between AT11 and AT7 (P<0.01). Pasture intake also decreased by 259 g DM/d between AT7 and AT7D (P<0.01), which means a substitution rate of 0.72 between pasture and dehydrated lucerne. Milk production, and milk fat and protein concentrations were not affected by AT (P>0.10). Milk production response to dehydrated lucerne was 0.82 kg of milk/kg DM. Grazing time decreased by 2.5 h/d between AT11 and AT7 or AT7D (P<0.001). The pasture intake rate averaged 272 g DM/h and tended to increase between AT11 and AT7 (P = 0.08). It is concluded that an access time to pasture of 7 h/d may be sufficient for lactating dairy goats receiving 600 g/d of concentrate and grazing high-quality pastures. Supplying 400 g/d of dehydrated lucerne has a much stronger positive effect on milk production than an additional grazing period of 4 h after evening milking.

Intake, milk yield and grazing behaviour of strip-grazing Alpine dairy goats in response to daily pasture allowance

Grazing management has an important impact on dairy ruminants’ performance. References on the intake and milk yield of dairy goats under strip-grazing systems in temperate regions are scarce. In order to study the effect of pasture allowance on pasture intake (PI), milk yield and grazing behaviour, a trial was carried out in spring with 36 Alpine goats in mid-lactation. Three daily pasture allowances (PA=1.7, 2.6 and 3.5 kg dry matter (DM)/day, namely Low, Medium and High, respectively) were compared in a 3 × 3 Latin square design replicated six times during three successive 14-day periods. Goats individually received 268 g DM of concentrate twice daily at each milking and had access 11 h/day to pasture (from 0830 to 1600 h and from 1730 to 2100 h). Pasture intake increased with PA, and more so between Low and Medium than between Medium and High (+216 v. +101 g DM/kg DM of PA). Milk yield was lower on Low than on Medium and High (2.79 v. 3.13 kg/day), as were milk fat and protein yields. Grazing time averaged 476 min/day and was lowest on Low and greatest on Medium. Pasture intake rate was 30 g DM/h lower on Low and Medium than on High. It is concluded that under temperate conditions, when goats are supplemented with 536 g DM of concentrate and have enough access time to pasture (11 h/day), a medium pasture allowance close to 2.6 kg DM/day may be sufficient to maximise milk yield.

The Behaviour and Productivity of Mid-Lactation Dairy Cows Provided Daily Pasture Allowance over 2 or 7 Intensively Grazed Strips.

Simple SummaryRecent technological advances will soon allow pastoral dairy farmers to manage their cattle using increasingly intense and complex grazing regimes. Ensuring there is merit in the implementation of more intense grazing regimes will minimise the potential misuse of this technology and the associated consequences of misuse for animal welfare and productivity. Two groups of dairy cattle were provided with the same amount of fresh pasture over either 2 or 7 feeds per day. Cows that received pasture over 7 feeds produced less milk and spent less time ruminating, but more time resting, per day. However, feeding frequency did not affect the time cows spent feeding or estimated pasture consumption. Increasing the frequency of feeding restricts the amount of pasture available to cattle at any one time. In response, cattle adjust their ingestive behaviour (e.g., adopt high intake rates) which may negatively impact digestive processes. The success of more intensive pastoral dairy production requires grazing regimes that support the natural ingestive, digestive, and social behaviours of cattle, rather than necessitating cattle to substantially alter their behaviour in accordance with the grazing regime imposed upon them.Research into the effects of intense grazing regimes on cattle behaviour and productivity will support the ethical intensification of pastoral dairy production. Two treatments were applied to two herds of 30 mid-lactation cows over 28 days. Cows were offered an estimated 12 kg DM/cow (above 5 cm from ground level) of irrigated pasture per day. The control herd received their daily pasture allocation in two equal grazings while the experimental herd received theirs over seven smaller grazings. Backgrazing beyond the current allocation (morning or afternoon) was prevented. Individual records were taken daily for milk production and behaviour (MooMonitor+). Milk composition, energy corrected milk (ECM), and live weight were recorded weekly. Feeding mid-lactation dairy cows over seven smaller grazing allocations reduced the time cows spent ruminating (p < 0.001), milk yield (p < 0.001), and ECM (p < 0.05). However, milk composition, live weight, time feeding, and pasture consumption were not affected by feeding frequency (p > 0.05). Cattle may have adapted their ingestive behaviour in response to the more intensive strip-grazing regime utilised in this study, with negative consequences for digestive processes and consequently milk production. Intense grazing regimes need to support the ingestive, digestive, and social behaviours of cattle.

Milk production and grazing behaviour responses of Alpine dairy goats to daily access time to pasture or to daily pasture allowance on temperate pastures in spring

In a context of price volatility and low feed self-sufficiency of goat farms in Western Europe, grazing may play an important role. Knowledge about the impact of grazing management – particularly daily access time to pasture or pasture allowance – on dairy goats’ performance is scarce. Two 3-week trials were carried out in spring with 36 Alpine goats in mid-lactation. In Trial 1, 3 access times to pasture during daytime (AT: 4, 6 or 8 h/day, from 08:00 to 12:00, from 10:00 to 16:00 and from 08:00 to 16:00, respectively) were compared. In each treatment, goats individually received 370 g DM of dehydrated forage (lucerne and maize) after the evening milking, 278 g DM of concentrate twice daily, at each milking, and a daily pasture allowance of 2.0 kg DM/goat, measured at 4 cm above ground level. In Trial 2, 3 daily pasture allowances (PA: 1.6, 2.3 or 3.0 kg DM/goat, measured at 4 cm above ground level) were compared. Goats individually received 278 g DM of concentrate twice daily, at each milking, and access time to pasture was of 13 h/d (from 07:30 to 16:00 and from 17:00 to 21:30). Milk production was similar between medium and high levels of AT or PA, and was lower for the lowest level of AT or PA (−300 g/d at the lowest AT and −500 g/d at the lowest PA). Milk fat and protein concentrations only slightly varied between the different levels of AT or PA. In both trials, goats maintained an intense grazing activity. Grazing time (from 240 to 540 min/d) and proportion of time spent grazing (from 66 to 99%) were mainly affected by AT, and only marginally by PA.

Evaluation of blood metabolites in dairy cows grazing under two pasture allowances and supplemented with corn silage under restricted grazing conditions

This study was conducted to evaluate the influence of management tools - daily pasture allowance and corn silage supplementation - during periods of forage shortage on the metabolism of dairy cows grazing low-mass pasture in temperate regions. Forty lactating Holstein cows were used during an experimental period of seventy days (April 15 to June 23, 2012). Blood metabolites and milk production were determined in fall-calving dairy cows grazing under two daily pasture allowances (PA) (moderate, 17 kg vs. high, 25 kg dry matter (DM)) and supplemented with corn silage (CS) (low, 4.5 kg vs. high, 9.0 kg DM). All cows received 3 kg DM of concentrate. Plasma concentrations of β-hydroxybutyrate (BHBA), non-esterified fatty acids (NEFA), and urea were determined using an automated spectrophotometer, and milk production was electronically measured at each milking time during the trial. The experimental design was completely randomized using a 2 × 2 factorial arrangement of treatments. The increase in daily PA decreased plasma concentrations of BHBA (0.91±0.36 vs. 1.12±0.43 mmol L−1), provided an increase in milk yield (23.18±3.26 vs. 21.99±3.37 kg d−1), and did not modify the concentrations of NEFA and urea. The increased CS supplementation increased mildly the plasma concentrations of BHBA (1.07±0.36 vs. 0.96±0.44 mmol L−1) and NEFA (92.77±54.14 vs. 92.77±55.31 µmol L−1), and decreased the concentrations of urea (4.08±1.40 vs. 4.64±1.30 mmol L−1), but did not change milk production. The positive effect of increasing PA was associated with a high herbage intake, while the lack of response to increasing CS supplementation was attributed to a high substitution of pasture intake (0.9 kg DM pasture/kg DM CS). Low corn silage supplementation is recommended.

Phyto-oestrogens and their metabolites in milk produced on two pastures with different botanical compositions

Phyto-oestrogens are a group of secondary plant metabolites that may bind to oestrogen receptors and exert oestrogenic or anti-oestrogenic effects in humans and can protect against cancer diseases. When ingested by dairy cows, phyto-oestrogens can be metabolised and transferred to the milk. The objective of this study was to assess the effects of grazing a recently established pasture containing red clover (Trifolium pratense L.) and an older pasture containing a variety of sown and unsown plant species on milk concentrations of phyto-oestrogens. Sixteen Norwegian Red dairy cows [mean (standard deviation); body weight 599 (45.1)kg, stage of lactation 73 (15.0) d in milk, milk yield 29.9 (2.90) kg/d at the start of the experiment] were divided into two groups and grazed either a short-term pasture (SP) or a long-term pasture (LP). The SP was representative of organically managed leys in Norway, which are frequently, approximately every third year, renewed by soil tillage and seeding, whereas LP was representative of organically managed grasslands that are less frequently renewed. The SP contained meadow fescue (Festuca pratensis Huds.) (mean 34%), timothy (Phleum pratense L.) (mean 19%), red clover (mean 28%), shepherd׳s-purse (Capsella bursa-pastoris (L.) Medik.) (mean 6%), pineappleweed (Matricaria matricarioides Porter ex Britton) (mean 5%) and scentless mayweed (Tripleurospermum perforatum (Mérat) Laínz) (mean 4%), and LP contained mainly white clover (Trifolium repens L.) (mean 21%), smooth meadowgrass (Poa pratensis L.) (mean 19%), timothy (mean 17%), meadow fescue (mean 15%), perennial ryegrass (Lolium perenne L.) (mean 6%), tufted hairgrass (Deschampsia cespitosa (L.) P. Beauv.) (mean 5%), northern dock (Rumex longifolius DC.) (mean 4%), common couch (Elytrigia repens (L.) Desv. Ex Nevski) (mean 4%), red clover (mean 3%) and dandelion (Taraxacum spp.) (mean 3%). In addition to a daily pasture allowance of 20kg dry matter per cow, supplements of 3.0kg barley (Hordeum vulgare L.) concentrate were fed. Herbage, concentrates and milk was sampled during the last week of three experimental periods and analysed for phyto-oestrogens using LC-MS/MS technology. Herbage from SP had 19 times higher concentration of isoflavones than herbage from LP, whereas only small differences were found for lignans. Milk produced on SP had 14 times higher concentrations of the mammalian isoflavonoid equol, and the concentrations of equol were higher than found in most other studies. This study confirms that grazing pastures containing red clover increases concentrations of isoflavones and especially equol in bovine milk compared to grazing pastures with other botanical composition. The higher milk concentrations of the lignan metabolite enterodiol in milk from SP compared to LP could not be related to differences in intake of the analysed lignans and may therefore be related to unidentified lignans.

Pasture intake and milk production of dairy cows grazing annual ryegrass with or without corn silage supplementation

The use of corn silage to supplement grazing dairy cows can improve milk production and nutrient intake when the substitution rate is low. This enhancement occurs by increasing the total DM intake. The hypothesis tested in this study was that increasing corn silage supplementation level of dairy cows grazing annual ryegrass at medium pasture allowance could increase substitution rate and decrease milk production response. Three supplementation levels (0, 4 and 8 kg DM/day) were compared for dairy cows strip-grazing annual ryegrass (Lolium multiflorum cv. Common). The study was arranged as a 3 × 3 Latin square design, repeated three times using nine Holstein lactating cows. The supplement was a 7 : 1 mixture based on the DM of corn silage and soybean meal and was offered individually twice daily after the morning and afternoon milkings. Each treatment group of cows grazed separately at a common daily pasture allowance of 35 kg DM/cow to ground level. Each experimental period was 12 days, with an 8-day adaptation and a 4-day measurement period. The individual pasture intake was measured from Day 9 to Day 12 using the n-alkane technique. The daily pattern of grazing and ruminating times were measured by visual observations. The supplement DM intake was lower than expected, averaging 3.0 and 4.3 kg DM/day for cows receiving 4 and 8 kg DM/day, respectively. Pasture and supplement energy value were 7.0 and 6.3 MJ of NEL/kg DM, respectively. The substitution rate averaged 0.2 and was unaffected by the supplementation level. The pasture intake was similar for all treatments (14.0 kg of DM/day), and total DM intake and milk production (+0.5 kg of milk/kg DM of supplement) increased linearly with increasing supplementation level. The daily grazing time was shorter by 111 min/day for the supplemented cows than for the unsupplemented cows. Cows grazing the annual ryegrass at medium pasture allowance improved their total DM intake and individual performance when supplemented with corn silage.

Meta-analysis of the effect of pasture allowance on pasture intake, milk production, and grazing behavior of dairy cows grazing temperate grasslands

Daily pasture allowance (PA) is defined as the product of pregrazing pasture mass and offered area, and is the major grazing management factor determining pasture utilization per unit area and daily performance of grazing dairy cows. The objective of the present study was to perform a meta-analysis reviewing the effect of PA on pasture intake, milk production, milk composition, and grazing behavior of dairy cows. Experiments studying the effect of PA on pasture intake or milk production, which eventually included milk composition or grazing behavior data, or both, were selected to create a database. Papers were selected only if at least 2 PA were compared under the same experimental conditions, particularly the same pasture mass (i.e., where PA levels were only obtained through changes in daily offered area). The final database included 97 PA comparisons reported in 56 papers. For analytical purposes, the database was subdivided into 3 subsets that varied according to the estimation height (EH) at which PA was determined; that is, PA above ground level (PA0 subset), PA above 2.5 to 3.5cm (PA3 subset), and PA above 4 to 5cm (PA5 subset). Statistical analyses were conducted independently on the PA0, PA3, and PA5 subsets and on the whole database (global analysis) by using linear and nonlinear mixed-model procedures. The curves, either exponential, quadratic, or linear, describing the effects of PA on pasture intake, milk production, or grazing behavior of dairy cows are conceptually similar, whatever the EH. The equations describing these curves are, however, specific for each EH. Accordingly, from typical low to high PA, the increase in pasture intake (0.13 vs. 0.21 vs. 0.28kg/kg of PA), milk production (0.11 vs. 0.17 vs. 0.24kg/kg of PA), and milk solids production (0.008 vs. 0.010 vs. 0.013kg/kg of PA) per kilogram of increase in PA was lower for PA0 than for PA3, and for PA3 than for PA5. Grazing time increased from low to medium PA and did not vary from medium to high PA. Pasture intake rate seemed to increase from low to medium PA because of greater bite mass, whereas it increased from medium to high PA because of greater biting rate. The present meta-analysis demonstrated that the general relationship between PA and any dependent variable is quite strong and independent of EH. This suggests no specific relationship for some parts of the world or methodology approach, with a high portability of the global equations calculated here. These results are useful for improving grazing management and modeling on pasture-based dairy systems.

Fatty acid composition, fat-soluble vitamin concentrations and oxidative stability in bovine milk produced on two pastures with different botanical composition

Previous research has shown that grazing pastures compared to feeding preserved forages has large impact on milk fatty acid (FA) composition, but differences between grazing red clover (Trifolium pratense L.) or white clover (Trifolium repens L.) are small, whereas the herbage proportions of dicotyledon botanical families is positively correlated with the milk-fat proportions of total polyunsaturated FA when grazing pastures in the Alps. The objective of the present study was to investigate the influence of botanically different pastures on bovine milk composition and milk susceptibility to oxidation. Two groups of 8 multiparous Norwegian Red dairy cows [mean (standard deviation); 599 (45.1)kg body weight, 73 (15.0)d in milk, 29.9 (2.90)kg milk/d at experiment start] grazed either a short-term pasture (SP) or a long-term pasture (LP). Both pastures were organically managed, meaning that no artificial fertilizers or herbicides were applied. The SP was representative for pastures, which are frequently, i.e. at least every third year, renewed by soil tillage and seeding, whereas LP was representative for pastures, which are less frequently renewed. The SP contained mainly meadow fescue (Festuca pratensis Huds.), timothy (Phleum pratense L.) and red clover and LP contained smooth meadow grass (Poa pratensis L.), white clover and a variety of unsown species. Sixteen cows were blocked according to milk yield, days in milk and sire, and randomly within block allocated to the 2 dietary treatments with a daily pasture allowance of 15–20kg dry matter per cow, supplemented with 3.0kg barley (Hordeum vulgare L.) concentrate. Milk was sampled during the last week of 3 experimental periods and analysed for FA composition by gas chromatography, concentrations of fat-soluble vitamins by high performance liquid chromatography, and oxidative stability in a light-exposure experiment by measuring the formation of hydroperoxides and by front-face fluorescence spectroscopy. Pasture type had no effect on milk yield, milk gross composition, and only minor effects on milk FA composition. Milk from SP had higher concentration of α-tocopherol than LP. The formation of hydroperoxides in milk was lower for SP than LP after 24h light exposure, but no differences were found after 48h. Front-face fluorescence spectroscopy revealed slightly higher formation of components in the area of 409–480nm wavelength for SP than LP, which may be related to milk-lipid oxidation. The experimental pastures differed mainly in herbage proportions of red clover and white clover and less in proportions of non-legume dicotyledons. This explains small differences in milk FA composition and milk susceptibility to oxidation.

Effect of increasing pasture allowance and grass silage on animal performance, grazing behaviour and rumen fermentation parameters of dairy cows in early lactation during autumn

The aim of the present study was to determine the effect of daily pasture allowance and grass silage supplementation on milk production performance, grazing behaviour, rumen function, and blood and urinary metabolites in early lactation of dairy cows grazing on low herbage mass pastures. The study was conducted with 32 Holstein–Friesian cows over a time period of 63 days. Prior to experimental treatment, milk production, body weight, and days in milk averaged 23.8±0.70kg/day, 537±9.6kg, and 37.1±1.02, respectively. Four dietary treatments resulted from the combination of two pasture allowances above ground level (low 17 vs. high 25kg of dry matter (DM)/cow/day) and grass silage supplementation offered at levels of 4.5 and 9.0kg DM/cow/day. All of the cows received 2.6kg DM of concentrate supplementation. Total DM intake was determined using the controlled-release chromium capsules method and milk composition was analysed weekly using infrared spectroscopy (Foss 4300 Milko-scan). Pasture allowance and grass silage supplementation had no significant effect on milk production (average 23.4kg/day) or on milk composition. Grazing time and ruminating time were not significantly affected by pasture allowance. The rate of intake was significantly reduced (P<0.001) from 32.0 to 19.1g DM/min by increasing grass silage supplementation. Increasing pasture allowance increased (P<0.001) daily live weight change (−0.061 and 0.553kg/day for low and high, respectively). High pasture allowance decreased plasma β-hydroxybutyrate (P<0.05) and increased plasma urea–N (P<0.05) (3.95 and 4.10mmol/L for low and high, respectively). Supplementation with grass silage had no significant effect on blood and urinary metabolites. Rumen ammonia and total volatile fatty acid concentrations were not significantly affected by increasing pasture allowance or increasing grass silage supplementation. This value, the lowest measured, is below the range recommended for optimal microbial growth. Rumen microbial nitrogen efficiency did not appear to be affected by pasture allowance or grass silage supplementation in that the purine derivative/creatinine ratio was similar for the four dietary treatments.

The effect of pasture allowance and concentrate supplementation type on milk production performance and dry matter intake of autumn-calving dairy cows in early lactation

This study was undertaken to evaluate the influence of daily pasture allowance and concentrate type on milk production performance, dry matter intake, grazing behavior and blood and urinary metabolites of autumn-calving dairy cows in early lactation. The study lasted 61 days using 28 Holstein Friesian cows. Prior to experimental treatment, milk production averaged 23.1 kg/day (± 3.64 kg/day) and mean live weight was 595 kg (± 48.1 kg). The four dietary treatments were a combination of two pasture allowances above ground level (low, 25.5 vs. high, 38.5 kg of dry matter/cow/day) and two concentrate types offered at the level of 5 kg/cow/day (corn-based concentrate vs. sugar beet pulp-based concentrate). Total dry matter intake of individual cows was estimated using the n-alkenes method and milk composition was analyzed weekly using infrared spectroscopy (Foss 4300 Milko-scan). Pasture allowance significantly ( P < 0.01) increased milk production 22.0 kg/day for low and 24.7 for high, respectively. The concentrate energy source had no effect ( P > 0.05) on milk production, milk fat (3.73%) or protein (3.09%). Pasture dry matter (DM) intake and total DM intake (kg/day) increased from 9.6 to 11.1 and from 13.9 to 15.4 with increased in pasture allowance ( P < 0.05). Live weight and body condition score were unaffected by either pasture allowance or concentrate type. Plasma NEFA and plasma urea nitrogen (PUN) did not differ with pasture allowance averaging 0.298 mmol/l and 8.23 mmol/l, respectively ( P > 0.05). High pasture allowance increased ( P < 0.05) urinary purine derivatives (allantoin + uric acid/Creatinine (PD/C)) ratio. Concentrate type had no effect on any milk production parameter and the effect of daily pasture allowance was the most significant influencing factor.

Effects of body condition score at calving and feeding various types of concentrate supplements to grazing dairy cows on early lactation performance

The objective of the experiment reported here was to establish the effect of canola meal supplements of different protein degradability on the performance of grazing dairy cows with different body condition scores (BCS) in early lactation, and to compare this with feeding a cereal grain supplement instead. The experiment included a total of 72 cows in six treatments in a 2 * 3 factorial design, incorporating two BCS at calving (4.1 and 5.6 units on an 8-point scale), and three early lactation pelleted supplements. The three pelleted supplements were 100% wheat (wheat supplement), 50% wheat and 50% untreated mechanically extracted canola meal (a rumen degradable protein supplement), and 50% wheat and 50% canola meal that had been subjected to additional heat and pressure (a rumen undegradable protein supplement). It was hypothesised that fat cows would perform best with the treated canola meal supplement, which provided most rumen undegradable protein, whereas the type of protein would not be important for thin cows, and neither would the type of supplemental energy. Cows grazed pasture after calving at a daily pasture allowance of 35–40 kg DM/cow and were offered 6 kg concentrates each day. Cows were fed these diets for 92 days on average, from calving until three weeks into the mating period. Both canola meal-based supplements resulted in higher ( P < 0.05) daily milk production than the wheat supplement (33.3 and 34.0 v. 30.5 kg/cow) in both thin and fat cows. Body condition score at calving positively ( P < 0.05) affected average milk yield (31.6 v. 33.7 kg/cow), however, there was a time × BCS × supplement interaction whereby the fatter cows benefited from the use of either of the canola meal supplements for a longer period than did the thin cows. Average milk fat concentrations were greater ( P < 0.05) where the canola meal supplements were fed (39.3 and 39.6 v. 35.6 g/kg), and in fat cows relative to thin cows (40.0 v. 36.3 g/kg). Serum urea concentrations were significantly ( P < 0.05) lower in cows fed wheat than in those fed either of the canola meal supplements (3.1 v. 4.3 and 4.2 mmol/L). Serum β-hydroxybutyrate concentrations were lowest ( P < 0.05) where wheat pellets were fed, and highest ( P < 0.05) where treated canola meal pellets were fed, and plasma glucose concentrations were highest with wheat and lowest with the treated canola meal pellets. Thus, the hypothesis was not supported by the results of this experiment.

Effects of melatonin on the yield and composition of milk from grazing dairy cows in New Zealand

The aim was to determine whether administration of melatonin would alter the yield and composition of milk from grazing dairy cows in summer. Twelve sets of spring-calving identical twin Friesian cows were used in the experiment. In late-November (late spring), one twin from each set was given slow-release melatonin implants behind the ears (108 mg melatonin/cow). Two further implantations occurred at 4-weekly intervals to maintain increased circulating concentrations of melatonin for 12 weeks. The other twin served as a control. Milk yield and composition were measured twice prior to treatment and then four times over the following 12 weeks. Concentrations of melatonin, prolactin and insulin-like growth factor 1 (IGF-1) were measured in blood plasma twice before treatment and then either seven (melatonin and prolactin) or three (IGF-1) further times during the experiment. Management procedures for all cows were similar and cows grazed a daily pasture allowance of approximately 30 kg DM/cow as their sole feed source. In melatonin-treated cows there was a decrease in mean concentrations of prolactin in plasma, but concentrations of IGF-1 did not change. Melatonin reduced milk yield by 6 weeks after treatment and by the end of the 12-week experimental period milk yield in melatonin-treated cows had fallen by 23%. Melatonin also reduced concentrations of lactose in milk, but increased concentrations of fat, protein and casein, changes that were broadly similar to those that occur in late lactation in seasonally calving dairy cows. Thus, the results suggest that some of the variation in the volume and quality of milk throughout the season in New Zealand dairy systems may be due to changes in photoperiod mediated by increased concentrations of plasma melatonin in association with decreased concentrations of plasma prolactin.