Effects of acetate, propionate, and pH on volatile fatty acid thermodynamics in continuous cultures of ruminal contents.

To investigate the effects of acetate, propionate, and pH on thermodynamics of volatile fatty acids (VFA) in the rumen, a dual-flow continuous culture study was conducted to quantify production of major VFA, interconversions among the VFA, and H2 and CH4 emissions in a 4 × 4 Latin square design. The 4 treatments were (1) control: pH buffered to an average of 6.75; (2) control plus 20 mmol/d of infused acetate (InfAc); (3) control plus 7 mmol/d of infused propionate (InfPr); and (4) a 0.5-unit decline in pH elicited by adjustment of the buffer (LowpH). All fermentors were fed 40 g of a pelleted diet containing whole alfalfa pellets and concentrate mix pellets (50:50) once daily. After 7 d of treatment, sequential, continuous infusions of [2-13C] sodium acetate (3.5 mmol/d), [U-13C] sodium propionate (2.9 mmol/d), and [1-13C] sodium butyrate (0.22 mmol/d) were carried out from 12 h before feeding for 36 h. Filtered liquid effluent (4 mL) was sampled at 0, 2, 4, 6, 8, 12, 16, and 22 h after feeding, and assessed for VFA concentrations, with another filtered sample (20 mL) used to quantify aqueous concentrations of CH4 and H2. Headspace CH4 and H2 gases were monitored continuously. Ruminal microbes were isolated from the mixed effluent samples, and the microbial community structure was analyzed using the 16S rRNA amplicon sequencing technique. The digestibility of neutral detergent fiber, acid detergent fiber, and starch and microbial C sequestrated from VFA were not affected by treatments. The LowpH treatment increased net propionate production and decreased H2 and CH4 headspace emissions, primarily due to shifts in metabolic pathways of VFA formation, likely due to the observed changes in bacterial community structure. Significant interconversions occurred between acetate and butyrate, whereas interconversions of other VFA with propionate were relatively small. The InfAc and InfPr treatments increased net acetate and propionate production, respectively; however, interconversions among VFA were not affected by pH, acetate, or propionate treatments, suggesting that thermodynamics might not be a primary influencer of metabolic pathways used for VFA formation.

Test of conditions that affect in vitro production of volatile fatty acids and gases.

In vitro methods have been developed to measure digestibility, but such methods may not accurately reflect gas production or volatile fatty acid (VFA) profiles. The objective of this study was to determine the effect of different in vitro conditions on VFA and gas production. The experimental design was a 4 × 2 × 2 factorial CRD with four replicates. Treatments were four ratios of medium to rumen fluid by volume (5:95, 25:75, 50:50, and 75:25), two concentrations (w/v) of added timothy hay (0.5% or 1%), with or without added sodium acetate (increased initial concentration by 50 mM). Total volume of medium and rumen fluid was 10 mL per tube. Measurements of gas production and VFA were recorded at 0, 4, 16, 24, and 48 h. Statistical analyses used a mixed model including all fixed effects and interactions with tube as a random effect, and time nested within tube. Total gas production increased (P < 0.001) with higher medium proportion. The final pH increased (P < 0.0001) as medium proportion increased. Medium proportion positively affected (P < 0.05) overall average concentration of both acetate production and propionate production. Higher hay concentration increased (P < 0.0001) total gas produced from 0 to 48 h, increased total acetate production (P < 0.01), propionate production (P < 0.001), and decreased pH between 24 and 48 h (P < 0.0001). Sodium acetate addition increased (P < 0.0001) pH between 24 and 48 h. Acetate:propionate (A:P) concentration decreased over time (P < 0.0001). Initial rumen fluid A:P ratio was 3.7 but average A:P ratio of produced VFA started at 2.2 and increased to 2.50 (SE = ±0.51). The A:P ratio differed for VFA produced in vitro compared to initial rumen fluid, but no tested treatments were found to change A:P ratio.

Representing interconversions among volatile fatty acids in the Molly cow model.

The Molly cow model uses fixed stoichiometric coefficients for predicting volatile fatty acid (VFA) production from the fermented individual dietary nutrient fractions of forage and concentrate. We previously showed that predictions of VFA production had large errors and hypothesized that it was due to a lack of representation of carbon exchange among VFA. The objectives of the present study were to add VFA interconversion equations based on thermodynamics to the Molly cow model and evaluate the effect of these additions on model accuracy and precision of VFA predictions. Previously described thermodynamic equations were introduced to represent interconversions among VFA. The model was further modified to predict de novo acetate, propionate, and butyrate production coefficients based on forage-to-concentrate ratios rather than discrete, fixed sets of coefficients for forage-based, concentrate-based, and mixed diets. Both the original model and the modified one were reparameterized and evaluated against a common data set containing 8 studies reporting pH, VFA concentration, and VFA production rates using isotope dilution techniques and 62 studies reporting VFA concentrations and pH. Evaluations after parameter estimation revealed that predictions of VFA production rates were not improved, with root mean squared prediction errors (RMSPE) of 77, 60, and 51% for acetate, propionate, and butyrate, respectively, for the revised model versus 75, 63, and 55, respectively, for the original model. The RMSPE for predictions of VFA concentrations were reduced from 28, 46, and 40% to 22, 31, and 26% for acetate, propionate, and butyrate, respectively, simply by rederiving the VFA coefficients, but minimal further improvement was achieved with the addition of thermodynamically driven interconversion equations (RMSPE of 21, 32, and 27% for acetate, propionate, and butyrate, respectively). Thus, the results indicate that thermodynamically driven interchanges among VFA, as represented in this study, may not be a primary determinant for the accuracy of predictions of net production rates. Including the effect of pH on VFA absorption reduced the mean bias of propionate production and slope bias of acetate production, but not the overall RMSPE. The larger prediction errors for VFA production as compared with concentrations suggest the data quality may not be high, or that our representation of VFA production and absorption as well as ruminal digestion is inadequate. Additional data are required to discriminate among these hypotheses.

Corn silage versus corn silage:alfalfa hay mixtures for dairy cows: effects of dietary potassium, calcium, and cation-anion difference.

Corn silage (CS) has replaced alfalfa hay (AH) and haylage as the major forage fed to lactating dairy cows, yet many dairy producers believe that inclusion of small amounts of alfalfa hay or haylage improves feed intake and milk production. Alfalfa contains greater concentrations of K and Ca than corn silage and has an inherently higher dietary cation-anion difference (DCAD). Supplemental dietary buffers such as NaHCO(3) and K(2)CO(3) increase DCAD and summaries of studies with these buffers showed improved performance in CS-based diets but not in AH-based diets. We speculated that improvements in performance with AH addition to CS-based diets could be due to differences in mineral and DCAD concentrations between the 2 forages. The objective of this experiment was to test the effects of forage (CS vs. AH) and mineral supplementation on production responses using 45 lactating Holstein cows during the first 20 wk postpartum. Dietary treatments included (1) 50:50 mixture of AH and CS as the forage (AHCS); (2) CS as the sole forage; and (3) CS fortified with mineral supplements (CaCO(3) and K(2)CO(3)) to match the Ca and K content of the AHCS diet (CS-DCAD). Feed intake and milk production were equivalent or greater for cows fed the CS and CS-DCAD diets compared with those fed the AHCS diet. Fat percentage was greater in cows fed the CS compared with the AHCS diet. Fat-corrected milk (FCM; 3.5%) tended to be greater in cows fed the CS and CS-DCAD diets compared with the AHCS diet. Feed efficiencies measured as FCM/dry matter intake were 1.76, 1.80, and 1.94 for the AHCS, CS, and CS-DCAD diets, respectively. The combined effects of reduced feed intake and increased FCM contributed to increased feed efficiency with the CS-DCAD diet, which contained 1.41% K compared with 1.18% K in the CS diet, and we speculate that this might be the result of added dietary K and DCAD effects on digestive efficiency. These results indicate no advantage to including AH in CS-based diets, but suggest that improving mineral supplementation in CS-based diets may increase feed efficiency.

A fecal test for assessing phosphorus overfeeding on dairy farms: evaluation using extensive farm data.

Managing P on dairy farms requires the assessment and monitoring of P status of the animals so that potential overfeeding may be minimized. Numerous published studies have demonstrated that for lactating dairy cows, increasing P concentrations in diets led to greater P excretion in feces. More recent work reported that inorganic P (P(i)) in 0.1% HCl extracts of feces (fecal extract P(i), g/kg) closely reflects dietary P changes. This has led to the proposal that 0.1% HCl fecal extract P(i) may serve as an indicator of the animal's P status (adequate or excessive) when compared with a benchmark value. Here, we present the results of an extensive evaluation of the proposed fecal P indicator test. With samples (n=575) from >90 farms, fecal total P (TP, g/kg) and fecal extract P were positively correlated with dietary P (X, g/kg): TP=1.92X - 0.17 (R2=0.36); fecal extract P=1.82X - 2.54 (R2=0.46). Fecal extract P was responsive to dietary P changes, whereas the remaining P, calculated as TP minus fecal extract P, was not. A provisional benchmark value of fecal extract P representing near-adequate P status was set at 4.75g/kg. Assessment of the farm data using the benchmark indicated that 316 out of 575 data points were associated with possible P overfeeding. Advantages of the fecal-based test over feed-based analysis to assess P status are discussed. The fecal extract P method is a simple and practical test that can be used as an assessment tool for helping dairy producers improve P management and reduce their environmental footprint.

Modeling nutrient fluxes and plasma ketone bodies in periparturient cows.

A mechanistic model was developed to study the interrelationship between glucose and lipid metabolism in periparturient cows. The driving variables were dry matter intake, feed composition, calf birth weight, milk production, and milk components. The response variables were body fat content and concentrations of plasma glucose, glycerol, nonesterified fatty acids (NEFA), and total ketone bodies (KB). Fetal growth and milk synthesis were assigned the highest priority for glucose demand in the model. The rate of fat mobilization was expressed as a function of glucose deficiency. The model assumed first-order kinetics for utilization of NEFA and KB. Model prediction errors were 19, 43, 48, and 36% of mean predictions for glucose, glycerol, NEFA, and KB, respectively. A linear bias was observed in KB and glycerol predictions. The model may be useful for understanding and explaining ketosis development.

Evaluation of a mechanistic model of glucose and lipid metabolism in periparturient cows.

A mechanistic model was previously developed to quantitatively describe glucose and lipid metabolism in periparturient cows. The objectives of the current study were to evaluate the accuracy and precision of the model by comparing predictions to data collected in an independent experiment; to identify the critical metabolic processes for ketosis development; and to use the model to evaluate the relative importance of dry matter intake, calf birth weight, milk yield, and body condition score on nutrition management. Residuals (observed - predicted) were regressed on model predictions using the independent data for the model inputs, and prediction error was calculated. Each model parameter (e.g., the rate of glucose consumption by peripheral tissues) was increased independently by 1 standard deviation to identify the critical metabolic processes for ketosis development. Critical control points to prevent ketosis were identified by increasing the driving variables of the model by 1 standard deviation to estimate the response in ketone body formation. The root mean square prediction error was 0.527 mM for ketone body predictions. The sensitivity analysis indicated that in the first few days of lactation, the rate of nonesterified fatty acid utilization had a greater effect on ketone body concentrations in periparturient cows than the other parameters tested in the model. The model was consistent with the knowledge that over-fattening during the prepartum period should be avoided to help prevent ketosis.

Effect of a transition diet on production performance and metabolism in periparturient dairy cows.

The objectives of this study were to characterize the change in blood metabolites over time, and to evaluate the effect of dietary energy concentration on ketone body accumulation in periparturient cows. Twenty-eight multiparous Holstein cows were listed in order of their anticipated due dates and assigned randomly to 1 of 2 groups: with or without a transition diet. The control group received a nonlactating cow diet [1.54 Mcal/kg of net energy for lactation (NE(L)), 10.9% crude protein (CP), 53.1% neutral detergent fiber (NDF)] from 28 d before expected parturition, and a lactation diet (1.77 Mcal of NE(L)/kg, 16.8% CP, 29.9% NDF) after parturition. The treatment group received a transition diet (1.71 Mcal of NE(L)/kg, 16.8% CP, 35.2% NDF) from 17 d before parturition to 14 d after calving and was fed the same diets as cows in the control group during the third week of lactation. Blood from the coccygeal vein was sampled 3 times per week from 21 d before expected parturition to 21 d postpartum for analysis of glucose, nonesterified fatty acids (NEFA), beta-hydroxybutyrate, acetoacetate, acetone, and glycerol. There were no significant differences in dry matter intake, milk yield, milk components, body weight change, and body condition score change during the postcalving period. Plasma concentrations of different ketone bodies changed in parallel, stayed relatively constant precalving, peaked after parturition, and then decreased but remained high compared with concentrations late in gestation. Plasma concentrations of NEFA and glycerol changed in a pattern similar to those of the ketone bodies. Feeding a transition diet resulted in a greater area under the curve (AUC) for glucose in the last 17 d of gestation, but in no effect within the first 21 d in milk. Acetoacetate AUC was greater for treatment cows than for control cows across the first 21 d in milk. The AUC of NEFA and glycerol between d 15 and 21 postpartum were greater for treatment cows than for control cows. Feeding a transition diet both before and after parturition was associated with greater mobilization of adipose tissue and greater exposure to ketone bodies in early lactation compared with abruptly changing to a lactation diet after parturition.

A meta-analysis of fumarate effects on methane production in ruminal batch cultures.

The objective of this study was to understand the effects of fumarate addition on methane (CH4) and VFA production in the rumen through a meta-analysis of its effects on ruminal batch cultures. Because the reduction of fumarate to succinate can draw electrons away from ruminal methanogenesis, fumarate has been studied as a potential feed additive to decrease CH4 production in ruminants. The average decrease in CH4 in batch cultures was of 0.037 micromol/micromol of added fumarate, which is considerably lower than 0.25 micromol/micromol, the decrease predicted from the stoichiometry of the reactions involved. One reason that fumarate was not effective at decreasing CH4 in batch cultures was that only an average of 48% of added fumarate appeared to be converted to propionate. Secondly, the incorporation of reducing equivalents in the conversion of fumarate to propionate was almost entirely offset by their release from an average of 20% of added fumarate that appeared to be converted to acetate. Thermodynamic calculations indicated that the conversion of added fumarate to both propionate and acetate was feasible. Fumarate appears to be more effective in decreasing CH4 production and increasing propionate in continuous culture than in batch culture. This suggests that microbial adaptation to fumarate metabolism can be important. Variation in populations of fumarate-reducers, methanogens, and protozoa could all be involved. Fumarate supplementation for an extended period may result in the amplification of otherwise small populations of fumarate-reducers. Addition of some of these organisms may be helpful to improve fumarate conversion to propionate. Strategies based on enhancing the rumen's capacity to convert fumarate to propionate by maintaining a low fumarate concentration have been effective. Thermodynamic considerations should be taken into account when designing strategies for CH4 abatement through the addition of external electron acceptors.

Salvage of blood urea nitrogen in sheep is highly dependent on plasma urea concentration and the efficiency of capture within the digestive tract.

The aims of this study were 1) to determine whether transfer of blood urea to the gastrointestinal tract (GIT) or the efficiency of capture of urea N within the GIT is more limiting for urea N salvage, and 2) to establish the relationship between plasma urea concentration and recycling of urea N to the GIT. We used an i.v. urea infusion model in sheep to elevate the urea entry rate and plasma concentrations, thus avoiding direct manipulation of the rumen environment that otherwise occurs when feeding additional N. Four growing sheep (28.1 +/- 0.6 kg of BW) were fed a low-protein (6.8% CP, DM basis) diet and assigned to 4 rates of i.v. urea infusion (0, 3.8, 7.5, or 11.3 g of urea N/d; 10-d periods) in a balanced 4 x 4 Latin square design. Nitrogen retention (d 6 to 9), urea kinetics([(15)N2]urea infusion over 80 h), and plasma AA were determined. Urea infusion increased apparent total tract digestibility of N (29.9 to 41.3%) and DM (47.5 to 58.9%), and N retention (1.45 to 5.46 g/d). The plasma urea N entry rate increased (5.1 to 21.8 g/d) with urea infusion, as did the amount of urea N entering the GIT (4.1 to 13.2 g/d). Urea N transfer to the GIT increased with plasma urea concentration, but the increases were smaller at greater concentrations of plasma urea. Anabolic use of urea N within the GIT also increased with urea infusion (1.43 to 2.98 g/d; P = 0.003), but anabolic use as a proportion of GIT entry was low and decreased (35 to 22%; P = 0.003) with urea infusions. Consequently, much (44 to 67%) of the urea N transferred to the GIT returned to the liver for resynthesis of urea (1.8 to 9.2 g/d; P < 0.05). The present results suggest that transfer of blood urea to the GIT is 1) highly related to blood urea concentration, and 2) less limiting for N retention than is the efficiency of capture of recycled urea N by microbes within the GIT.

Milk production of dairy cows fed differing concentrations of rumen-degraded protein.

Thirty-two multiparous and 16 primiparous Holstein cows in midlactation averaging 126 d in milk were used to determine the effects of rumen-degraded protein (RDP) concentration on lactation performance. Cows were assigned to diets in a repeated Latin square design with 3-wk experimental periods. Diets were formulated to provide 4 concentrations of dietary RDP [6.8, 8.2, 9.6, and 11.0% of dry matter (DM)] while rumen-undegraded protein remained constant (5.8% of DM). Diets contained 50% corn silage and 50% concentrate (DM basis). Ingredients within diets were equal across treatments except for ground corn, soybean meal, and ruminally protected soybean meal. Dry matter intake was not affected by treatment. Milk yield, fat yield, and protein yield all increased linearly when cows were fed diets with greater RDP. Milk fat and protein concentration each increased by 0.16 percentage units for cows fed 11% RDP compared with 6.8% RDP. Milk protein yield increased by 0.19 g/d for every 1 g/d increase in crude protein supplied mainly as RDP. As RDP increased, the efficiency of N use declined linearly. Milk urea N increased linearly when cows were fed increasing amounts of RDP, indicating increased losses of N via urine. Feeding deficient RDP diets to dairy cows can decrease nitrogen excretion, but it also decreases lactation performance. These data show an environmental benefit from underfeeding RDP to dairy cows according to National Research Council requirements, but at a financial cost to the dairy producer.

Using blood urea nitrogen to predict nitrogen excretion and efficiency of nitrogen utilization in cattle, sheep, goats, horses, pigs, and rats.

The objectives of this study were to evaluate the potential for using blood urea N concentration to predict urinary N excretion rate, and to develop a mathematical model to estimate important variables of N utilization for several different species of farm animals and for rats. Treatment means (n = 251) from 41 research publications were used to develop mathematical relationships. There was a strong linear relationship between blood urea N concentration (mg/100 mL) and rate of N excretion (g x d(-1) x kg BW(-1)) for all animal species investigated. The N clearance rate of the kidney (L of blood cleared of urea x d(-1) x kg BW(-1)) was greater for pigs and rats than for herbivores (cattle, sheep, goats, horses). A model was developed to estimate parameters of N utilization. Driving variables for the model included blood urea N concentration (mg/100 mL), BW (kg), milk production rate (kg/d), and ADG (kg/d), and response variables included urinary N excretion rate (g/d), fecal N excretion rate (g/d), rate of N intake (g/d), and N utilization efficiency (N in milk and gain per unit of N intake). Prediction errors varied widely depending on the variable and species of animal, with most of the variation attributed to study differences. Blood urea N concentration (mg/100 mL) can be used to predict relative differences in urinary N excretion rate (g/d) for animals of a similar type and stage of production within a study, but is less reliable across animal types or studies. Blood urea N concentration (mg/100 mL) can be further integrated with estimates of N digestibility (g/g) and N retention (g/d) to predict fecal N (g/d), N intake (g/d), and N utilization efficiency (grams of N in milk and meat per gram of N intake). Target values of blood urea N concentration (mg/100 mL) can be backcalculated from required dietary N (g/d) and expected protein digestibility. Blood urea N can be used in various animal species to quantify N utilization and excretion rates.

Comparison of analytical methods and the influence of milk components on milk urea nitrogen recovery.

The objectives of this study were to compare analytical instruments used in independent laboratories to measure milk urea nitrogen (MUN) and determine whether any components in milk affect the recovery of MUN. Milk samples were collected from 100 Holstein cows fed one ration in a commercial dairy herd with a rolling herd average of 9500 kg. Half of each sample was spiked with 4 mg/dL of urea N, while the other half was not, to determine recovery. Both milk samples (spiked and not spiked) were sent to 14 independent laboratories involved in the MUN Quality Control Program through National Dairy Herd Improvement Association and analyzed for MUN, fat, protein, lactose, somatic cell count (SCC), and total solids. The laboratories analyzed MUN using CL-10 (n = 3), Skalar (n = 2), Bentley (n = 3), Foss 4000 (n = 3) or Foss 6000 (n = 3) systems. When recovery of MUN was evaluated among the 5 analytical methods, the mean recoveries for the Bentley, Foss 6000, and Skalar systems were 92.1 (SE = 2.76%), 95.4 (SE = 10.1%), and 95.1% (SE = 7.61%), respectively, and did not differ from each other. However, MUN recovery was 85.0% (SE = 2.8%) for the CL-10 system and 47.1% (SE = 9.9%) for the Foss 4000 system, both of which differed from the other 3 systems. Recoveries from Foss 4000, Foss 6000, and Skalar varied among laboratories using the same instrument. As initial MUN concentration increased, recovery decreased using the Bentley and CL-10 systems. Increasing milk fat resulted in a decrease in recovery using the Foss 6000 system. For 4 of the 5 methods, recovery of MUN was not associated with specific milk components. Recovery of MUN was inconsistent for laboratories using the Foss 4000 and the Foss 6000 method and using these systems may result in an overestimation or underestimation of MUN.

A comparison of instruments and laboratories used to measure milk urea nitrogen in bulk-tank milk samples.

The objective of this study was to compare the instruments and laboratories that are currently used for analysis of milk urea nitrogen (MUN) for bulk-tank milk samples. Two replicate samples from each bulk tank on 10 different dairy farms were sent to 12 Dairy Herd Improvement Association (DHIA) laboratories throughout the US for MUN analysis. Two laboratories used 2 different methods for MUN analysis for a total of 14 analyses on 20 samples (n = 280). Values of MUN were analyzed using a random effects model with farm, laboratory, and farm x laboratory variance components. Greater than 98% of the variance in measured MUN was attributed to farm-to-farm variance for analysis of MUN by the Bentley, CL 10, Foss 6000, and Skalar instruments. However, for the laboratories using the Foss 4000 system, <60% of the variance in MUN was attributed to farm-to-farm variance. Laboratories using the Bentley, CL 10, Foss 6000, and Skalar instruments provided slightly different results for MUN analysis, but >95% of sample measurements fell within 1.75 mg/ dL of each other. The laboratories using Foss 4000 differed from each other, and 95% of samples fell within 5 mg/dL of the CL 10 measurement. Laboratories using the Foss 4000 instrument did not consistently provide measurements of MUN that were similar to each other or to the measurements of the other instruments.

Effects of milk urea nitrogen and other factors on probability of conception of dairy cows.

The objective of this study was to evaluate the relationships between milk urea nitrogen (MUN) and other factors and the probability of conception in dairy cows. Data were retrieved from the Lancaster Dairy Herd Improvement Association (DHIA). A total of 713 dairy herds and 10,271 dairy cows were included in the study. Logistic regression was used to determine the within-herd effects of MUN, milk production, lactation number, and breeding season on the probability of conception for each of 3 services. Within herds, MUN displayed a slight negative association with probability of conception at first service. For example, there was a 2- to 4-percentage unit decrease in conception rate at first service with a 10-mg/dL increase in MUN. In among-herd regression analysis, there was no effect of MUN on probability of conception. These results suggest that MUN may be related to conditions affecting reproduction of individual cows within a herd. Diet formulation usually would affect MUN equally among all cows at a similar stage of lactation in a herd. Because there was no effect of MUN among herds, diet formulation did not appear to affect conception rate.

Phosphorus feeding levels and critical control points on dairy farms.

A viable and cost-effective approach to managing P on dairy farms is to minimize excess P in diets, which in turn leads to less excretion of P in manure without impairing animal performance. A questionnaire survey was conducted, coupled with on-site feed and fecal sample collection and analysis on dairy farms in New York, Pennsylvania, Delaware, Maryland, and Virginia. The purpose was to assess dietary P levels and to identify critical control points pertaining to P feeding management. Survey responses, 612 out of 2500 randomly selected farms, revealed a wide range of dietary P concentrations for lactating cows, from 3.6 to 7.0 g/kg of feed DM. The mean was 4.4 g/kg, which was 34% above the level recommended by the NRC for 27.9 kg milk/d, the mean milk yield in the survey. Higher P concentrations in diets were not associated with higher milk yields (n = 98, R2 = 0.057 for the survey farms; n = 92, R2 = 0.043 for farms selected for on-site sampling). However, higher dietary P led to higher P excretion in feces (n = 75, R2 = 0.429), with much of the increased fecal P being water soluble. Phosphorus concentrations in diet samples matched closely with P concentrations in formulated rations, with 67% of the feed samples deviating <10% from the formulations. On 84% of the survey farms, ration formulation was provided by professionals rather than producers themselves. Most producers were feeding more P than cows needed because it was recommended in the rations by these consultants. In conclusion, P fed to lactating cows averaged 34% above NRC recommendations; to reduce excess dietary P, ration formulation is the critical control point.

Whole-farm phosphorus balance on western dairy farms.

Environmental concerns have focused attention on animal agriculture and its contribution to P accumulation in soils and runoff to surface waters. Monitoring P inputs and outputs on farms is a means of calculating the potential P build-up in farm soils. The objective of this study was to determine whole-farm P balance and the relative importance of the farm components (herd, manure storage, cropping systems) that contribute to it in dairies of the western United States. Whole-farm balances were computed for 41 commercial dairies in Utah and Idaho using the Maryland Nutrient Balancer. The average whole-farm P balance in the study was 6.6 tonne/yr with an average herd size of 466 cows. Imported feed made up 85.4% of the total P inputs and exported animal products (milk and meat), and manure and compost made up 53.1 and 45.9%, respectively, of the total P outputs. Farms were divided into those that grew crops and those that did not. Whole-farm balance (kg of P balance per animal) for farms that grew crops had more unaccounted for P (difference between P inputs and output) than farms that grew no crops. They also had more imported fertilizer and less imported feed and exported manure and compost. Multiple regression analysis of the relative effects of herd management, manure storage, and cropping system on whole-farm balance per product found that herd P utilization efficiency was the most important factor in determining whole-farm P balance on farms where crops were grown. Crop uptake of available P was the only other subsystem important for these farms. Increased conversion of feed P to P in product is an important way to decrease whole-farm P balance.

Whole-farm nitrogen balance on western dairy farms.

Environmental legislation has made it necessary for livestock producers to be able to quantify and adjust the N balance on their farms. Whole-farm N balance and efficiencies were computed for 41 commercial dairies in Utah and Idaho using the University of Maryland Nutrient Balancer. The average N balance, or unaccounted for N, was 81 tonnes per year for the average herd size of 466 cows with 35.8% of the inputs accounted for in the outputs. The major inputs for farms that grew crops (n = 23, herd size = 284 total cows) were imported feed (57.4% of all inputs) and nitrogen fixation (30% of inputs). The major outputs were animal products (primarily milk and some meat, 80% of outputs). For farms that grew no crops (n = 18, herd size = 700 total cows), 98% of the inputs were from imported feed. Of the outputs, 57% of the N was in animal products and 42.9% in manure and compost. Whole-farm balance per product for those farms that grew crops was most affected by herd N utilization efficiency (kg feed N per kg product N), crop N utilization efficiency, and availability of manure N applied to crops, while manure N storage efficiency was of lesser importance. For farms that grew no crops, whole-farm N balance per product was most affected by herd N utilization efficiency and manure N storage efficiency. Maximizing conversion of feed N to product N was the best way to reduce whole-farm N balance.

Dairy herd management practices that impact nitrogen utilization efficiency.

Improving the efficiency of feed N utilization by dairy cattle is the most effective means to reduce nutrient losses from dairy farms. The objectives of this study were to quantify the impact of different management strategies on the efficiency of feed N utilization for dairy farms in the Chesapeake Bay Drainage Basin. A confidential mail survey was completed in December 1998 by 454 dairy farmers in PA, MD, VA, WV, and DE. Nitrogen intake, urinary and fecal N, and efficiency of feed N utilization was estimated from survey data and milk analysis for each herd. Average efficiency of feed N utilization for milk production by lactating dairy cows (N in milk/N in feed x 100) was 28.4% (SD = 3.9). On average, farmers fed 6.6% more N than recommended by the National Research Council, resulting in a 16% increase in urinary N and a 2.7% increase in fecal N. Use of monthly milk yield and component testing, administration of bovine somatotropin (bST), and extending photoperiod with artificial light each increased efficiency of feed N utilization by 4.2 to 6.9%, while use of a complete feed decreased efficiency by 5.6%. Increased frequency of ration balancing and more frequent forage nutrient testing were associated with higher milk production, but not increased N utilization efficiency. Feeding protein closer to recommendations and increasing production per cow both contributed to improving efficiency of feed N utilization.