Sites of digestion and bacterial protein synthesis in dairy heifers fed fresh oats with or without corn or barley grain

A meta-analysis of the relationship between milk protein production and absorbed amino acids and digested energy in dairy cattle

Milk protein production is the largest draw on AA supplies for lactating dairy cattle. Prior NRC predictions of milk protein production have been absorbed protein (MP)-based and used a first-limiting nutrient concept to integrate the effects of energy and protein, which yielded poor accuracy and precision (root mean squared error [RMSE] >21%). Using a meta-data set gathered, various alternative equation forms considering MP, absorbed total EAA, absorbed individual EAA, and digested energy (DE) supplies as additive drivers of production were evaluated, and all were found to be superior in statistical performance to the first limitation approach (RMSE = 14%-15%). Inclusion of DE intake and a quadratic term for MP or absorbed EAA supplies were found to be necessary to achieve intercept estimates (nonproductive protein use) that were similar to the factorial estimates of the National Academies of Sciences, Engineering, and Medicine (2021). The partial linear slope for MP was found to be 0.409, which is consistent with the observed slope bias of -0.34 g/g when a slope of 0.67 was used for MP efficiency in a first-limiting nutrient system. Replacement of MP with the supplies of individual absorbed EAA expressed in grams per day and a common quadratic across the EAA resulted in unbiased predictions with improved statistical performance as compared with MP-based models. Based on Akaike's information criterion and biological consistency, the best equations included absorbed His, Ile, Lys, Met, Thr, the NEAA, and individual DE intakes from fatty acids, NDF, residual OM, and starch. Several also contained a term for absorbed Leu. These equations generally had RMSE of 14.3% and a concordance correlation of 0.76. Based on the common quadratic and individual linear terms, milk protein response plateaus were predicted at approximately 320 g/d of absorbed His, Ile, and Lys; 395 g/d of absorbed Thr; 550 g/d of absorbed Met; and 70 g/d of absorbed Leu. Therefore, responses to each except Leu are almost linear throughout the normal in vivo range. De-aggregation of the quadratic term and parsing to individual absorbed EAA resulted in nonbiological estimates for several EAA indicating over-parameterization. Expression of the EAA as g/100 g total absorbed EAA or as ratios of DE intake and using linear and quadratic terms for each EAA resulted in similar statistical performance, but the solutions had identifiability problems and several nonbiological parameter estimates. The use of ratios also introduced nonlinearity in the independent variables which violates linear regression assumptions. Further screening of the global model using absorbed EAA expressed as grams per day with a common quadratic using an all-models approach, and exhaustive cross-evaluation indicated the parameter estimates for BW, all 4 DE terms, His, Ile, Lys, Met, and the common quadratic term were stable, whereas estimates for Leu and Thr were known with less certainty. Use of independent and additive terms and a quadratic expression in the equation results in variable efficiencies of conversion. The additivity also provides partial substitution among the nutrients. Both of these prevent establishment of fixed nutrient requirements in support of milk protein production.

Changing the grazing session from morning to afternoon or including tannins in the diet was effective in decreasing the urinary nitrogen of dairy cows fed a total mixed ration and herbage

Our aim was to evaluate whether increasing soluble carbohydrates in the herbage by changing the time of the grazing session or including Acacia mearnsii tannin in the diet would affect intake, digestion, N partitioning, and productive performance of dairy cows fed a diet combining ryegrass herbage with partial total mixed ration (PMR). We hypothesized that both strategies could reduce the concentration of NH₃-N in the rumen, reducing urinary N excretion. Nine Holstein cows were used in a triplicate 3 × 3 Latin square experiment with 3 experimental periods of 22 d. The cows were fed a fixed amount of PMR [60% of the predicted individual dry matter intake (DMI)], and an unrestricted amount of herbage in 1 grazing session of 5 h/d. The treatments were (1) morning grazing session and afternoon PMR meal (AM); (2) morning PMR meal and afternoon grazing session (PM); and (3) morning grazing session and afternoon PMR meal supplemented with 15.0 g of tannins/kg of PMR dry matter (TAN). Milk production was not affected by treatments. Although the protein concentration was lower for TAN than for PM, no differences were detected for the yield of any component between treatments. The concentration of individual or grouped fatty acids in milk fat was not affected by treatments, except for 16:1 cis-9 and Δ⁹-desaturase ratios 14:1/14:0 and 16:1/16:0, which were lower for TAN. Treatments did not affect total DMI, but PM tended to increase herbage DMI and reduce dry matter and crude protein digestibilities. Treatments did not affect cow eating and ruminating behavior except for the proportion of time spent eating PMR, which was higher for PM and TAN. Although no relevant effects of treatments on ruminal fermentation, purine derivatives excretion in urine, or N excretion in milk were detected, both PM and TAN decreased the total N excreted in urine by an average of 8% compared with AM. In conclusion, changing the grazing session from the morning to the afternoon and including tannins in the diet were effective in decreasing the excretion of urinary N but did not change the productive performance of dairy cows fed PMR and ryegrass herbage.

Effects of barley and corn as sources of silage and grain on dry matter intake, ruminal fermentation, and total-tract digestibility in growing beef heifers

The objective of this study was to evaluate the use of barley (BS) or corn (CS) silage when fed with dry-rolled barley grain (BG), corn grain (CG), or an equal blend (BCG) of BG and CG for backgrounding cattle. Ruminally cannulated heifers (n = 5) were assigned to an incomplete 6 × 6 Latin square design. Treatments contained either BS or CS in combination with BG, CG, or BCG. Samples were collected to determine dry matter intake, ruminal fermentation, total-tract nutrient digestibility, and nitrogen (N) balance. Interactions between cereal silage and cereal grain sources were detected for the molar proportions of propionate and butyrate where the magnitude of change with the type of cereal grain was increased in heifers fed CS relative to BS. Feeding CS increased estimated microbial N production (silage, P = 0.022) and fecal N excretion (silage, P = 0.042) over BS. Diets containing BG had greater dry matter, organic matter, starch, and gross energy digestibility values compared with CG, but values for BCG were not different (grain, P ≤ 0.043). Based on limited silage × grain interactions, use of CS in backgrounding diets may increase microbial N production relative to BS and dry-rolled CG may reduce apparent total-tract nutrient digestibility relative to dry-rolled BG.

The nutritional evaluation of forage-based mixed rations in New Zealand using an in vitro gas production technique. 1: analytical survey

Greenhouse gas (GHG) emissions from livestock are an important consideration in environmental science. Estimating GHG production can be problematic at a farm or animal level, and requires controlled conditions to produce real data. An in vitro gas production technique (IVGPT) was developed to evaluate forage-based total mixed rations in digestion kinetics and GHG production. Two hundred and sixty samples of complete mixed rations (MR), which included a pasture component used in commercial lactating dairy herds, were collected around NZ across three calendar years, 2017-2019. Twenty of the 260 samples were 100% total mixed rations (TMR) with no pasture content. The samples were submitted for proximate analysis as well as IVGPT to generate GHG production figures. The results showed an average total gas production (TGP) of 129.82 ml/g dry matter (DM), 78.6% true digestibility (TDMD), 125.06 mg/g DM microbial biomass (MB), 20.16 g CH4/kg DM, and 12.8 MJME/kg DM. The average nutrient composition was dry matter (DM) 31.55%, crude protein (CP) 21.85%, neutral detergent fibre (NDF) 44.35%, and starch 7.03%. The IVGPT CH4 production was negatively correlated to NDF (r=-0.312), ADF (r=-0.193), TGP (r=-0.216), and was positively correlated with TDMD (r=0.250), apparent digestibility (ADMD) (r=0.614), starch (r=0.117) and volatile fatty acids (r=0.538). The MR diet showed a strong positive relationship with ADMD digestibility (P=0.01) and a negative relationship with fibre content (NDF, P=0.01 and ADF, P=0.01). However, CH4 production reduced linearly with increasing TGP (P=0.01). The results indicated that a greater CH4 production may be related to higher digestibility of mixed ration.

Rumen metabolism, omasal flow of nutrients, and microbial dynamics in lactating dairy cows fed fresh perennial ryegrass (Lolium perenne L.) not supplemented or supplemented with rolled barley grain

The objective of this study was to evaluate the effect of rolled barley grain (RB) supplementation on rumen metabolism, omasal flow of nutrients, and microbial dynamics in lactating dairy cows fed fresh perennial ryegrass (Lolium perenne L.; PRG)-based diets. Ten ruminally cannulated Holstein cows averaging (mean ± standard deviation) 49 ± 23 d in milk and 513 ± 36 kg of body weight were assigned to 1 of 2 treatments in a switchback design. The treatment diets were PRG only (G) or PRG plus 3.5 kg of dry matter RB (G+RB). The study consisted of three 29-d periods where each period consisted of 21 d of diet adaptation and 8 d of data and sample collection. A double marker system was used to quantify nutrient flow entering the omasal canal along with labeled ¹⁵N-ammonium sulfate to measure bacterial, protozoal, and nonmicrobial N flow. Rumen evacuation techniques were used to determine nutrient and microbial pool size, allowing the calculation of fractional rates of digestion and microbial growth. There was no difference in daily milk yield or energy-corrected milk yield between treatments. Milk fat concentration and milk urea N decreased, whereas milk protein concentration increased in cows fed the G+RB diet. During the omasal sampling phase, dry matter intake was higher in cows fed the G+RB diet. Ruminal and total-tract neutral detergent fiber digestibility was lower in G+RB cows; however, no difference was observed in reticulorumen pH. The rumen pool size of fermentable carbohydrate was increased in cows fed the G+RB diet; however, the fractional rate of digestion was decreased. Flow of nonammonia N and bacterial N at the omasal canal increased in cows fed the G+RB diet compared with the G diet. Protozoa N flow was not different between diets; however, protozoa appeared to supply a much larger amount of microbial N and exhibited shorter generation time than previously considered. Feed N ruminal digestibility, corrected for microbial contribution, was similar for both treatments (88.4 and 89.0% for G and G+RB, respectively). In conclusion, RB supplementation did not benefit overall animal performance; however, it reduced ruminal neutral detergent fiber digestibility and increased bacterial N flow. The results demonstrate the large dependence of cows consuming PRG-based diets on microbial N as the main source of nonammonia N supply. Additional quantitative research is required to further describe the supply of nutrients and microbial dynamics in cows consuming PRG-based diets in an effort to determine most limiting nutrients.

Combination of legume-based herbage and total mixed ration (TMR) maintains intake and nutrient utilization of TMR and improves nitrogen utilization of herbage in heifers

Diets combining herbage and total mixed rations (TMR) are increasingly used in temperate regions for feeding ruminants, but little information is available regarding the effects on nutrient intake and digestion of this feeding management in beef cattle. The aim of this study was to determine the effects of combining TMR (10% CP and 13% ADF), and legume-based herbage (14% CP and 27% ADF) on intake, nutrient digestion, ruminal fermentation, microbial N flow and glucose and nitrogen metabolism in heifers. The experiment was a 3×3 Latin square design replicated three times; each period lasted 18 days (10 adaptation days and 8 measurement days). Nine cross-bred (Aberdeen Angus×Hereford) heifers (214±18 kg) fitted with permanent rumen catheters and housed in individual metabolic cages were assigned to one of three treatments: 24 h access to TMR ( T ), 24 h access to herbage ( H ) or combined diets with 18 h access to TMR and 6 h access to herbage ( T+H ). Data were evaluated using a mixed model. Animals fed T+H (TMR 71% and herbage 29%) diets tended to have a higher dry matter intake as a proportion of their BW than animals fed T. The T+H diet did not change ruminal fermentation (pH, N-NH3 and volatile fatty acids) or the N metabolism relative to the T diet, but increased the glucagon concentration and altered glucose metabolism. Conversely, animals fed T+H had increased purine derivatives excretion, increased N use efficiency for microbial protein synthesis and decreased plasma urea and urinary N excretion relative to animals fed H diet. The use of combined diets led to consumption of nutrients similar to a TMR diet, without reducing nutrient use and could improve N utilization compared with the herbage-only diet.

Effect of starchy or fibrous carbohydrate supplementation of orchardgrass on ruminal fermentation and methane output in continuous culture

A 4-unit dual-flow continuous culture fermentor system was used to assess the effects of supplementing orchardgrass (Dactylis glomerata L.) with 2 levels [5 and 10% of total dry matter (DM) fed] of starchy (barley grain, BAR) or fibrous (beet pulp, BP) carbohydrate sources on nutrient digestibility, volatile fatty acid (VFA) production, bacterial protein synthesis, and CH4 output. Treatments were randomly assigned to fermentors in a 4×4 Latin square design with a 2×2 factorial arrangement using 7 d for microbial adaptation and 3 d for sample collection. Treatments included (1) 57g of DM orchardgrass + 3g of DM BAR, (2) 54g of DM orchardgrass + 6g of DM BAR, (3) 57g of DM orchardgrass + 3g of DM BP, or (4) 54g of DM orchardgrass + 6g of DM BP. Feedings occurred at 0900, 1030, 1400, and 1900h throughout four 10-d periods. Gas samples for CH4 analysis were collected 6 times daily at 0725, 0900, 1000, 1355, 1530, and 1630h. Fermentor samples for pH, NH3-N, and VFA analysis were taken on d 8, 9, and 10. Samples were also analyzed for DM, organic matter, crude protein, purines, neutral detergent fiber, and acid detergent fiber to determine nutrient digestibilities and estimation of bacterial protein synthesis. Apparent and true DM and organic matter digestibilities were not affected by supplement source. Apparent neutral and acid detergent fiber digestibilities were greater for BAR than BP. Conversely, apparent crude protein digestibility was greater for BP than BAR. Mean and maximum pH tended to be greatest for BAR than BP. Minimum pH was greater at the lower level (5% of diet DM) of supplementation. Barley produced greater concentrations of total VFA and acetate, whereas BP had greater daily outputs of CH4. Significant supplement type × level interactions were found for bacterial N flow and efficiency. Overall, supplementing orchardgrass with BP improved crude protein digestibility, reduced fiber digestibility and total VFA concentration, but increased CH4 output compared with BAR.

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.

Implantation of an ultrafiltration device in the ileum and spiral colon of steers to continuously collect intestinal fluid

Collection of fluid from the lumen of the gastrointestinal tract is commonly necessary for research projects, but presents challenges including intestinal motility and potential for leakage of intestinal contents. In this study, ultrafiltration collection devices were surgically implanted in the ileum and spiral colon of 12 steers for repeated collection of intestinal fluid over 48 hours. There were no significant complications associated with surgery or during the post-operative period, nor were there any significant pathologic changes found at necropsy 3 or 4 days post-surgery. Over 48 hours, we obtained 88% of the desired 212 samples. Only two devices failed to routinely collect samples. Use of ultrafiltration probes is a novel, consistent and humane method to repeatedly sample the gastrointestinal contents.

Supplementation with non-fibrous carbohydrates reduced fiber digestibility and did not improve microbial protein synthesis in sheep fed fresh forage of two nutritive values

To determine whether non-fibrous carbohydrate (NFC) supplementation improves fiber digestibility and microbial protein synthesis, 18 Corriedale ewes with a fixed intake level (40 g dry matter (DM)/kg BW0.75) were assigned to three (n = 6) diets: F = 100% fresh temperate forage, FG = 70% forage + 30% barley grain and FGM = 70% forage + 15% barley grain + 15% molasses-based product (MBP, Kalori 3000). Two experimental periods were carried out, with late (P1) and early (P2) vegetative stage forage. For P2, ewes were fitted with ruminal catheters. Forage was distributed at 0900 h, 1300 h, 1800 h and 2300 h, and supplement added at 0900 h and 1800 h meals. Digestibility of the different components of the diets, retained N and rumen microbial protein synthesis were determined. At the end of P2, ruminal pH and N-NH3 concentration were determined hourly for 24 h. Supplementation increased digestibility of DM (P < 0.001) and organic matter (OM; P < 0.001) and reduced NDF digestibility (P = 0.043) in both periods, with greater values in P2 (P = 0.008) for the three diets. Daily mean ruminal pH differed (P < 0.05) among treatments: 6.33 (F), 6.15 (FG) and 6.51 (FGM). The high pH in FGM was attributed to Ca(OH)2 in MBP. Therefore, the decreased fiber digestibility in supplemented diets could not be attributed to pH changes. The mean ruminal concentration of N-NH3 was 18.0 mg/dl, without differences among treatments or sampling hours. Microbial protein synthesis was greater in P2 (8.0 g/day) than in P1 (6.1 g/day; P = 0.006), but treatments did not enhance this parameter. The efficiency of protein synthesis tended to be lower in supplemented groups (16.4, 13.9 and 13.4 in P1, and 20.8, 16.7 and 16.2 g N/kg digestible OM ingested in P2, for F, FG and FGM, respectively; P = 0.07) without differences between supplements. The same tendency was observed for retained N: 2.55, 1.38 and 1.98 in P1, and 2.28, 1.23 and 1.10 g/day in P2, for F, FG and FGM, respectively; P = 0.05). The efficiency of microbial protein synthesis was greater in P2 (P = 0.007). In conclusion, addition of feeds containing NFCs to fresh temperate forage reduced the digestibility of cell walls and did not improve microbial protein synthesis or its efficiency. An increase in these parameters was associated to the early phenological stage of the forage.

The Effect of Carbohydrate Source on Nitrogen Capture in Dairy Cows on Pasture

The objective of this study was to determine if feeding carbohydrate supplements with faster degradation rates than corn to dairy cows grazing ryegrass would improve nitrogen capture, milk production, and components. Treatments were grain supplements based on: 1) corn (CORN), 2) barley and molasses (BM), or 3) citrus pulp and molasses (CM). For BM and CM, the diet composition was the same as that of CORN except that a portion of the corn was replaced with barley and molasses or citrus pulp and molasses, respectively, on a dry matter basis. Cows grazed ryegrass (Lolium multiflorum Lam.) pasture. Yield of milk, 3.5% fat-corrected milk, energy-corrected milk, and milk fat, as well as milk fat percentage, were not different among treatments. True milk protein percentage was higher for CORN (2.81%) compared with CM (2.70%), but was not different for BM (2.77%). However, true milk protein yield was not different among treatments. Milk urea N was higher for BM (11.43 mg/dL) compared with both CORN and CM (average: 9.95 mg/dL). There were no differences among CORN, BM, and CM treatments for overall BUN (average: 10.60 mg/dL). At 0400 h, however, cows on CORN had higher BUN than cows on CM (11.43 vs. 9.96 mg/dL), but there were no differences between CORN and BM (average: 11.21 mg/dL) or BM and CM (average: 10.48 mg/dL), and there were no differences among treatments at other time points. The CM diet might have shown more advantage if the pasture crude protein content was higher. Partial replacement of corn with citrus pulp for grazing cows should be further studied using pasture with higher crude protein content. Although cows receiving CM and BM did not produce more milk than cows on CORN, if barley or citrus pulp is less expensive than corn, they may be viable replacements for a portion of the corn supplement for grazing cows.

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.

Opportunities for future Australian dairy systems: a review

During the last decade, Australian dairy farmers have been challenged to increase total factor productivity (the ratio between the rate of increase in total output and the rate of increase in the use of all inputs) in order to attenuate the negative effects of a steady decline in the terms of trade over the same period of time. Overall, the increase in total factor productivity has been low (1.5%) and farmers are questioning the most appropriate production system for the future. In an attempt to address this central question, we first identified the nature of the key pressures dairy farmers in Australia are likely to face in the future, namely labour and feed related issues. We then discuss major opportunities for developing new dairy production systems based on increased efficiency in the use of land and cows and on increasing the efficiency of labour management and lifestyle. We do not attempt to provide the best futuristic option for dairy systems in Australia. Instead, this review discusses key areas of the production system with potential to impact positively on any or all the physical, economic and labour-related aspects of modern dairy farming. By so doing, this review highlights the research questions that need to be addressed now in order to provide Australian dairy farmers with improved tools to manage their production systems in the future.

Effect of Corn Particle Size on Site and Extent of Starch Digestion in Lactating Dairy Cows

Two experiments were conducted to determine the effects of corn particle size (CPS) on site and extent of starch digestion in lactating dairy cows. Animals were fitted with ruminal, duodenal, and ileal cannulas. Dry corn grain accounted for 36% of dry matter intake. In experiment 1, 6 cows were used in a duplicate 3 x 3 Latin square design. Semiflint corn was used. Corn processing methods were grinding, medium rolling, and coarse rolling. The mean particle size of the processed corn was 730, 1807, and 3668 microm, respectively. Rumen digestibility of starch linearly decreased from 59% with ground corn to 36% with coarsely rolled corn. Similarly, small intestine digestibility linearly decreased with increased CPS, and consequently, the amount of starch digested in the small intestine was not affected by corn processing. In experiment 2, 4 cows were used in a 2 x 2 crossover design. Dent corn was used. Corn processing methods were grinding and coarse rolling. The mean particle size of the processed corn was 568 and 3458 microm, respectively. Rumen digestibility of starch decreased from 70% with ground corn to 54% with coarsely rolled corn. Small intestine digestibility of starch was not significantly affected by CPS, and the amount of starch digested in the small intestine tended to be greater for rolled than for ground corn. In both experiments, starch total tract digestibility decreased with increased CPS. In conclusion, CPS is an efficient tool to manipulate rumen degradability of cornstarch. In midlactation cows, the decrease in the amount of starch digested in the rumen between grinding and coarse rolling is partly compensated for by an increase in the amount of starch digested in the small intestine with dent genotype, but with semiflint genotype postruminal digestion is not increased and rumen escape starch is not utilized by the animal.

Pasture intake and substitution rate effects on nutrient digestion and nitrogen metabolism during continuous culture fermentation

A continuous culture system was used to investigate ruminal digestion in response to increased pasture intake and three different substitution rates (SR) in a 4 x 4 Latin square design. The treatments were 1) low pasture (55 g dry matter (DM)/d, 2) medium pasture (MP, 65 g DM/d), 3) high pasture (75 g DM/d), and 4) pasture (45 g DM/d) plus concentrate (PC, 30 g DM/d). Treatments were designed to produce a low (0.33), medium (0.67), and high (1.00) SR (g of pasture/g of concentrate) by contrasting the low, medium, and high pasture intake treatments with the pasture plus concentrate treatment, respectively. Pasture was fed at 0630, 1000, 1730, and 2100 h, and concentrate at 0600 and 1700 h. Digestibility of DM and neutral detergent fiber were not affected by the amount of pasture. As the amount of pasture increased, pH decreased linearly, and total volatile fatty acid and NH3-N concentrations, and nonammonia N and bacterial N flows increased linearly. Concentrate supplementation did not affect DM digestibility at high SR but increased DM digestibility at low SR. Concentrate supplementation reduced pH and NH3-N concentrations at the three SR. Concentrate supplementation reduced the ratio of rumen degradable N to rumen degradable organic matter; however, the mechanism depended on the SR. High SR, concentrate supplementation reduced rumen degradable N, which reduced NIH-N concentration without affecting bacterial N flow. At low SR, concentrate supplementation increased rumen degradable organic matter, which reduced NH3-N concentration and increased bacterial N flow. Based on these results, at low SR, concentrate supplementation may enhance animal performance because of higher total DM intake and synthesis of microbial protein.

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.

Ruminal digestion by dairy cows grazing winter oats pasture supplemented with different levels and sources of protein

Six Holstein cows fitted with ruminal cannulae were used in two simultaneous 3 x 3 Latin squares to study the effects of protein supplements on ruminal fermentation and in situ crude protein degradability. Cows rotationally grazed a winter oats (Avena sativa L.) pasture and were supplemented with one of three concentrate supplements: 1) low protein sunflower meal (L-SM); 2) high protein sunflower meal (H-SM); or 3) high protein feather meal (H-FM). Concentrates (6.5 kg/d) were offered in equal portions twice daily during milking. Ruminal pH and total volatile fatty acids concentration were unaffected by treatments. Supplementation with L-SM and H-FM decreased ruminal NH3-N concentration compared with H-SM. The concentrate with feather meal had lower effective rumen degradability of crude protein than concentrates containing sunflower meal. Effective rumen degradability of crude protein of pasture averaged 82.7%. Thirty-six multiparous Holstein cows (71 d in milk) were used in a complementary experiment to study the effect of treatments on intake, milk yield, and milk composition. Pasture (13.2 kg/d) and total (19.6 kg/d) dry matter intake (estimated using Cr2O3 as fecal marker) and milk yield (20.5 kg/d) were unaffected by level or source of protein supplemented. Intake of rumen undegradable protein in grazing dairy cows was higher when the amount of sunflower meal was increased or when feather meal was used in the supplement. However, higher rumen undegradable protein intake did not increase milk production, suggesting that rumen undegradable protein was not limiting for cows on pasture producing less than 22 kg of milk.