Prior Forage Type Influences Ruminal Responses to a Wheat Grain Challenge in Lactating Dairy Cows

Simple Summary

High-producing dairy cows require more than just pasture to meet the energy demands of milk production. Wheat is an excellent energy source for milk production; however, cows require careful adaptation and monitoring to avoid ruminal upset when large amounts of wheat are introduced. The results of this study show that careful selection of the forage that precedes wheat could allow safer and more aggressive grain introduction strategies to be used in the dairy industry.

Abstract

To increase the dry matter and metabolisable energy intake of cows, dairy farmers often supplement pasture with concentrates and conserved fodder. Feeding large amounts of highly fermentable concentrates to cows can result in metabolic issues, such as ruminal acidosis, and thus safer but more efficient introduction strategies are desirable. We assessed the role that forages play in ruminal, behavioural and production responses to a wheat grain challenge in dairy cows with no previous wheat adaptation. Multiparous lactating Holstein dairy cows (n = 16) were fed a forage-only diet of either lucerne (Medicago sativa) hay, perennial ryegrass (Lolium perenne L.) hay or one of two cultivars of zero-grazing fresh perennial ryegrass herbage (Bealey or Base), for 3 weeks. The forage diet was then supplemented with crushed wheat grain at 8 kg dry matter/cow day⁻¹, with no adaptation period. Wheat comprised between 32 and 43% of total dry matter intake. Cows fed hay maintained a higher mean ruminal fluid pH than those fed herbage, on both the forage-only diet (6.43 vs. 6.17) and the forage plus wheat diet (6.03 vs. 5.58). Following supplementation of wheat, cows fed herbage exhibited minimum ruminal fluid pH levels indicative of acute ruminal acidosis, at 5.15 and 5.06 for cultivars Bealey and Base, respectively. Furthermore, for both herbage cultivars, adding wheat resulted in a ruminal fluid pH under 6 for >20 h/day. The ruminal environment of cows fed lucerne hay remained most stable throughout the grain challenge, spending the least amount of time below pH 6.0 (9.0 h/day). Hay created a ruminal environment that was better able to cope with the accumulation of acid as wheat was digested. A combination of increased ruminating time and a slower rate of fermentation, due to higher neutral detergent fiber and lower metabolisable energy concentrations in the hays, is likely responsible for the higher ruminal fluid pH values. Forage plays a critical role in wheat introduction strategies; aggressive adaptation strategies could be implemented when a hay such as lucerne is used as the base forage.

Challenges of feeding dairy cows in Australia and New Zealand

There is a continuing evolution of feeding systems in both Australian and New Zealand dairy industries and this presents challenges for the future. Since the turn of the century, the two countries have diverged in industry growth characteristics, with Australian dairying having contracted, with 10% less milk being produced because of 20% fewer cows producing 15% more per cow, whereas New Zealand dairying has expanded, producing 83% more milk driven by a 54% increase in cow numbers and a 31% increase in milk production per cow. Solutions to optimise feed efficiency included the common themes of (1) growing more forage on farm, (2) increasing its utilisation and (3) more efficient use of supplements resulting in increases in DM intake, and they remain relevant. In New Zealand, many of the recent research activities have aimed at improving feed supply while limiting environmental impacts driven by increasing societal concern surrounding the environmental footprint of a growing and intensifying agricultural sector. In Australia, many of the recent research activities have aimed at improving feed efficiency, with a focus on understanding situations where partial mixed ration feeding systems (Australian Farm Systems 3 and 4) are sustainable. Simply growing more feed on farm can no longer be a sole objective; farms must be operated with a view to reduce the environmental footprint, with New Zealand dairy farmers increasingly needing to farm within nitrogen limits. The present review revisits and reinforces many of the concepts developed in previous reviews, but also examines the evolution of feeding systems in both countries and opportunities to improve feed efficiency and profit, while satisfying public expectations around environmental stewardship. We also identify some of the gaps in the current knowledge that warrant further research.

Offering a forage crop at pasture did not adversely affect voluntary cow traffic or milking visits in a pasture-based automatic milking system

Feed is a strong incentive for encouraging cows in automatic milking systems (AMS) to voluntarily move around the farm and achieve milkings distributed across the 24 h day. It has been reported that cows show preferences for some forages over others, and it is possible that offering preferred forages may increase cow traffic. A preliminary investigation was conducted to determine the effect of offering a forage crop for grazing on premilking voluntary waiting times in a pasture-based robotic rotary system. Cows were offered one of two treatments (SOYBEAN or GRASS) in a cross-over design. A restricted maximum likelihood procedure was used to model voluntary waiting times. Mean voluntary waiting time was 45.5±6.0 min, with no difference detected between treatments. High and mid-production cows spent 55 min/milking for low-production cows, whereas waiting time increased as queue length increased. Voluntary waiting time was 23% and 80% longer when cows were fetched from the paddock or had a period of forced waiting before volunteering for milking, respectively. The time it took cows to return to the dairy since last exiting was not affected by treatment, with a mean return time of 13.7±0.6 h. Although offering SOYBEAN did not encourage cows to traffic more readily through the premilking yard, the concept of incorporating forage crops in AMS still remains encouraging if the aim is to increase the volume or quantity of home-grown feed rather than improving cow traffic.

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 3.5 cm (PA₃ subset), and PA above 4 to 5 cm (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. 0.28 kg/kg of PA), milk production (0.11 vs. 0.17 vs. 0.24 kg/kg of PA), and milk solids production (0.008 vs. 0.010 vs. 0.013 kg/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.

Potential impacts of negative associative effects between concentrate supplements, pasture and conserved forage for milk production and dairy farm profit

A case study and whole-farm modelling approach was used to examine the potential impacts of negative associative effects on milk production and economic performance of two dairy farms in northern Victoria. The two case studies differed in herd and farm size, calving pattern, forages grown and use of labour, but both had production systems based on grazed pasture, grain fed in the dairy at milking and conserved hay fed out in the paddock. The feeding system of each farm was altered by implementing a partial mixed ration (PMR), where cows grazed once a day and received supplements in a well formulated mix once a day. Negative associative effects between feeds were included in the biophysical modelling by deriving a relationship from published studies between declining neutral detergent fibre digestibility and increasing grain intake. Before applying a PMR system, both farms were profitable and earning competitive rates of return after tax, with mean real internal rate of return higher than 5%, and positive mean annual operating profit and mean net present value, at a discount rate of 5%. Feeding a PMR enabled both farms to increase profitability and internal rate of return, particularly if milk production was increased as well, but only when associative effects were less than those in the feeding system based on grain fed in the dairy and hay in the paddock. Increased profitability was also associated with higher standard deviation in annual operating profit, internal rate of return and net present value, in other words risk increased under the PMR feeding system, as the businesses would be more vulnerable to fluctuating supplementary feed prices.

Comparison of ryegrass and red clover on the fermentation pattern, microbial community and efficiency of diet utilisation in the rumen simulation technique (Rusitec)

An in vitro experiment was designed to investigate the effects of incubating two forages with a different energy/nitrogen (N) ratio [perennial ryegrass (GR) vs red clover (RC)] on the efficiency of N utilisation by rumen microbes. Second-cut forages were incubated in artificial rumen fermenters (n = 8). Ryegrass represented a supply of quickly available N and energy for the rumen microorganism which led to a rapid fermentation and bacterial growth 2–4 h after feeding. Ryegrass also promoted greater numbers of anaerobic fungi, methanogens and cellulolytic bacteria, which tended to increase neutral detergent fibre disappearance, gas production, volatile fatty acid and methane production than observed using RC diets. On the contrary, RC provided slowly degradable N and energy, which led to a relatively slow bacterial growth (4–8 h after feeding). In terms of diet utilisation, RC diets promoted a higher N outflow (mainly as undegraded-N) and efficiency of microbial protein synthesis per organic matter disappeared. Even so, microbial protein yield was similar on both diets indicating a better N capture by microorganisms fed GR than in those fed RC diets. The use of 15N-labelled forages demonstrated that this high ammonia incorporation by bacteria-fed GR occurred mainly during the early fermentation coinciding with the highest bacterial growth. In conclusion, this experiment demonstrated that the use of isotopic labelling combined with molecular techniques provided an insight into forage utilisation by the rumen microbes; GR diets led to a better efficiency of N utilisation compared with RC; moreover the lower N outflow on GR diets may be partially compensated for a higher proportion of microbial protein leaving the system and the greater volatile fatty acid production. These findings seem to indicate that RC grazing may increase the N pollution compared with GR without substantial improvements on the rumen function, however this must be confirmed in vivo.

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.

Increasing amounts of crushed wheat fed with Persian clover herbage reduced ruminal pH and dietary fibre digestibility in lactating dairy cows

Sixteen cows in early lactation were individually fed diets consisting of fresh Persian clover (Trifolium resupinatum)-dominant pasture, offered to all cows at 3.7 kg DM/100 kg liveweight (LW); either alone or supplemented with amounts of crushed wheat ranging from ~0.3 to 0.9 kg DM/100 kg LW (four treatments with four cows per treatment). Cows fed Persian clover alone consumed 19 kg DM/day and total DM intake increased (P < 0.001) in a linear manner as the amount of wheat consumed increased, with no significant effects on clover intake. As the proportion of wheat in the diet increased, dietary neutral detergent fibre (NDF) concentrations declined from 28 to 24%, and in vivo NDF (P = 0.055) and acid detergent fibre (ADF; P = 0.015) digestibilities also declined. There were no significant effects of proportion of wheat in the diet on apparent digestibility of DM, organic matter or gross energy. The extent to which negative associative effects on NDF digestion was associated with the clover could not be determined as it was not possible to distinguish between the NDF derived from clover or wheat, but the decline in ADF digestibility suggested that most of the response lay with the clover since the wheat only contained relatively small amounts of ADF. Ruminal fluid pH was below 6.0 for more than 18 h/day in all cows. There was no effect of wheat in the diet on average ruminal fluid pH, but lowest values during the day were negatively related (P < 0.05) to the proportion of wheat in the diet. As the proportion of wheat in the diet increased, ruminal fluid ammonia-N concentration (P < 0.001) and the acetate + butyrate to propionate ratio (P < 0.001) decreased. The proportion of wheat in the diet did not affect nylon bag estimates of NDF degradation rates for grain or forage. Although most data indicated that effects of proportion of wheat in the diet on the utilisation of consumed nutrients were small, the marginal milk response to additional wheat averaged only 0.9 kg energy-corrected milk/kg DM wheat.

Utilisation of forage rape (Brassica napus) and Persian clover (Trifolium resupinatum) diets by sheep: effects on whole tract digestibility and rumen parameters

This study investigated the effect of increasing the proportion of forage rape or Persian clover in the diet on dry matter intake (DMI), whole tract digestibility and rumen function. Two experiments were conducted simultaneously; each in a 3 by 3 Latin square design with rumen-fistulated sheep fed either increasing amounts (10, 25 or 40% DM of the diet) of forage rape (experiment 1) or Persian clover (experiment 2). The remaining diet comprised short-rotation ryegrass, cereal-based pellets and maize silage, formulated to give similar metabolisable energy (MJ/kg DM), neutral detergent fibre and protein values between treatments. Purine derivatives excreted in the urine were used to estimate microbial protein synthesis. The study found similar DMI within forage rape and Persian clover treatments, although total DMI was 16% higher for the latter (29 v. 25 g/kg liveweight.day, respectively). The in vivo DM (mean = 0.75) and neutral detergent fibre (mean = 0.65) digestibility coefficients were similar (P > 0.05) within and between experiments. The mean (±s.e.) rumen pH in sheep fed forage rape-based diets (6.17 ± 0.10) was higher than for Persian clover-based diets (5.70 ± 0.09). The efficiency of microbial N supply expressed as microbial N (g/day) per kg of digestible organic matter fermented in the rumen was also similar (P > 0.05) within and between forage rape and Persian clover treatments (mean = 23.4 g microbial N/kg digestible organic matter fermented). The results indicate that increasing the proportion of high nutritive value forage rape or Persian clover within the diet of sheep did not affect the efficiency of feed utilisation.

Increasing amounts of crushed wheat fed with pasture hay reduced dietary fiber digestibility in lactating dairy cows

Sixteen cows in mid-lactation (milk yield of 23.8 +/- 2.3 kg/d) were individually fed diets consisting of chopped perennial ryegrass hay, offered at 3 kg of dry matter (DM)/100 kg of body weight (BW), fed either alone or supplemented with amounts of crushed wheat ranging from 0.4 to 1.6 kg of DM/100 kg of BW (increasing at nominal intervals of 0.4 kg of DM/100 kg of BW; 5 nominal treatments in total). Three cows were allocated to each treatment except the mid-range wheat treatment, which had 4 cows. Results were analyzed by regression because the intake of the wheat by cows within treatments varied. The hay was used to reflect the characteristics of summer pastures in southeastern Australia. Feed intake and fecal output were measured to determine digestion coefficients, feeds were incubated in nylon bags in the rumen, and rumen variables were monitored. Estimates of metabolizable energy (ME) of the hay from in vivo or in vitro digestibility were also compared. The digestibility of neutral detergent fiber (NDF) was depressed linearly as the amount of crushed wheat consumed increased to 36% of DM intake. The extent to which negative associative effects on NDF digestion were associated with the hay could not be determined, as it was not possible to distinguish between the NDF from hay and that from wheat. However, acid detergent fiber (ADF) digestion also declined, suggesting that most of the response lay with the hay because ADF was negligible in the wheat. Most data indicated that effects of proportion of wheat in the diet on the utilization of consumed nutrients were small. Despite substitution of wheat for hay reducing the forage intake of cows, there was a positive linear effect on marginal milk responses (1.3 kg of energy-corrected milk/kg of DM wheat). Mean rumen fluid pH declined as the proportion of wheat in the diet increased. The lowest pH for any individual cow during a 24-h period was 5.4, and the amount of time that rumen fluid pH was <6.0 ranged from 0 to 14 h depending on the amount of wheat consumed. It was concluded that these perturbations of the rumen environment were probably sufficient to result in negative associative effects. In addition, estimates of the ME content of the hay were higher when calculated from in vitro compared with in vivo digestibility, which has implications when estimating the amount of feed required for production.

Grazing behaviour and pattern of intake of dairy cows grazing kikuyu (Pennisetum clandestinum) grass pastures in relation to sward height and length of grazing session

To investigate how grazing time, herbage dry matter intake (DMI) and intake rate (IR) are influenced by intensive grazing management, dairy cows strip-grazing subtropical grass pastures (Pennisetum clandestinum) at two compressed sward heights (10 and 13 cm) and at five grazing durations (1, 2, 4, 8 and 15 h) and replicated over 3 days were studied. The study was conducted in summer and the cows were observed every 20 min from 1600 to 0700 hours to calculate the time spent (min/h) grazing, ruminating and resting. Total time spent grazing was 45 min longer for cows grazing the 13-cm sward than for those grazing the 10-cm sward over the 15-h grazing period. The rate of increase in grazing time was 0.64 h/h grazing duration up to 4 h after introduction to fresh pasture. IR of cows grazing the 13-cm sward was significantly higher than those grazing the 10-cm sward (0.17 v. 0.12 kg DM/min spent grazing). The difference in IRs between sward height treatments resulted from the higher DMI in the 13-cm sward within the first 4 h of grazing compared with the 10-cm sward, although following the first 4-h grazing period IR was similar for both sward heights. Grazing time increased with sward height up to a maximum of 4 h after introduction to fresh pasture and had also maximised herbage DMI by this time. These results have important practical implications for dairy cow grazing management systems because they show that dairy managers could remove cows after 4 h with little compromise in production and will help in developing optimum supplementary feeding strategies when pasture availability limits DMI.

Feed conversion efficiency as a key determinant of dairy herd performance: a review

This paper focuses on dairy herd performance in the United Kingdom and southern Australia, where feed costs have been estimated to comprise between 40 and 67% of the total costs of production. The efficiency of conversion of grazed pasture, home grown forages and purchased feeds into milk has a major bearing on farm profit. Feed conversion efficiency (FCE), defined as ‘kg milk of standardised composition with respect to protein and fat concentrations produced per kg feed dry matter consumed’, is a useful measure of the performance of a farm feeding system, but is seldom used by dairy farmers. It could also be defined as ‘g protein plus fat produced per kg feed dry matter consumed’, given that farmers are often paid for these components. The value of estimating FCE on an annual or shorter-term basis is discussed in relation to accepted principles of feed utilisation and dairy cow energy requirements. The implications of feed intake, conversion of ingested nutrients into absorbed nutrients and the subsequent utilisation of these nutrients for milk production or other purposes, as well as the effects of stage of lactation on FCE, are reviewed. Measuring FCE and identifying opportunities for improvement is relatively straightforward in housed feeding systems, but is more problematic under grazing. Hence, approaches and the key assumptions in estimating FCE in grazing situations, as well as possible limitations of these estimates, are discussed. Finally, a case study examining the potential impact of improved nutritional strategies on FCE and on margin over feed costs is presented. It is concluded that, to remain profitable, dairy farmers need to have a sound knowledge of cow nutrition, along with appropriate measures of FCE to monitor the performance of their milk production system. Such indicators of the biological performance of the farming system are most useful when used in conjunction with appropriate measures of economic performance.

Profitable feeding of dairy cows on irrigated dairy farms in northern Victoria

Milk production per cow and per farm in the irrigated region in northern Victoria have increased dramatically over the past 2 decades. However, these increases have involved large increases in inputs, and average productivity gains on farms have been modest. Before the early 1980s, cows were fed predominantly pasture and conserved fodder. There is now large diversity in feeding systems and feed costs comprise 40–65% of total costs on irrigated dairy farms. This diversity in feeding systems has increased the need to understand the nutrient requirements of dairy cows and the unique aspects of nutrient intake and digestion in cows at grazing. Principles of nutrient intake and supply to the grazing dairy cow from the past 15 years’ research in northern Victoria are summarised and gaps in knowledge for making future productivity gains are identified. Moreover, since the majority of the milk produced in south-eastern Australia is used in the manufacture of products for export, dairy companies have increased their interest in value-added dairy products that better meet nutritional requirements or provide health benefits for humans. Finally, some examples of the impacts of farm system changes on operating profit for some case study farms in northern Victoria are presented to illustrate the need for thorough analysis of such management decisions.

Associative effects between feeds when concentrate supplements are fed to grazing dairy cows: a review of likely impacts on metabolisable energy supply

Efficient conversion of grazed pasture and supplementary feeds into milk is essential to the profitability of dairy farming in southern Australia as costs of production continue to rise. The application of diet formulation approaches to the nutritional management of grazing dairy cows provides unique challenges in predicting the interactions that occur between grazed herbage and supplementary feeds. How feed intake and associative effects between feeds in lactating dairy cows grazing pasture might affect estimated metabolisable energy supplied by the diet are examined. The effects of increasing feed intake in high-producing dairy cows on decreasing the digestibility of energy, and the compensatory effects of reduced methane production, are reviewed. The factors affecting intake of grazing cows and possible effects on digestibility are considered, and include characteristics of the pasture/forage and amounts and forms of concentrates. Estimates of the potential magnitude of negative associative effects in dairy cows have been made using 2 datasets from recent experiments. Finally, the potential importance of understanding and predicting the magnitude of associative effects for efficient pasture-based dairy production is discussed. It is concluded that although associative effects between feeds in grazing dairy cows cannot be predicted with certainty, and although they involve complex interactions among a number of variables, ignoring their occurrence can lead to significant errors in both feed evaluation and ration formulation.

Effects of grain or hay supplementation on the chewing behaviour and stability of rumen fermentation of dairy cows grazing perennial ryegrass-based pasture in spring

Cows grazing highly digestible pasture in early spring can have very low rumen fluid pH that can negatively impact on the efficiency of microbial digestion. In this experiment, cows in early lactation grazed perennial ryegrass (Lolium perenne L.)-based pasture alone at low or high allowances, or at a low allowance supplemented with either a cereal grain pellet, a chopped hay cube or a cube containing cereal grain and chopped hay to determine the effects of the addition of neutral detergent fibre as hay on chewing behaviour and stability of rumen fermentation. The hypotheses tested were that: (i) supplementing high digestibility ryegrass pasture with pelleted cereal grain would increase the proportion of the day that the pH of rumen fluid was below 6.0 and would decrease the rate of degradation of neutral detergent fibre in pasture and hay; and (ii) the inclusion of chopped hay with the cereal grain supplement in cubes would reduce the proportion of the day that the rumen fluid pH was below 6.0 and restore the rate of degradation of neutral detergent fibre in pasture and hay to that in unsupplemented cows. Rumen fluid pH was highest (P<0.05) in cows fed a chopped hay cube and lowest (P<0.05) in cows fed a cube containing cereal grain and chopped hay, with no significant (P>0.05) difference between those fed pasture only or cereal grain pellets. The introduction of grain, with or without hay, did not substantially alter the pattern or magnitude of changes in rumen fluid pH, but did reduce (P<0.05) the rate of degradation of neutral detergent fibre in pasture (5.3 v. 7.6%/h) and hay (2.7 v. 5.0%/h) in the rumen. Rumen degradation rates of pasture dry matter and neutral detergent fibre were not improved by adding chopped hay. Total volatile fatty acid concentrations in rumen fluid were highest (P<0.05) on the high pasture allowance treatment and lowest (P<0.05) on the low pasture allowance and the chopped hay cube treatments. Supplementation with grain reduced (P<0.05) the proportion of acetate and increased (P<0.05) the proportion of propionate in total volatile fatty acids. There were no significant (P>0.05) effects of dietary treatment on time spent grazing or on rate of biting while grazing. Cows in treatments receiving grain supplements (532 min) and those in the high pasture allowance treatment (566 min) spent more (P<0.05) time ruminating than those in the low pasture allowance (415 min) and chopped hay cube (465 min) treatments. The relative contributions of low pH and of starch to the reduction in rates of dry matter and neutral detergent fibre degradation in the rumen cannot be determined from this experiment, however, it is likely that both factors contribute to a relative increase in the metabolic activity of non-cellulolytic microorganisms.

Increasing the intake of highly digestible Persian clover herbage reduces rumen fluid pH and the rate of degradation of neutral detergent fibre in grazing dairy cows

An experiment was conducted to investigate the hypothesis that increasing the intake of Persian clover (Trifolium resupinatum L.) would decrease rumen fluid pH and the rate of loss of neutral detergent fibre from nylon bags. It was further hypothesised that the reduction in the rate of disappearance of neutral detergent fibre with increased intake would be less in highly digestible clover than in highly digestible ryegrass or pasture hay. Sixteen rumen fistulated cows, in late lactation, were used in a completely randomised, split-plot design for 33 days. There were 4 pasture allowance treatments (9, 16, 32 and 53 kg dry matter (DM)/cow.day of Persian clover) with 4 cows per treatment. Cows grazed in individual plots, pasture intakes were measured, and rumen fluid and in sacco measurements were undertaken. Pasture DM intake increased asymptotically as pasture allowance increased from 9 to 53 kg DM. Cows grazing at the 9 kg pasture allowance spent less time grazing than cows at higher allowances (294 v. 368, 421 and 414 min, P<0.05). Cows grazing at the 2 lower allowances spent less time grazing at night than cows at the 2 higher allowances. There was no effect (P>0.05) of pasture allowance on time spent ruminating, which averaged 236 min. Estimated rates of intake increased (P<0.05) with pasture allowance. Average daily rumen fluid pH decreased linearly (P<0.05) with pasture intake, with the averages for the pasture allowance treatments being 6.03, 5.95, 5.83 and 5.79 as pasture allowance increased. The patterns of rumen fluid pH over 24 h indicated that it was only late in the night that treatment differences were detected, with the lower pasture allowance treatments recording higher rumen fluid pH values than the higher pasture allowance treatments. There was no effect of pasture allowance on average daily rumen fluid ammonia-N concentrations (25 mg/100 mL). Total volatile fatty acids concentrations averaged 139, 152, 163 and 168 mmol/L as pasture allowance increased from 9 to 53 kg DM/cow.day. The proportion of acetate in total volatile fatty acids generally declined (71.4, 70.4, 67.4 and 69.2%; s.e.d. = 1.14) and the proportion of propionate generally increased (15.2, 15.5, 17.6 and 17.0%; s.e.d. = 0.77) as pasture allowance increased from 9 to 53 kg DM/cow, respectively. Rate of neutral detergent fibre loss from nylon bags was highest in clover, and lowest in hay, and was higher in cows grazing at 9 and 16 kg allowances compared with cows at 32 and 53 kg (P<0.05). There was a linear relationship (P<0.05) between rate of neutral detergent fibre loss and rumen fluid pH for clover and ryegrass, but not hay (P>0.05). There were no differences (P>0.05) in total rumen contents (75.6 kg; s.e.d. = 6.95), or DM (7.3 kg; s.e.d. = 0.73) and neutral detergent fibre (2.7 kg; s.e.d. = 0.32) pools, of cows grazing at different allowances. Offering cows increasing allowances of Persian clover pasture reduced rumen digesta retention times, as rumen pool sizes did not change. Average daily pH fell with increasing allowance due to differences in daily intake and pH patterns, and increasing pasture allowance decreased the rate of disappearance of neutral detergent fibre. It is suggested that increased outflow rates, driven by differences in daily pH patterns and changes in substrate composition, were responsible for the decline in disappearance of neutral detergent fibre from nylon bags, but the effects of the factors cannot be separated.