Effects of dietary phosphorus concentration during the transition period on plasma calcium concentrations,feed intake,and milk production in dairy cows

Our aim was to evaluate the effects of a low or high dietary phosphorus (P) concentration during the dry period, followed by either a high or low dietary P concentration during the first 8 wk of lactation, on plasma Ca concentrations, feed intake, and lactational performance of dairy cattle. Sixty pregnant multiparous Holstein Friesian dairy cows were assigned to a randomized block design with repeated measurements and dietary treatments arranged in a 2 × 2 factorial fashion. The experimental diets contained 3.6 (Dry-HP) or 2.2 (Dry-LP) g of P/kg of dry matter (DM) during the dry period, and 3.8 (Lac-HP) or 2.9 (Lac-LP) g of P/kg of DM during 56 d after calving period. In dry cows, plasma Ca concentrations were 3.3% greater when cows were fed 2.2 instead of 3.6 g of P/kg of DM. The proportion of cows being hypocalcemic (plasma Ca concentrations <2 mM) in the first week after calving was lowest with the low-P diets both during the dry period and lactation. Plasma Ca concentrations in wk 1 to 8 after calving were affected by dietary P level in the dry period and in the lactation period, but no interaction between both was present. Feeding Dry-LP instead of Dry-HP diets resulted in 4.1% greater plasma Ca values, and feeding Lac-LP instead of Lac-HP diets resulted in 4.0% greater plasma Ca values. After calving, plasma inorganic phosphate (Pi) concentrations were affected by a 3-way interaction between sampling day after calving, and dietary P levels during the dry period and lactation. From d 1 to d 7 postpartum, cows fed Lac-HP had increased plasma Pi concentrations, and the rate appeared to be greater in cows fed Dry-LP versus Dry-HP. In contrast, plasma Pi concentrations decreased from d 1 to d 7 postpartum in cows fed Lac-LP, and this decrease was at a higher rate for cows fed Dry-HP versus Dry-LP. After d 7, plasma Pi concentrations remained rather constant at 1.5 to 1.6 mM when cows received Lac-HP, whereas with Lac-LP plasma Pi concentrations reached stable levels (i.e., 1.3–1.4 mM) at d 28 after calving. Milk production, DM intake, and milk concentrations of P, Ca, fat, protein, and lactose were not affected by any interaction nor the levels of dietary P. It is concluded that the feeding of diets containing 2.2 g of P/kg of DM during the last 6 wk of the dry period and 2.9 g of P/kg of DM during early lactation increased plasma Ca levels when compared with greater dietary P levels. These low-P diets may be instrumental in preventing hypocalcemia in periparturient cows and do not compromise DM intake and milk production. Current results suggest that P requirements in dairy cows during dry period and early lactation can be fine-tuned toward lower values than recommended by both the National Research Council and the Dutch Central Bureau for Livestock Feeding. Caution however is warranted to extrapolate current findings to entire lactations because long-term effects of feeding low-P diets containing 2.9 of g/kg of DM on production and health needs further investigation.     

Altering dietary cation-anion difference in lactating dairy cows to reduce phosphorus excretion to the environment

Four early-lactating dairy cows were randomly allocated to 4 diets with dietary cation-anion difference [DCAD; (Na + K) - (Cl- + S2-) mEq/100 g dry matter)] values of +14, +18, +24, and +45. Diets were formulated to be isoenergetic and isonitrogenous, and supplied similar levels of P (0.46%) and Ca (0.77%). The salts, MgCl2, MgSO4, K2CO3, and NaHCO3 were used to alter DCAD. The main objective of the study was to ascertain whether a decrease in DCAD would reduce fecal P excretion in lactating dairy cattle. The experiment was conducted as a 4 x 4 Latin square design with 21-d periods. During the last 5 d, diets were offered at a restricted level and samples of blood, milk, feces, and urine were collected. Measures of acid-base status of the cows were linearly related to DCAD, but the animals did not experience metabolic acid stress. Neither fecal P nor urinary P was affected by DCAD, and there was no change in overall P balance. Plasma P tended to increase and blood concentrations of ionized Ca were enhanced as DCAD decreased; P excretion in milk showed a quadratic response to DCAD. Milk yield and milk composition were unaffected by changes in DCAD. Although DCAD may have influenced P homeostasis in lactating cows, there was no evidence that, within the range of + 14 to + 45 mEq/ 100 g dry matter, DCAD could be used as a nutritional strategy to reduce manure P from dairy cattle.     

Effects of restricted dietary phosphorus supply to dry cows on periparturient calcium status

Restricted dietary P supply to transition dairy cows has recently been reported to beneficially affect the Ca balance of periparturient cows. The objective of the present study was to determine whether this effect on the Ca balance can be reproduced when limiting the P-restricted feeding to the last 4 wk of gestation. A total of 30 dairy cows in late pregnancy were randomly assigned to a dry cow diet with either low or adequate P content (0.16 and 0.30% P in DM, respectively) to be fed in the 4 wk before expected calving. After calving, all cows received the same lactating cow ration with adequate P content (0.46% P in DM). Blood was collected daily from 4 d antepartum until calving, at calving (d 0), 6 and 12 h after calving (d +0.25 and d +0.5, respectively) and on days +1, +2, +3, +4 and +7 relative to calving. Blood gas analyses were conducted to determine the concentration of ionized Ca in whole blood ([Ca²⁺]), and plasma was assayed for concentrations of inorganic phosphorus ([Pi]), total calcium, parathyroid hormone ([PTH]), 1,25-dihydroxyvitamin D ([1,25-(OH)₂D₃]), and CrossLaps ([CTX]), a biomarker for bone resorption (Immunodiagnostic Systems GmbH). Repeated-measures ANOVA was conducted to study treatment, time, and lactation number effects. The mean [Ca²⁺] in P-deprived cows remained above the threshold of 1.10 mmol/L throughout the study, and values were higher compared with cows on adequate P supply between d 0 and d +2 and on d +4. The [Ca²⁺] differed between treatments at the sampling times d 0, d +0.25, d +0.5, d +2, and d +4. Plasma [PTH] and [1,25-(OH)₂D₃] did not differ between treatments, but P-deprived cows had greater [CTX] than cows with adequate P supply at d +1, d +2, and d +7. These results indicate that restricted dietary P supply to during the last 4 wk of the dry period improves the Ca homeostasis of these cows in the first days of lactation, an effect that seems to be primarily driven by increased bone tissue mobilization.     

不同种类的膳食蛋白对血浆同型半胱氨酸浓度的影响

目的探讨不同种类膳食蛋白对血浆同型半胱氨酸(Hcy)水平的影响。方法 48只健康Wistar大鼠随机分为6组:酪蛋白组、乳清蛋白组、卵白蛋白组、大豆分离蛋白组、小麦蛋白组、玉米蛋白组。分别给予含有不同膳食蛋白的饲料喂养10 d后处死动物,采集血液样品,测定血浆中同型半胱氨酸(Hcy)、半胱氨酸(Cys)、谷胱甘肽(GSH)及半胱氨酰甘氨酸(Cys-Gly)等指标的浓度。结果小麦蛋白组的体质量增长低于蛋白含量相同的酪蛋白组、乳清蛋白组、卵白蛋白组及大豆分离蛋白组,玉米蛋白组的体质量呈现负增长,且与其他各组比较差异有统计学意义(P0.05)。小麦蛋白组和玉米蛋白组血浆Hcy浓度低于其他蛋白组,且差异均有统计学意义(P0.05)。与酪蛋白组比较,乳清蛋白组的血浆Hcy浓度也有所降低。结论膳食蛋白的种类能够影响大鼠血浆中的Hcy水平,可能与蛋白中氨基酸组成有关。高Cys含量的膳食蛋白能提高促进Hcy代谢的酶活性,增加Hcy的代谢消耗,从而降低血浆中的Hcy水平。     

Influence of phosphorus intake on excretion and blood plasma and saliva concentrations of phosphorus in dairy cows

Phosphorus (P) balance, and blood plasma P and saliva P concentrations were measured in multiparous dairy cows through two lactations and two dry periods. The cows were fed three amounts of P at either 100, 80 or 67% of the Dutch P recommendation, actually resulting in dietary P concentrations of 3.2 to 3.9, 2.6 to 2.9 and 2.2 to 2.6 g P/kg dry matter during lactation for the three treatments, respectively. On the basis of plasma P values as low as 0.9 mmol/l and saliva P values as low as 5.1 mmol/l during the second lactation period within the experiment, the 67% group was considered to be deficient in P. By decreasing milk production, and thus lowering P losses with milk, P retention in the 67% group remained near zero. The P supply with the 80% ration was considered to be just sufficient. At high milk yield and marginal dietary P concentrations, plasma P and saliva P concentrations were decreased. The higher P intake in high-compared with low-producing cows resulted in a constant absolute fecal P excretion, due to the fact that the apparent P digestibility was raised with increasing milk yield. There was a direct relationship between milk P output and the percentage of apparent P digestibility for individual animals.     

Effects of restricted dietary phosphorus supply during the dry period on productivity and metabolism in dairy cows

Phosphorus in bovine nutrition is under ongoing scrutiny because of concerns with excessive amounts of P excreted in manure contributing to environmental pollution. Feeding rations with excessive P content, however, still remains common practice, particularly during the transition period, as limited P supply in late gestation and early lactation is thought to present a risk for health and productivity of high-yielding dairy cows. The objectives of this study were to investigate the effect of restricted P supply during the last 4 wk of pregnancy on Ca and P homeostasis during the transition period in high-yielding dairy cows, and to identify possible effects on metabolism and productivity throughout the following lactation. Thirty late-pregnant multiparous dairy cows were randomly assigned to either a dry cow diet with low (LP) or adequate P (AP) content [0.16 and 0.30% P in dry matter (DM), respectively] to be fed in the 4 wk before calving. After calving all cows received the same ration with adequate P content (0.46% P in DM). Blood, milk, and liver tissue samples were obtained during the dry period and the following lactation, DM intake (DMI), body weight, milk production, and disease occurrence were monitored. Plasma was assayed for the concentrations of P, Ca, Na, and K, metabolic parameters, and liver enzyme activities. Liver tissue was analyzed for mineral, triglyceride, cholesterol, and water contents. Repeated-measures ANOVA was used to identify treatment, time, and treatment × time interaction effects. Cows fed LP had lower plasma P concentrations ([Pi]) than AP cows during restricted P feeding, reaching a nadir of 1.1 mmol/L immediately before calving. After calving, plasma [Pi] of LP cows was at or above the level of AP cows and within the reference range for cattle. Symptoms assumed to be associated with hypophosphatemia were not observed, but plasma Ca was higher from 1 wk before to 1 wk after calving in LP cows, which was associated with a numerically lower incidence of clinical and subclinical hypocalcemia in LP cows. Both treatments had a similar 305-d milk yield (12,112 ± 1,298 kg for LP and 12,229 ± 1,758 kg for AP cows) and similar DMI. Plasma and liver tissue biochemical analysis did not reveal treatment effects on energy, protein, or lipid metabolism. The results reported here indicate that restricted dietary P supply during the dry period positively affected the Ca homeostasis of periparturient dairy cows but did not reveal negative effects on DMI, milk production, or metabolic activity in the following lactation. Restriction of P during the dry period was associated with hypophosphatemia antepartum but neither exacerbated postparturient hypophosphatemia, which is commonly observed in fresh cows, nor was associated with any clinical or subclinical indication of P deficiency in early lactation.     

Effects of dietary crude protein concentration on late-lactation dairy cow performance and indicators of nitrogen utilization

The objectives of this study were to measure performance responses and to evaluate indictors of N utilization in late-lactation cows fed diets with incremental reductions in crude protein (CP) concentration. Holstein cows (n = 128; 224 ± 54 d in milk) were stratified by parity and days pregnant (86 ± 25 d) and randomly assigned to 1 of 16 pens in a randomized complete block design. For 3 wk, all cows received a covariate diet containing 16.9% CP [dry matter (DM) basis]. For the subsequent 12 wk, pens were randomly assigned to 1 of 4 treatments that contained 16.2, 14.4, 13.1, or 11.8% CP (DM basis). Diets were offered once daily and contained 32.5% corn silage, 32.5% alfalfa silage, 13.5% high-moisture corn, and 21.5% concentrate mix. A reduction in dietary CP was achieved by replacing soybean meal with soy hulls in the concentrate mix (DM basis). Dry matter intake, milk urea N (MUN; mg/dL), and the yield of milk urea N (g/d) decreased linearly with dietary CP. Compared with a 16.2% CP diet, a 14.4% CP diet did not alter milk yield throughout the study, but the 13.1 and 11.8% CP diets reduced milk yield after 4 and 1 wk, respectively. Furthermore, milk protein percentage was reduced for all dietary CP less than 16.2%, but this negative effect was temporary and disappeared after 7 wk for the 14.4% CP diet. In contrast, MUN adjusted to a new steady state within 1 wk for all dietary treatments. Modeling quadratic responses with a plateau led to predictions of no reduction in fat- and protein-corrected milk (32.6 kg/d) and yields of fat (1.31 kg/d), lactose (1.49 kg/d), and true protein (1.12 kg/d) until dietary CP decreased below 15.5, 15.3, 15.9, and 16.2%, respectively. In this study, MUN and the yield of MUN were highly correlated with N intake, milk protein yield, and fat- and protein-corrected milk. Surprisingly, N use efficiency (milk protein N/intake N) was not correlated with any variables related to N utilization and reached an apparent upper limit of approximately 30%. Although this observation may be associated with feeding diets deficient in metabolizable protein, late-lactation cows in this study adjusted to low dietary CP concentration better than anticipated as milk production was 2.6, 3.6, 6.4, and 8.0 kg/d higher than National Research Council (2001)-predicted metabolizable protein-allowable milk for dietary CP of 16.2, 14.4, 13.1, and 11.8%, respectively.     

Influence of source and amount of dietary fat on digestibility in lactating cows

Digestibility of commercial fat supplements was determined in two experiments with high (59% of diet DM) forage diets. Experiment 1 was a preliminary trial in which six Jersey cows were in two 3 x 3 Latin squares to evaluate two formulations of calcium soap at two intake levels (500 and 1000 g/d). The two formulations were compared with control (0 supplement) within squares; the squares differed in amount of soap supplemented. Mean apparent digestibilities of fat were not influenced by source or amount of fat supplemented and averaged 82.5, 84.3, and 83.4% for control, 500, and 1000 g/d. In Experiment 2, six Jersey cows were in a 6 x 6 Latin square to compare effects of various commercial fats on digestibility of diet components at 2.85 and 5.7% (DM) added fat. Higher fat decreased digestibility of P and fatty acid. Fatty acid intake affected fatty acid digestibility quadratically, and variability among cows increased at higher fatty acid intake. Endogenous fecal fat was 55.9 g/d; apparent digestibility of fat was relatively constant at 80 to 82% between 2 and 5% of diet DM, whereas marginal true digestibility decreased linearly (4.4% units/100 g fatty acid consumed). There were no differences among fat sources in fatty acid digestibility. Rumen VFA were not influenced by dietary fat.     

Effects of dietary protein concentration and coconut oil supplementation on nitrogen utilization and production in dairy cows

The objective of this study was to investigate the effect of metabolizable protein (MP) deficiency and coconut oil supplementation on N utilization and production in lactating dairy cows. The hypothesis of the study was that a decrease in ruminal protozoal counts with coconut oil would increase microbial protein synthesis in the rumen, thus compensating for potential MP deficiency. The experiment was conducted for 10 wk with 36 cows (13 primiparous and 23 multiparous), including 6 ruminally cannulated cows. The experimental period, 6 wk, was preceded by 2-wk adaptation and 2-wk covariate periods. Cows were blocked by parity, days in milk, milk yield, and rumen cannulation and randomly assigned to one of the following diets: a diet with a positive MP balance (+44 g/d) and 16.7% dietary crude protein (CP) concentration (AMP); a diet deficient in MP (−156 g/d) and 14.8% CP concentration (DMP); or DMP supplemented with approximately 500 g of coconut oil/head per day (DMPCO). Ruminal ammonia tended to be greater and plasma urea N (20.1, 12.8, and 13.1 mg/dL, for AMP, DMP, and DMPCO diets, respectively) and milk urea N (12.5, 8.3, and 9.5 mg/dL, respectively) were greater for AMP compared with DMP and DMPCO. The DMPCO diet decreased total protozoa counts (by 60%) compared with DMP, but had no effect on the methanogens profile in the rumen. Total tract apparent digestibility of dry matter and CP was decreased by DMP compared with AMP. Fiber digestibility was lower for both DMP and DMPCO compared with AMP. Urinary N excretion was decreased (by 37%) by both DMP and DMPCO compared with AMP. The DMP and DMPCO diets resulted in greater milk N efficiency compared with AMP (32.0 and 35.1 vs. 27.6%, respectively). Milk yield was decreased by both DMP and DMPCO compared with AMP (36.2, 34.4, and 39.3 kg/d, respectively) and coconut oil supplementation suppressed feed intake and caused milk fat depression. Coconut oil supplementation decreased short-chain fatty acid (C4:0, C6:0, and C8:0) concentration and increased medium-chain (C12:0 and C14:0) and total trans fatty acids in milk. Overall, the MP-deficient diets decreased N losses, but could not sustain milk production in this study. Coconut oil decreased feed intake and similar to DMP, suppressed fiber digestibility. Despite decreased protozoal counts, coconut oil had no effect on the methanogen population in the rumen.     

Interactions of dietary cation-anion balance and phosphorus: effects on growth and serum inorganic phosphorus in dairy calves

Thirty-six male and female Holstein and Jersey calves were assigned at weaning to a randomized complete block design in a 2 x 3 factorial arrangement to evaluate the influence of two dietary cation-anion balances (-14 and +39 meq(Na + K)-(Cl + S) per 100 g diet DM) and three amounts of dietary P (.22, .29, and .37%) on performance and P metabolism from 9 to 19 wk of age. Feed intake, average daily gain, and serum inorganic P were higher on the anionic diets and increased with increasing dietary P. Body weights were higher on the .37% P diets by wk 3 and on the anionic diets by wk 6. The interaction of dietary P and cation-anion balance was responsible for significant differences in calf performance; the anionic diet exhibited marked improvement over the cationic diet at the lowest P concentration. Results indicate that the availability of P for young dairy calves may be higher with anionic than cationic diets.     

Nitrogen and phosphorus retention and excretion in late-gestation dairy heifers

The objectives of this study were to evaluate the effects of limit feeding diets containing concentrates or by-products in place of forages on manure and nutrient excretion in growing, gravid heifers. Eighteen Holstein heifers confirmed pregnant were grouped by due date and fed 1 of 3 diets (n = 6 per treatment) for the last 14 wk of pregnancy. Diets were high forage, fed ad libitum (HF); by-product based (BP), fed at the same rate as HF-fed heifers; or low forage (LF), fed at 86% of the HF diet. Diets were designed to supply equal quantities of P, N, and metabolizable energy. Total collection of feces and urine was conducted in wk 14, 10, 6, and 2 prepartum. The HF ration was 90.7% forage, 13.7% crude protein (CP), and contained orchardgrass hay, corn silage, corn grain, soybean meal 44%, and a vitamin-mineral premix. The BP diet was 46.2% forage and 14.0% CP, with 70% of the grain mix space replaced with soybean hulls and cottonseed hulls in a 1:1 ratio, with intake limited to 93% of the dry matter intake (DMI) of HF. The LF ration was 45.3% forage and 17.8% CP, with intake limited to 86% of the DMI of HF. The effect of diet was analyzed with repeated measures, using preplanned contrasts to compare HF with BP and LF with HF and BP. As designed, heifers fed HF and BP had greater DMI than the heifers limit-fed LF, and there was no effect of diet on average daily gain or BW. Intake and digestibility of N were lower, and fecal N excretion was higher, in heifers fed HF and BP than heifers fed LF. Mean feces excretion on both a wet and dry basis was greater for HF heifers compared with BP heifers and less for LF heifers than for HF and BP heifers. Despite differences in urinary output, diet had no effect on urea N excretion, but there was a trend for heifers fed HF and BP rations to excrete less urinary N compared with those fed LF. Compared with HF and BP heifers, LF heifers tended to have lower fecal P excretion and had higher urinary P excretion. Measured manure and urine excretion from heifers fed LF was greater than current American Society of Agricultural and Biological Engineers values, whereas heifers fed HF excreted less manure and urine than predicted. Heifers achieving similar rates of gain from diets differing in forage, grain, and by-product content excreted widely varying quantities of manure.     

Effects of corn silage hybrid and dietary concentration of forage NDF on digestibility and performance by dairy cows

Eight intact multiparous cows and four ruminally and duodenally cannulated primiparous cows were fed four diets in a replicated 4 x 4 Latin square design: 1) 17% forage neutral detergent fiber (NDF) with brown midrib corn silage (BMRCS), 2) 21% forage NDF with BMRCS, 3) 17% forage NDF with conventional corn silage (CCS), and 4) 21% forage NDF with CCS. Diets contained 17.4% crude protein and 38.5% NDF. Each period consisted of 4 wk for intact cows and 2 wk for cannulated cows. For intact cows, DM intake was higher for BMRCS than CCS, and milk urea N was higher for 21 than 17% forage NDF. Milk protein yield tended to be higher and milk urea N lower for cows fed BMRCS than those fed CCS. Milk yield and milk protein percentage were similar among treatments. For the cannulated cows, ruminal mat consistency was similar among treatments. Based on a 72 h in situ incubation, BMRCS was lower in indigestible NDF than CCS. The BMRCS resulted in a higher proportion of ruminal propionate than CCS. Cows fed 21% forage NDF had a higher proportion of acetate and a lower proportion of propionate than cows fed 17% forage NDF. The total tract digestibility of nutrients and efficiency of bacterial N synthesis were similar among treatments, except that BMRCS resulted in lower intestinal fatty acid digestibility than CCS, and 17% forage NDF tended to result in higher total tract fatty acid digestibility than 21% forage NDF. Ruminal NDF digestibility was similar among dietary treatments. The increased milk production observed from feeding BMRCS in some studies may be explained by higher DM intake rather than increased total tract digestibility of the diets.     

Effects of dietary fiber and phytic acid on mineral availability.

In general, it has been shown that dietary fiber may bind metallic cations in both in vitro and in vivo studies. However, there clearly are many unresolved questions on the effects of high-fiber diets on mineral availability. On one side, the effects of fiber on the utilization of nutrients vary greatly with the amount and type of fiber. In addition, there are many agents in both food and the digestive tract that may affect the mineral binding to fiber: some agents may inhibit binding, while others will enhance it. Also, there are several major difficulties in drawing conclusions from the in vitro and in vivo studies due to the different experimental conditions, methods used to follow the mineral balance, etc. Finally, it must be borne in mind that fiber and phytic acid occur together in fiber-rich diets and, thus, it is difficult to separate the effects of fiber and phytate in the utilization of most essential polyvalent metallic ions. The studies summarized in this review show that the recommendation for increasing dietary fiber in Western communities would not be expected to have any adverse effect on mineral absorption if we increase not only the intake of fiber, but also the dietary intake of other food components such as protein (both vegetable and animal protein) and ascorbic, citric, and oxalic acids (in fruits and vegetables). The adequate intake of minerals, fat, and simple sugars are maintained with this type of diet. The recommendations should be best interpreted in such a way as to prevent the consumption of excessive amounts of phytate, particularly for those whose mineral needs are great. Further studies are still needed in this field in order to understand the conflicting results published in the literature regarding the effects of fiber on the utilization of minerals; however, the studies reviewed in this article may give us an idea of the complexity of mineral availability in fiber-rich, phytate-rich diets.     

Effects of amounts and degradability of dietary protein on lactation,nitrogen utilization,and excretion in early lactation Holstein cows

Five treatment diets varying in crude protein (CP) and rumen undegradable protein (RUP) were calculated to supply a postruminal lysine to methionine ratio of about 3:1. Diets were fed as a total mixed ration to 65 Holstein cows that were either primiparous (n = 28) or multiparous (n = 37) from 21 to 120 d in milk to determine effects on lactation and nitrogen utilization. Crude protein % and calculated RUP (% of CP) of diets [on a dry matter (DM) basis] were: 1) 19.4, 40 (HPMU), 2) 16.5, 34 (LPLU), 3) 16.8, 40 (LPMU), 4) 16.8, 46 (LPHU), 5) 17.2, 43 (LPHU + UREA), which is the result of adding 0.4% of the diet DM as urea to LPHU. The corn silage-based treatment diets contained an average of 24% acid detergent fiber and 1.6 Mcal/kg net energy of lactation. Milk urea nitrogen (MUN) concentrations and body weights (BW) were used to calculate predicted amounts of urinary nitrogen (N) using the relationship: urinary N (g/d) = 0.0259 x BW (kg) x MUN (mg/dl). Cows fed HPMU had greater CP and RUP intakes, which resulted in higher concentrations of plasma urea nitrogen, rumen ammonia, MUN, and predicted urinary N. Milk yield, fat yield, fat percent, protein yield, and protein percent were not significantly different among treatments. Parity primarily affected parameters that were related to body size and not measurements of N utilization. The interaction of treatment and parity was not significant for any measurements taken. In this study, cows fed LPHU had significantly lower MUN and predicted urinary N without limiting production. These results demonstrate the potential to optimize milk production while minimizing N excretion in lactating dairy cattle.     

Short communication: Effect of abomasal inorganic phosphorus infusion on phosphorus absorption in large intestine,milk production,and phosphorus excretion of dairy cattle

The objective of the study was to evaluate the effect of inorganic phosphorus (Pi) infusion on P absorption in large intestine, milk production, and phosphorus excretion. Four ruminally and ileally cannulated crossbred cows were used in a 4 × 4 Latin square with 21-d periods. Cows were fed a total mixed ration containing 0.21% P, providing 50% of the cows' P requirement. Cobalt-EDTA was used as marker to measure large intestine digesta flow. On d 13 to 21 of each period, each cow was infused daily with 0, 20.1, 40.2, or 60.3 g of Pi into the abomasum and total collection was conducted on d 18 to 21. Ileal samples were collected every 9 h on d 18 to 21. Feed, digesta, and fecal samples were analyzed for total P and Pi using the molybdovanadate yellow method and blue method, respectively. All data were analyzed using PROC GLIMMIX in SAS 9.3 (SAS Institute Inc., Cary, NC) using contrasts to evaluate linear, quadratic, and cubic effects of Pi infusion dose. Dry matter intake, apparent dry matter digestibility, milk yield, and milk total P were unaffected by Pi infusion. Ileal flow and fecal excretion of total P and Pi increased linearly with increasing infused Pi. In the large intestine, net absorption of TP and Pi was increased linearly with increasing infused Pi. The magnitude of absorption from the large intestine was greater than reflected in current models, raising questions that could be evaluated with longer infusion periods or dietary alteration.     

Effects of feed intake and dietary urea concentration on ruminal dilution rate and efficiency of bacterial growth in steers

Four multiple-fistulated steers (340 kg) were fed a diet containing 50% ground grass hay, 20% dry distillers grains, and 30% concentrate at two intakes (7.2 or 4.8 kg DM/d). Urea (.4 or 1.2% of the diet) was infused continuously into the steers' rumens. The experimental design was a 4 X 4 Latin square with a 2 X 2 factorial arrangement of treatments. Infusing urea at .4 or 1.2% of the diet resulted in ruminal NH3 N concentrations of 4.97 and 9.10 mg/dl, respectively. Feeding steers at high rather than low intake decreased ruminal and total tract digestibilities of organic matter, NDF, and ADF but did not increase ruminal escape of N. However, apparent N escape from the rumen calculated using purines, but not 15N, as a bacterial marker was higher when 1.2 vs. .4% urea was infused. Feeding at high rather than at low intake increased the total pool of viable bacteria per gram organic matter fermented in the rumen. Although ruminal fluid outflows and particulate dilution rates were greater when steers were fed at high than at low intakes, efficiencies of bacterial protein synthesis were unaffected by intake. The possibility of increased N recycling within the rumen with feeding at the higher intake is discussed.     

Nitrogen and phosphorus partitioning in lactating Holstein cows fed different sources of dietary protein and phosphorus

To evaluate dietary N and P partitioning, 36 Holstein cows grouped by parity were assigned at calving to diets supplemented with soybean meal (S) or a combination of S and blood meal (B). Diets S and B were formulated to contain 16.2% CP and 0.35% P using mono- and dicalcium phosphate (PM) or wheat bran (WB) as the supplemental source of P. Actual dietary P contents were 0.38, 0.36, 0.34, and 0.34% for SPM, BPM, SWB, and BWB. Two-day total collections of feces, urine, and milk were conducted between 30 and 45 d in milk (DIM), then all cows were fed a control diet until 120 DIM. Between 120 and 150 DIM, cows were again fed the diet assigned at calving, then 2-d total collections of feces, urine, and milk were conducted. Milk production was similar for cows fed diets containing WB (SWB or BWB) when compared with cows fed PM. However, DMI tended to be lower, and P intake and total P excretion were lower in response to WB (20.7 kg/d, 71.9 g/d, and 40.3 g/d) compared with cows fed PM (23.0, 86.7, and 46.8 g/d). Apparent digestibility of dietary P did not differ due to source of supplemental P, averaging 45% across diets. The lower P intake by cows fed WB resulted in lower absorbed P and lower retained P (32.2 and 7.5 g/d) compared with those fed PM (40.6 and 13.4 g/d). Apparent N digestibility, urinary N, and N retention were not affected by P source. Blood meal decreased apparent N digestibility and absorbed N, and also decreased P retention compared with S. In later lactation, cows retained proportionately more absorbed N and P in body tissue and secreted less in milk than they did in early lactation. Results indicated the organic source of P (phytate-P) in WB can be used to provide a substantial portion of the P needed in dairy cattle diets after peak lactation, but the amount of WB in the diet during early lactation should be limited to prevent suppression of DMI and P retention.     

Effects of dietary neutral detergent fiber concentration and alfalfa hay quality on chewing, rumen function, and milk production of dairy cows.

Six ruminally fistulated Holstein cows (80 d postpartum) were used in a 6 x 6 Latin square to evaluate the effects of dietary NDF concentration and alfalfa hay quality on chewing activities, digestive parameters, and productivity of dairy cattle. Cows received one of six diets formulated to provide three concentrations of dietary NDF (31, 34, and 37%) and two sources of first-cutting, long alfalfa hay. Alfalfa hay was harvested either in early bloom (19.4% CP, 38.8% NDF) or midbloom (16.7% CP, 47.6% NDF) stages of maturity. Dietary NDF concentrations were achieved by adjusting the forage to concentrate ratios. Mean ad libitum DMI was 22.3 kg/d. Increased NDF concentration of the diet corresponded to a linear decrease in milk production (from 26.5 to 24.8 kg/d) and a linear increase in the fat content of milk (from 2.68 to 3.30%). Hay quality had no effect on milk production and composition when diets were formulated for specific NDF concentrations. Total chewing time increased from 767 to 796 and 853 min/d as fiber content of the diet increased from 31 to 34 and 37%, mainly because of increased time spent eating. Rumination time adjusted for fiber intake decreased linearly from 59.0 to 54.2 min/kg NDF as fiber intake increased, and it was higher for the early bloom than for the midbloom hay (57.3 vs. 55.5 min/kg NDF). Effects of decreased forage quality because of the increased maturity of the alfalfa hay can be minimized by formulating diets for specific NDF concentration. For diets formulated with barley-based concentrates, dietary NDF concentrations should be higher than currently recommended with allowance for greater proportions of NDF from concentrates.     

Effects of diets differing in dietary cation-anion difference and calcium concentration on calcium homeostasis in neutered male sheep

Feeding low dietary cation-anion difference (DCAD) diets is one strategy to prevent milk fever in cows. The mechanism of action, as well as whether the calcium (Ca) supply of such diets combined with this feeding regimen should meet the requirements, is still unclear. Small ruminants are commonly used as models for cows. The goal of the present study was to demonstrate basic effects of DCAD against a background of different Ca supplies in a sheep model. Twenty-three castrated male East Friesian milk sheep, aged 11 to 12 mo, were randomly assigned to 4 different feeding groups. The ration of each group was either high (highDCAD) or low in DCAD (lowDCAD) combined with adequate (nCa) or restricted Ca supply (lowCa). At baseline, serum and urine were collected from all sheep and a peripheral quantitative computed tomography of the left metatarsus was performed. After a 14-d adaptation period to the different diets, the experiment started (d 0). Urine, feces, and serum were collected on d 0, 4, 7, 14, and 22, and peripheral quantitative computed tomography was performed on d 0 and 22. On d 22, the sheep were killed and sampled for functional studies. LowDCAD was significantly associated with lower urine pH, higher urinary Ca excretion, higher ionized Ca in blood, and higher serum Ca concentrations. Blood pH and bone parameters did not differ significantly between groups. It is unclear from which compartment the high amounts of Ca excreted with urine in the lowDCAD groups originated. Interestingly, lowDCAD resulted in higher renal mRNA abundance of parathyroid hormone receptor but unaffected mRNA abundance of Ca transporters. As neither renal abundance of these transporters nor Ca excretion were influenced by dietary Ca supply, our results support the hypothesis that increased urinary Ca observed with low DCAD diets represents a loss rather than an excretion of surplus Ca.