Effect of altering the non-structural: structural carbohydrate ratio in a pasture diet on milk production and ruminal metabolites in cows in early and late lactation

Estimation of Nitrogen Use Efficiency for Ryegrass-Fed Dairy Cows: Model Development Using Diet- and Animal-Based Proxy Measures

This study aimed to identify suitable predictors of nitrogen (N) use efficiency (NUE; milk N/N intake) for cows that differed in breeds and were fed with ryegrass pasture, using existing data from the scientific literature. Data from 16 studies were used to develop models based on the relationships between NUE and dietary and animal-based factors. Data from a further 10 studies were used for model validation. Milk urea N (MUN) and dietary water-soluble carbohydrate-to-crudeprotein ratio (WSC/CP) were the best and most practical animal- and diet-based proxies to predict NUE. The results indicate that it might be necessary to adopt separate models for different breeds when using WSC/CP to predict NUE but not when using MUN.

Production and economic responses to intensification of pasture-based dairy production systems

Production from pasture-based dairy farms can be increased through using N fertilizer to increase pasture grown, increasing stocking rate, importing feeds from off farm (i.e., supplementary feeds, such as cereal silages, grains, or co-product feeds), or through a combination of these strategies. Increased production can improve profitability, provided the marginal cost of the additional milk produced is less than the milk price received. A multiyear production system experiment was established to investigate the biological and economic responses to intensification on pasture-based dairy farms; 7 experimental farmlets were established and managed independently for 3 yr. Paddocks and cows were randomly allocated to farmlet, such that 3 farmlets had stocking rates of 3.35 cows/ha (LSR) and 4 farmlets had stocking rates of 4.41 cows/ha (HSR). Of the LSR farmlets, 1 treatment received no N fertilizer, whereas the other 2 received either 200 or 400 kg of N/ha per year (200N and 400N, respectively). No feed was imported from off-farm for the LSR farmlets. Of the 4 HSR farmlets, 3 treatments received 200N and the fourth treatment received 400N; cows on 2 of the HSR-200N farmlet treatments also received 1.3 or 1.1 t of DM/cow per year of either cracked corn grain or corn silage, respectively. Data were analyzed for consistency of farmlet response over years using mixed models, with year and farmlet as fixed effects and the interaction of farmlet with year as a random effect. The biological data and financial data extracted from a national economic database were used to model the statement of financial performance for the farmlets and determine the economic implications of increasing milk production/cow and per ha (i.e., farm intensification). Applying 200N or 400N increased pasture grown per hectare and milk production per cow and per hectare, whereas increasing stocking rate did not affect pasture grown or milk production per hectare, but reduced milk production per cow. Importing feed in the HSR farmlets increased milk production per cow and per hectare. Marginal milk production responses to additional feed (i.e., either pasture or imported supplementary feed) were between 0.8 and 1.2 kg of milk/kg of DM offered (73 to 97 g of fat and protein/kg of feed DM) and marginal response differences between feeds were explained by metabolizable energy content differences (0.08 kg of milk/MJ of metabolizable energy offered). The marginal milk production response to additional feed was quadratic, with the greatest milk production generated from the initial investment in feed; 119, 99, and 55 g of fat and protein were produced per kilogram of feed DM by reducing the annual feed deficit from 1.6 to 1.0, 1.0 to 0.5, and 0.5 to 0 t of DM, respectively. Economic modeling indicated that the marginal cost of milk produced from pasture resulting from applied N fertilizer was less than the milk price; therefore, strategic use of N fertilizer to increase pasture grown increased farm operating profit per hectare. In comparison, operating profit declined with purchased feed, despite high marginal milk production responses. The results have implications for the strategic direction of grazing dairy farms, particularly in export-oriented industries, where the prices of milk and feed inputs are subject to the considerable volatility of commodity markets.

Invited review: An evaluation of the likely effects of individualized feeding of concentrate supplements to pasture-based dairy cows

In pasture-based dairy systems, supplementary feeds are used to increase dry matter intake and milk production. Historically, supplementation involved the provision of the same amount of feed (usually a grain-based concentrate feed) to each cow in the herd during milking (i.e., flat-rate feeding). The increasing availability of computerized feeding and milk monitoring technology in milking parlors, however, has led to increased interest in the potential benefits of feeding individual cows (i.e., individualized or differential feeding) different amounts and types of supplements according to one or more parameters (e.g., breeding value for milk yield, current milk yield, days in milk, body condition score, reproduction status, parity). In this review, we consider the likely benefits of individualized supplementary feeding strategies for pasture-based dairy cows fed supplements in the bail during milking. A unique feature of our review compared with earlier publications is the focus on individualized feeding strategies under practical grazing management. Previous reviews focused primarily on research undertaken in situations where cows were offered ad libitum forage, whereas we consider the likely benefits of individualized supplementary feeding strategies under rotational grazing management, wherein pasture is often restricted to all or part of a herd. The review provides compelling evidence that between-cow differences in response to concentrate supplements support the concept of individualized supplementary feeding.

Nutrition × reproduction interaction in pasture-based systems: is nutrition a factor in reproductive failure?

Dairy cow fertility has declined in recent decades, coincidental with large increases in milk production. Cows take longer to return to oestrus, display poorer signs of oestrus, have greater early embryo loss, and may have poorer conception rates. The problem is often considered to be nutritional, at least in part, and, therefore, can be corrected through dietary adjustment. Although acknowledged as highly digestible, high quality pastured forages tend to be low in non-structural carbohydrates (NSC), high in rumen degradable protein and the temporal supply may not be adequate for cow demand at key times; diet adjustment is often recommended to overcome these limitations. The interaction between nutrition and reproduction is poorly defined, however, and study results are often contradictory. Hypothesised limitations to pastured forages within a grazing system will be discussed, and the likely impact of nutritional adjustment on pre- and post-ovulatory reproductive processes examined. The effect of energy balance, carbohydrate type, protein and fat on reproductive outcomes will be considered. Nutrition is an important component of successful reproduction, but dietary adjustment to improve pregnancy rates is complicated, and merely offering pastured cows a supplement is unlikely to result in large effects. Conclusions indicate that care must be taken in interpreting associative analyses and in applying results from different farming systems.

In vitro digestion of fresh alfalfa under different conditions of ruminal pH

BACKGROUND

Relatively low ruminal pH values have been frequently registered in dairy cows grazing alfalfa, which can be involved in reducing feed digestion. An in vitro experiment was carried out to study the effect of ruminal pH (6.4, 6.1, 5.8 and 5.5) on the digestion of fresh alfalfa.

RESULTS

Decreasing the pH, in vitro gas production (ivGP) decreased (P < 0.05). The lowest ivGP was registered at pH 5.5 and it was product of a higher lag time and a lower digestion rate. Dry matter disappearance (DMD) was not affected by pH at 48 h (P > 0.05). Neutral detergent disappearance (NDFD) at 48 h decreased below pH 6.1. The NDFD was reduced by 62% at pH 5.5 with respect to results at pH 6.4 and 6.1 (where the highest DMD and NDFD were observed).

CONCLUSION

As expected, low rumen pH decreased alfalfa digestion. However, limits to ruminal digestion activity differed from those usually proposed for TMR diets. It is apparent that different relationships between rumen pH and NDFD exist when cows graze fresh alfalfa or grasses. Moreover, our results suggest the convenience to complement the data obtained through ivGP, DMD and NDFD. While ivGP and DMD seem to be more useful at early digestion times, NDFD may be a good predictor of final digestion.

Effects of grass silage and soybean meal supplementation on milk production and milk fatty acid profiles of grazing dairy cows

The effects of supplementation with grass silage and replacement of some corn in the concentrate with soybean meal (SBM) on milk production, and milk fatty acid (FA) profiles were evaluated in a replicated 4 x 4 Latin square study using 16 dairy cows grazing pasture composed of ryegrass, Kentucky bluegrass, and white clover. Each experimental period lasted for 3 wk. The 4 dietary treatments were PC, 20 h of access to grazing pasture, supplemented with 6 kg/d of corn-based concentrate mixture (96% corn; C); PCSB, 20 h of access to grazing pasture, supplemented with 6 kg/d of corn- and SBM-based concentrate mixture (78% corn and 18% SBM; CSB); SC, 7 h of access to grazing pasture during the day and 13 h of ad libitum access to grass silage at night, supplemented with 6 kg/d of C concentrate; and SCSB, 7 h of access to grazing pasture during the day and 13 h of ad libitum access to grass silage at night, supplemented with 6 kg/d of CSB concentrate. The concentrate mixtures were offered twice each day in the milking parlor and were consumed completely. Grass silage supplementation reduced dietary crude protein and concentration of total sugars, and dietary SBM inclusion increased dietary crude protein concentration and decreased dietary starch concentration. Milk yield and energy-corrected milk were increased by SBM supplementation of cows with access to grass silage. Milk protein concentration was lower in cows offered grass silage, regardless of whether SBM was fed. Dietary SBM inclusion tended to increase milk fat concentration. Plasma urea N was reduced by silage feeding and increased by SBM supplementation. Supplementation with grass silage overnight could represent a useful strategy for periods of lower pasture availability. Dietary inclusion of SBM in solely grazing cows had no effects on milk production and composition, exacerbated the inefficient capture of dietary N, and increased diet cost. Grass silage supplementation affected milk FA profiles, increasing both the FA derived from de novo synthesis and those derived from rumen microbial biomass, and decreasing the sum of C18 FA (mostly derived from diet or from mobilization of adipose tissue reserves). Milk fat concentrations of conjugated linoleic acid cis-9, trans-11, vaccenic acid (18:1 trans-11), and linolenic acid (18:3n-3) were unaffected by grass silage supplementation, suggesting that partial replacement of pasture by unwilted grass silage does not compromise the dietary quality of milk fat for humans.

Review of the relationship between nutrition and lameness in pasture-fed dairy cattle

Lameness of dairy cattle fed predominantly on pasture is increasingly recognised as one of the most costly disease conditions affecting dairy herds in New Zealand and Australia. Numerous risk factors are involved in the aetiology of claw lameness, including environment and factors associated with the conformation of individual cows. The role of nutrition requires further definition. Australasian pastures are characterised by low levels of fibre and effective fibre, rapid rates of fibre degradation, high water content, and high concentrations of rumen degradable protein during the autumn, winter and spring months. Relationships between high-quality vegetative pastures and ruminal acidosis may increase the risk of laminitis, particularly where pasture is supplemented with grains or other feeds containing significant amounts of starch. This article reviews the incidence, prevalence and pathophysiology of ruminal acidosis and laminitis and considers mechanisms by which acidosis may occur in pasture-fed cows. Techniques for diagnosing ruminal acidosis are reviewed, and practical strategies to avoid it are proposed. Currently, there is little information on the incidence and prevalence of ruminal acidosis and laminitis in pasture-fed cattle. The evidence gathered in this review suggests that ruminal acidosis and laminitis should be considered in the aetiology of lameness in pasture-fed dairy herds.

The effect of concentrate supplementation on nutrient flow to the omasum in dairy cows receiving freshly cut grass

An experiment was carried out to determine the effect of increasing the amount of grain-based concentrate (0, 3, or 6 kg/d) on nutrient flow to the omasum, rumen fermentation pattern, milk yield, and nutrient use of dairy cows. Harvested timothy-meadow fescue grass was fed individually 3 times daily to 6 rumen-cannulated Holstein-Friesian cows in a duplicated 3 x 3 Latin square experiment. Grass was offered as 6 equal meals daily, and concentrates were fed as 2 equal meals daily. Nitrogen, microbial N, and neutral detergent fiber (NDF) flow from the rumen were measured using an omasal sampling technique in combination with a triple marker method [CoEDTA, Yb, and indigestible NDF (INDF) as markers]. Concentrate supplementation linearly decreased ruminal pH, N degradability, ammonia N concentration, and molar proportion of acetate and increased the molar proportion of butyrate. Supplementation of grass with concentrates linearly increased dry matter intake (DMI), microbial N synthesis, N, and NDF flow to the omasum, and ruminal and total tract NDF digestibility decreased linearly. Decreases in NDF digestibility in response to concentrates was primarily related to a decrease in the rate of digestion. Increased DMI overcame the negative effects of concentrate on NDF digestion, resulting in a linear increase in total metabolizable energy intake and milk production. Physical constraints were found not to limit grass DMI. Concentrate supplementation increased the apparent use of dietary N for milk production because of a reduction in N intake, rather than thorough improvements in N capture in the rumen.

Diurnal variation in ruminal pH on the digestibility of highly digestible perennial ryegrass during continuous culture fermentation

Dairy cows grazing high-digestibility pastures exhibit pronounced diurnal variation in ruminal pH, with pH being below values considered optimal for digestion. Using a dual-flow continuous culture system, the hypothesis that minimizing diurnal variation in pH would improve digestion of pasture when pH was low, but not at a higher pH, was tested. Four treatments were imposed, with pH either allowed to exhibit normal diurnal variation around an average pH of 6.1 or 5.6, or maintained at constant pH. Digesta samples were collected during the last 3 d of each of four, 9-d experimental periods. A constant pH at 5.6 compared with a constant pH of 6.1 reduced the digestibility of organic matter (OM), neutral detergent (NDF), and acid detergent fiber (ADF) by 7, 14, and 21%, respectively. When pH was allowed to vary (averaging 5.6), digestion of OM, NDF, and ADF were reduced by 15,30, and 36%, respectively, compared with pH varying at 6.1. There was little difference in digestion parameters when pH was either constant or varied with an average pH of 6.1. However, when average pH was 5.6, maintaining a constant pH significantly increased digestion of OM, NDF, and ADF by 5, 25, and 24% compared with a pH that exhibited normal diurnal variation. These in vitro results show that gains in digestibility and potential milk production can be made by minimizing diurnal variation in ruminal pH, but only when ruminal pH is low (5.6). However, larger gains in productivity can be achieved by increasing average daily ruminal pH from 5.6 to 6.1.

Effects of Supplemental Hay and Corn Silage Versus Full-Time Grazing on Ruminal pH and Chewing Activity of Dairy Cows

Grazing young, highly digestible swards with and without supplemental hay or corn silage (5.5 kg of DM/d) offered overnight was tested for its effects on ruminal pH and chewing activity. A double 3 x 3 Latin square arrangement with 6 rumen-cannulated Brown Swiss cows (29 kg/d of milk) was applied. Herbage intake was quantified by controlled-release alkane capsules. Chewing activity was determined using an automatic microcomputer-based system for digital recording of the jaw movements. Except during milking, ruminal pH was measured continuously over 7 d by applying a device consisting of an indwelling pH electrode and a data-recording unit integrated in the cannula's cover. The grazing system had no significant effect on body weight, milk yield or composition (except milk urea), or total DM intake (13.5, 13.8, and 15.7 kg/d with full-time grazing, hay, and corn silage supplementation). No differences occurred for ruminating time per day and time per kilogram of DM intake. Full-time grazing cows spent more time eating per day (+26%) and time per kilogram of DM intake (+31%) than the other cows. Ruminal pH and time with pH <5.8 at night did not differ. Throughout the day, hay-supplemented cows had a significantly lower pH (-0.23) than full-time grazing cows, and the period of pH <5.8 was longer compared with corn-silage fed cows (77 vs. 11 min). Nocturnal supplement feeding gave no advantage over full-time grazing, and supplemental hay led to lower daytime pH.

Rumen fermentation characteristics of dairy cows grazing different allowances of Persian clover- or perennial ryegrass-dominant swards in spring

An experiment was conducted in which cows in early lactation grazed Persian clover (Trifolium resupinatum L.) or perennial ryegrass (Lolium perenne L.)-dominant pastures at low or high pasture allowances in order to determine the effects of pasture type and level of feeding on rumen fermentation patterns. The hypotheses for grazing dairy cows were: (i) the consumption of Persian clover would result in a more rapid rate of degradation and less stable rumen fermentation patterns compared with perennial ryegrass; and (ii) the greater intake of cows grazing at high compared with low pasture allowances would also cause less stable rumen fermentation patterns. Stability of rumen fermentation refers to the level to which rumen fluid pH declines, especially for long periods of a day, indicating that the rumen is not coping with neutralising and/or removing acids. Cows grazing Persian clover had lower (P<0.05) average daily rumen fluid pH (5.7 v. 5.9), molar proportions of acetic acid (68.3 v. 70.6%) and ratios of lipogenic to glucogenic volatile fatty acid (4.6 v. 5.1) in the rumen than those grazing perennial ryegrass. They had higher (P<0.05) rumen fluid ammonia-N (26.3 v. 13.0 mg/100 mL) and total volatile fatty acid (165 v. 134 mmol/L) concentrations and molar proportions of butyric (11.3 v. 10.7%) and propionic (17.2 v. 16.1%) acids than cows grazing perennial ryegrass. Cows grazing at low pasture allowances had a higher (P<0.05) average daily rumen fluid pH (5.9 v. 5.7) and lower rumen fluid ammonia-N (18.6 v. 20.7 mg/100 mL) and total volatile fatty acid (143 v. 156 mmol/L) concentrations than cows grazing at high pasture allowances. Cows given Persian clover at the high allowance had a rumen fluid pH less than 6.0 for the entire day while rumen fluid pH was below 6.0 for at least 15 h of the day on all the other treatments. There was no effect (P>0.05) of pasture allowance on the degradation rate of perennial ryegrass dry matter, but the higher allowance of Persian clover resulted in the highest (P<0.05) rate of degradation of dry matter compared with either ryegrass treatment or the low allowance of Persian clover. The effective dry matter degradability of Persian clover was greater (P<0.05) than that of perennial ryegrass, and the effective dry matter degradability of herbage in cows grazing at low allowances was greater (P<0.05) than at higher allowances. However, future research should consider neutral detergent fibre degradation in grazing dairy cows with low rumen fluid pH levels.

Supplemental carbohydrate sources for lactating dairy cows on pasture

Two experiments were conducted to evaluate steam-flaked corn and nonforage fiber sources as supplemental carbohydrates for lactating dairy cows on pasture. Cows were allotted to a new paddock of an orchardgrass (Dactylis glomerata L.) pasture twice daily in one group in both trials. In experiment 1, 28 Holstein cows, averaging 216 d in milk, were randomly assigned to either a cracked-corn (CC) or a steam-flaked (SFC) supplement in a split plot design. The supplement contained 66.7% of corn and a protein/mineral pellet. In experiment 2, 28 Holstein cows, averaging 182 d in milk, were randomly assigned to either a ground corn (GC) or a nonforage fiber (NFF)-based supplemented in a single reversal design. The GC supplement contained 85% ground corn plus protein, mineral, and vitamins. The NFF supplement contained 35% ground corn, 18% beet pulp, 18% soyhulls, 8% wheat middlings plus protein, mineral, and vitamins. In both experiments, cows were fed the grain supplement twice daily after each milking at 1 kg/4 kg milk. In experiment 1, milk production (24.3 kg/d) and composition did not differ between treatments; however, plasma and milk urea N were lower with the SFC supplement. In experiment 2, milk production (27.5 kg/d) was not affected by treatments, which may be related to the medium quality of pasture grazed. The GC supplement tended to reduce plasma and milk urea N and increased milk protein percentage (3.23 vs. 3.19%). Pasture dry matter intake, measured using Cr2O3, did not differ between treatments in either experiment 1 (15.1 kg/d) or experiment 2 (12.2 kg/d). Milk production did not differ when mid-late lactation cows on pasture were supplemented with SFC or NFF instead of dry corn.

Invited review: production and digestion of supplemented dairy cows on pasture

Literature with data from dairy cows on pasture was reviewed to evaluate the effects of supplementation on intake, milk production and composition, and ruminal and postruminal digestion. Low dry matter intake (DMI) of pasture has been identified as a major factor limiting milk production by high producing dairy cows. Pasture DMI in grazing cows is a function of grazing time, biting rate, and bite mass. Concentrate supplementation did not affect biting rate (58 bites/min) or bite mass (0.47 g of DM/bite) but reduced grazing time 12 min/d per kilogram of concentrate compared with unsupplemented cows (574 min/d). Substitution rate, or the reduction in pasture DMI per kilogram of concentrate, is a factor which may explain the variation in milk response to supplementation. A negative relationship exists between substitution rate and milk response; the lower the substitution rate the higher the milk response to supplements. Milk production increases linearly as the amount of concentrate increases from 1.2 to 10 kg DM/d, with an overall milk response of 1 kg milk/kg concentrate. Compared with pasture-only diets, increasing the amount of concentrate supplementation up to 10 kg DM/d increased total DMI 24%, milk production 22%, and milk protein percentage 4%, but reduced milk fat percentage 6%. Compared with dry ground corn, supplementation with nonforage fiber sources or processed corn did not affect total DMI, milk production, or milk composition. Replacing ruminal degradable protein sources with ruminal undegradable protein sources in concentrates did not consistently affect milk production or composition. Forage supplementation did not affect production when substitution rate was high. Fat supplementation increased milk production by 6%, without affecting milk fat and protein content. Increasing concentrate from 1.1 to 10 kg DM/d reduced ruminal pH 0.08 and NH3-N concentration 6.59 mg/dl, compared with pasture-only diets. Replacing dry corn by high moisture corn, steam-flaked or steam-rolled corn, barley, or fiber-based concentrates reduced ruminal NH3-N concentration 4.36 mg/dl. Supplementation did not affect in situ pasture digestion, except for a reduction in rate of degradation when high amounts of concentrate were supplemented. Supplementation with energy concentrates reduced digestibility of neutral detergent fiber and intake of N but did not affect digestibility of organic matter or flow of microbial N.

Prediction of ruminal pH from pasture-based diets

This study identified suitable predictors of ruminal pH and identified relationships between ruminal pH and animal measures for diets based on fresh pasture. Animal and dietary variables (121 treatment means from six countries) were collated from 23 studies of lactating dairy cows fed pasture. Mean daily ruminal pH ranged from 5.6 to 6.7 across studies. Within studies, a low ruminal pH was associated with higher (P < 0.05; r2 > 0.40) microbial N flow from the rumen, total and individual volatile fatty acid concentrations, milk and milk component yields, and dry matter intake, and with lower (P < 0.05; r2 > 0.30) concentrations of milk fat, fat:protein, and acetate:propionate. Large variation between studies meant that these ruminal and production variables could not be used to make reliable predictions of ruminal pH in future pasture-based studies or feeding scenarios. Ruminal pH was positively related (P < 0.05; r2 < 0.15) to forage neutral detergent fiber (NDF) and NDF content within study, and negatively related (P = 0.001; r2 = 0.14) to nonstructural carbohydrate across studies. No single dietary variable, or group of variables, could be used to make a reliable prediction of ruminal pH. Estimates of effective fiber for diets containing only pasture were made using the Cornell Net Carbohydrate and Protein System ruminal pH equation. Mean effectiveness of fiber in pasture was 43% of NDF, and ranged from 17 to 78% across studies. High flows of microbial nitrogen, milk, milk fat yield, and dry matter intake suggested that the performance of cows fed high quality pasture was not limited when mean ruminal pH decreased to 5.8.

Diurnal variation in pH reduces digestion and synthesis of microbial protein when pasture is fermented in continuous culture

Many models of digestion assume steady-state conditions and do not account for diurnal variation in the rumen environment. This experiment examined the relationships between diurnal pH, pasture digestion, and microbial protein synthesis. Four dual-flow continuous culture fermenters were used to test the effect of increasing time at suboptimal pH on parameters of digestion. Fermentation of high quality pasture was controlled at pH 5.4 (suboptimal) for four intervals during each 24-h period (0, 4, 8, and 12 h) according to a 4 x 4 Latin square design. During the remainder of each day, pH was controlled at 6.3 (optimal). Samples were collected during the last 3 d of each of the four 9-d experimental periods. A negative quadratic relationship was observed between time at suboptimal pH and apparent digestibility of organic matter and dry matter. The largest reduction in digestibility of organic matter, dry matter, and neutral detergent fiber was exhibited after 4 h at suboptimal pH. A negative linear relationship was found between time at suboptimal pH and microbial N flow, with the greatest decline in microbial N flow occurring at 12 h at suboptimal pH. These results suggest that the period of time that pH is below optimal may be more critical for digestion than the relationship between mean daily pH and optimal pH. Modeling non-steady-state ruminal conditions to account for diurnal variation in the ruminal environment may improve the prediction of digestion, especially fiber.

Digestion of ryegrass pasture in response to change in pH in continuous culture

The ruminal pH of dairy cows fed high quality pasture is often below values recommended to optimize digestion. Four continuous culture fermenters were used to determine the pH required for the optimal digestion of pasture. High quality pasture was fermented at four controlled levels of pH (5.4, 5.8, 6.2, and 6.6) according to a 4 x 4 Latin square design. Automatic infusion of 5 M NaOH and 5 M HCl controlled pH to +/- 0.1. Digesta samples were collected during the last 3 d of each of the four 9-d experimental periods. Digestion and synthesis of microbial protein were largely insensitive to pH across a broad range of pH (5.8 to 6.6), but a large reduction in both occurred when pH was 5.4. The digestibility of pasture dry matter and synthesis of microbial protein were optimized at pH 6.35 and 6.13, respectively. The proportions of individual volatile fatty acids were not changed as pH increased. Digestion of high quality pasture in continuous culture was comparable to that predicted by the Cornell Net Carbohydrate and Protein System between pH 6.2 and 6.6. However, the model underpredicted organic matter and fiber digestibility between pH 5.4 and 5.8, compared with values obtained in continuous culture. This suggests that when ruminal pH is less than 6.2, the model may over-predict the production response to supplementation of high quality pasture with an effective fiber source.