Intake and performance of lactating cows grazing diverse forage mixtures.

Twenty multiparous Holstein cows in midlactation grazed pastures of 4 forage mixtures in a 12-wk study repeated during 2 grazing seasons to determine if forage mixture complexity affected intake and productivity of lactating dairy cows. The forage mixtures were 1) orchardgrass plus white clover [2 species (SP)]; 2) orchardgrass, white clover, and chicory (3SP); 3) orchardgrass, tall fescue, perennial ryegrass, red clover, birdsfoot trefoil, and chicory (6SP); and 4) 6SP mixture plus white clover, alfalfa, and Kentucky bluegrass (9SP). Total herbage intake was similar among forage mixtures, averaging 12.0 kg/d across all forage mixtures and years. Milk production and composition were not affected by forage mixture or year, and averaged 34.6 kg/d, 3.4%, and 2.8% for milk production, milk fat percentage, and milk protein percentage, respectively. The conjugated linoleic acid content of milk fat was higher for cows that grazed the 3SP, 6SP, and 9SP mixtures than from cows that grazed the 2SP mixture (1.02 vs. 0.87 g of conjugated linoleic acid/100 g of fatty acids, respectively). Blood glucose, blood urea nitrogen, and nonesterified fatty acids were not affected by forage mixture and averaged 69.2 mg/dL, 13.4 mg/dL, and 277.5 muEq/L, respectively. The results of this study indicate that altering the forage mixture in pastures did not affect dry matter intake, milk production, or blood metabolite profiles of lactating cows. The use of complex mixtures of forages in grazing systems should not affect dairy cow performance.

Major advances in teaching dairy production.

A survey of 38 universities that grant 4-yr degrees as well as 12 institutions that grant technical degrees of 2 yr or less revealed that degree programs in dairy production remain popular, but have changed significantly over the last 25 yr. Enrollment in dairy production programs remains strong (1,189 and 417 students in baccalaureate and nonbaccalaureate degrees, respectively) even though this is viewed as a traditional industry. There are significant differences in size of programs across the United States, and some are struggling to maintain both the visibility and faculty numbers to keep pace with the industry. The percentage of students enrolled in 4-yr programs who are female has increased to the majority. More students hail from a nondairy farm background in our university programs today than in 1994. Computer and information technology has become a mainstream part of our educational programs. A high percentage of undergraduate students elect to engage in an internship or work experience, and there is a high correlation between internship and career paths selected by our students. The dairy industry initiated and financially supports the North American Intercollegiate Dairy Challenge; an educational activity among university teams to foster skills in analyzing a dairy farm business. This collaboration between universities and private industry is strong evidence that our undergraduate programs are relevant to the dairy industry. Extracurricular activities like dairy science clubs also remain popular, and are perceived by faculty members to be an important part of our educational experience. An analysis of nonbaccalaureate degree programs was not reported previously, but was a part of the present survey. In the nonbaccalaureate institutions that responded to the survey, there were 417 students enrolled in 12 dairy programs across the United States in 2004. This student population in nonbaccalaureate programs has a higher percentage of female enrollment than in 1994, but enrollment is still predominantly male. Computer and information technologies are an important part of their curricula and a very high percentage of these students remain in production agriculture upon graduation. Many of the challenges in undergraduate education described previously continue to be challenges in 2005. However, there are many reasons for optimism; as the number of students electing enrollment in dairy production remains strong, there is great interest in keeping the curricula relevant and interaction with and support by the dairy industry continues to be significant.

Grazing behavior affects daily ruminal pH and NH3 oscillations of dairy cows on pasture.

Grazing behavior of Holstein cows in late lactation at 2 pasture allowances without or with supplementation was studied in a single reversal design. Twenty multiparous cows (4 ruminally cannulated) grazed a bromegrass/orchardgrass pasture offered at 2 pasture allowances: 1) low, and 2) high, with 25 and 40 kg/d of DM per cow, respectively. Half of the cows were supplemented with a mineral/vitamin mixture (1 kg/ d of the mix in a corn/molasses carrier) and the other half supplemented with a corn-based concentrate (1 kg of concentrate per 4 kg of milk). Automatic behavior recorders were used to measure grazing time and number of bites. For the mineral/vitamin mixture-supplemented cows, grazing time and number of bites after the p.m. milking was greater and ruminal pH was numerically lower at the high pasture allowance. For the concentrate-supplemented cows, grazing behavior and ruminal pH did not differ between the 2 pasture allowances. Pattern of grazing time of mineral/vitamin mixture-supplemented and concentrate-supplemented cows influenced daily oscillations of ruminal pH and NH3-N concentration. Pasture allowance affected grazing behavior of mineral/vitamin mixture-supplemented cows; however grazing behavior of concentrate-supplemented cows was not affected by pasture allowance.

The effect of pasture allowance and supplementation on feed efficiency and profitability of dairy systems.

Partial budgeting was used to compare income over feed costs of high-yielding Holstein cows based on data from an experiment with 4 dietary treatments arranged in a 2 x 2 factorial. The factors were low (25 kg DM/cow per day) and high (40 kg DM/cow per day) pasture allowance (PA) and supplemental grain fed at 1 kg/4 kg of milk or no supplemental grain fed. The 4 treatments were low PA unsupplemented (LPAU), low PA concentrate supplementation (LPAC), high PA unsupplemented (HPAU), and high PA concentrate supplementation (HPAC). Two management systems were modeled. The first, a fixed herd size flexible rotation length model, and the second, a flexible herd size model where rotation length was fixed. The LPAC treatment yielded the highest income over feed costs, followed by the HPAC treatment. The treatment generating the lowest income was the HPAU system. The low PA systems generated more income than did the high PA systems for equivalent supplemental feeding strategies. The results also showed that feeding supplemental grain increased the income-over-feed costs compared with systems that did not feed supplemental grain. In most treatments, comparing the fixed herd against the flexible herd models, the flexible herd size model generated higher income due to the substitution of relatively low income per hectare of hay production for higher income from milk production. There were also differences in feed conversion efficiencies for milk production due to concentrate supplementation (1.04 unsupplemented vs. 1.21 supplemented), but PA did not affect the efficiency of milk production. Neither supplementation nor PA affected the feed conversion efficiency of milk fat yield. However, the efficiency of milk protein yield was affected by concentrate supplementation (0.028 unsupplemented vs. 0.034 supplemented) but not by pasture allowance.

Induced lactation in nonpregnant cows: profitability and response to bovine somatotropin.

Significant culling of high-producing cows with low fertility reduces profitability of dairy farms as those cows are replaced with heifers. Induced lactation of nonpregnant cows may be a management alternative to reduce culling and increase profits. The objectives of this study were to evaluate the efficacy of bovine somatotropin (bST) to increase milk production in cows induced into lactation with estrogen plus progesterone, and to determine the profitability of inducing cows into lactation vs. using replacement heifers entering the herd as first-lactation cows. Parity 1 or greater, nonpregnant, healthy Holstein cows (n = 28) were induced into lactation by administration of estradiol-17beta (0.075 mg/kg of body weight [BW] per d) and progesterone (0.25 mg/kg of BW per d) for 7 d. Milking began on d 18. Cows were randomly assigned to control or bST treatment groups on d 37 +/- 20 of milking, and milk production was compared for 70 d. After the 70-d comparison, all cows received bST for the duration of lactation. Cows receiving bST produced more milk (28.4 kg/d) than controls (24.1 kg/d), with variable yields among cows. For the economic analysis, induced cows were compared to first-lactation cows in the same herd using fair market value for costs and multiple component pricing for milk. Net present value for an induced cow (1966 dollars) was significantly greater than that for a first-lactation cow (1446 dollars). Our data suggest that bST use in induced cows is profitable. If a reliable method were developed and approved by the FDA, inducing nonpregnant cows into lactation could be used by dairy producers to increase profitability.

Milk fatty acid composition of cows fed a total mixed ration or pasture plus concentrates replacing corn with fat.

Thirty-one Holstein cows (six ruminally cannulated) were used to evaluate milk fatty acids (FA) composition and conjugated linoleic acid (CLA) content on three dietary treatments: 1) total mixed rations (TMR), 2) pasture (Avena sativa L.) plus 6.7 kg DM/d of corn-based concentrate (PCorn), and 3) pasture plus PCorn with 0.8 kg DM/d of Ca salts of unsaturated FA replacing 1.9 kg DM/d of corn (PFat). No differences were found in total (22.4 kg/d) or pasture (18.5 kg/d) dry matter intake, ruminal pH, or total volatile fatty acids concentrations. Fat supplementation did not affect pasture neutral detergent fiber digestion. Milk production did not differ among treatments (19.9 kg/d) but 4% fat-corrected milk was lower for cows fed the PFat compared to cows fed the TMR (16.1 vs. 19.5 kg/d) primarily because of the lower milk fat percentage (2.56 vs. 3.91%). Milk protein concentration was higher for cows fed the TMR than those on both pasture treatments (3.70 vs. 3.45%). Milk from the cows fed the PCorn had a lower content of short- (11.9 vs. 10.4 g/100 g) and medium-chain (56.5 vs. 47.6 g/100 g) FA, and a higher C18:3 percentage (0.07 vs. 0.57 g/100 g) compared with TMR-fed. Cows fed the PFat had the lowest content of short- (8.85 g/100 g) and medium-chain (41.0 g/100 g) FA, and the highest of long-chain FA (51.4 g/100 g). The CLA content was higher for cows in PCorn treatment (1.12 g/100 g FA) compared with cows fed the TMR (0.41 g/100 g FA), whereas the cows fed the PFat had the highest content (1.91 g/100 g FA). Pasture-based diets increased the concentrations of long-chain unsaturated FA and CLA in milk fat. The partial replacement of corn grain by Ca salts of unsaturated FA in grazing cows accentuated these changes. However, those changes in milk FA composition were related to a depression in milk fat.

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.

Economic analyses of feeding systems combining pasture and total mixed ration.

Partial budgeting was used to compare net incomes of high-yielding Holstein cows fed either a total mixed ration (TMR), a pasture-based diet, or a combination of both. Variables included in the analysis were milk income, feed, feeding, manure handling, fencing, and water system expenses (revenues and costs based on 2000 values). Base data were from 45 Holstein cows (109 days in milk), assigned to one of three dietary treatments: TMR (nongrazing with TMR ad libitum), pasture plus TMR (pTMR, with pasture in the day and TMR at night), or pasture plus concentrate (PC, pasture twice daily plus 1 kg of concentrate/4 kg milk). Data from those groups were projected to a case-study herd of 70 cows and subjected to sensitivity analysis at varying milk prices and feed and pasture costs. Although costs per kilogram of milk produced were lowest for PC cows, cows on TMR had the highest net income per cow per day (5.61 dollars) because of higher yields of milk (38.1 kg/d) and milk components (1.24 kg/d of fat, 1.13 kg/d of true protein), although expenses were highest among all systems (4.12 dollars). Cows on the PC had lower daily net income (5.31 dollars) due to lower yields of milk (28.5 kg/d) and milk components (0.89 kg/d of fat, 0.79 kg/d of true protein) even though expenses were also lowest (2.57 dollars). Cows fed the pTMR were intermediate in production (32.0 kg/d of milk, 1.06 kg/d of fat, 0.93 kg/d of true protein) but had similar daily net income per cow (5.28 dollars) to the PC cows but were lower than the TMR cows. Sensitivity analysis showed that the TMR system was more profitable than the pTMR and PC systems, with expenses considered, except at combinations of lower milk prices and higher feed costs. Differences between the pTMR and PC systems were less, with PC being more profitable in half of the scenarios, particularly at lower milk prices and higher feed costs.

Supplemental carbohydrate sources for lactating dairy cows on pasture.

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

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

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

Performance of high producing dairy cows with three different feeding systems combining pasture and total mixed rations.

Forty-five Holsteins cows in early to mid lactation were used to compare three feeding systems combining pasture and total mixed rations (TMR) on animal performance in a 21-wk repeated-measures experiment. The three treatments were: 1) pasture plus concentrate (PC), 2) pasture plus partial TMR (pTMR), and 3) TMR (non-pasture). Total dry matter intake, using chromic oxide as a marker, was 21.6, 25.2, and 26.7 kg/d for PC, pTMR, and TMR, respectively. Milk production was highest for TMR (38.1 kg/d), lowest on PC (28.5 kg/d), and intermediate for pTMR (32.0 kg/d). Cows on pTMR and TMR had higher milk fat and true protein percentages than cows on PC. Cows on PC gained less body weight and lost more body condition compared with cows on pTMR and TMR. Initial concentrations of plasma nonesterified fatty acids were higher on PC (302 microeq/L) than on pTMR (130 microeq/L) and TMR (225 microeq/L). Plasma and milk urea nitrogen were lower on both pTMR and TMR than on PC. Combining pasture and TMR resulted in higher milk production, milk fat and protein percentage, and maintenance in body condition score compared to pasture plus concentrate. The TMR feeding system resulted in the highest total dry matter intake and milk production.

Ruminal digestion and fermentation of high-producing dairy cows with three different feeding systems combining pasture and total mixed rations.

Six multiparous Holstein cows fitted with rumen cannulas were used to study the effect of three feeding systems combining pasture and total mixed rations (TMR) on ruminal digestion in a 21-wk repeated measures experiment. The three treatments were: 1) pasture plus concentrate (PC), 2) pasture plus partial TMR (pTMR), and 3) TMR (nonpasture). Ruminal NH3-N concentration was lower on both the pTMR and TMR treatments (10.2 +/- 0.5 mg/dL) than on the PC treatment (19.9 +/- 0.5 mg/dL). Ruminal pH was not affected by treatments and averaged 5.87. Neither total volatile fatty acid concentration (137.5 mmol/L) nor individual volatile fatty acid proportions (63.1,20.6, and 12.0 mol/ 100 mol for acetate, propionate, and butyrate, respectively) differed among treatments. The pTMR treatment reduced the total potentially degradable fraction of dry matter (85.5 vs. 82.3%) and the potentially digestible fraction of neutral detergent fiber (82.1 vs. 74.9%) of pasture compared to the PC treatment. Ruminal NH3-N losses were reduced when combining pasture and TMR; however this combination decreased the ruminal digestion of pasture, indicating the presence of associative effects in the rumen.

Milk response to concentrate supplementation of high producing dairy cows grazing at two pasture allowances.

Twenty multiparous Holstein cows (four ruminally cannulated) in five 4 x 4 Latin squares with 21-d periods were used to study the effect of concentrate supplementation when grazed at two pasture allowances. The four dietary treatments resulted from the combination of two pasture allowance targets (low, 25 vs. high, 40 kg of dry matter/cow per day) and two concentrate supplementation levels (zero vs. 1 kg of concentrate/4 kg of milk). Concentrate supplementation decreased pasture dry matter intake 2.0 kg/d at the low pasture allowance (17.5 vs. 15.5 kg/d) and 4.4 kg/d at the high pasture allowance (20.5 vs. 16.1 kg/d). Substitution rate was lower at the low pasture allowance (0.26 kg pasture/kg concentrate) than at the high pasture allowance (0.55 kg of pasture/kg of concentrate). Total dry matter intake of both supplemented treatments averaged 24.4 kg/d. Milk production of both supplemented treatments averaged 29.8 kg/d, but was increased with higher pasture allowance in the unsupplemented treatments (19.1 vs. 22.2 kg/d). Milk response to concentrate supplementation was 1.36 and 0.96 kg of milk/kg of concentrate for the low and high pasture allowances, respectively. Concentrate supplementation reduced milk fat percentage but increased milk protein percentage. Rumen pH and NH3-N concentration were decreased with concentrate supplementation. Substitution rate was likely related to both negative associative effects in the rumen (reductions in rumen pH, rate of pasture digestion, and NDF digestibility) and reductions in grazing time. The latter was more important, quantitatively explaining at least 80% of the reduction in pasture dry matter intake observed.

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.

Improved extraction methods for avoiding the interference of copper in the LC determination of ascorbic acid in multivitamin-mineral tablets.

A rapid sample extraction procedure for the determination of ascorbic acid (AA) by high performance liquid chromatography (HPLC) in multivitamin-mineral formulations containing interfering copper has been developed. The method takes special precautions to prevent degradation of AA in contact with high concentrations of interfering elements such as copper. Sample preparation involved addition of pyrogallol, citric acid solution and short time extraction (5 min) under an atmosphere of nitrogen to prevent oxidation of AA. Another sample cleanup based on extraction of the multivitamin-mineral tablet with the extraction solution as described above, but with addition of strong cation exchange sorbent, was also developed. The copper and other minerals in the formula were retained on an ion exchange sorbent and thus represses the speed of oxidation of AA. Extracts of multivitamin-mineral tablets were analyzed by ion-pair reversed phase HPLC and UV-diode array detection. Ten multivitamin formulations containing copper were analyzed by the two proposed methods. The analytical results of AA obtained using the simple sample extraction method and the method involving addition of cation exchange were 89-115% and 95-112%, respectively, of declared concentrations. The parameters for the validation of the methods are given.

Evaluating nitrogen management options for reducing nitrate leaching from northeast U.S. pastures.

Substantial amounts of nitrate nitrogen NO3-N can leach from intensively grazed pasture in the northeast U.S. where there is about 30 cm of groundwater recharge, annually. Management options for reducing NO3-N leaching were evaluated for this environment using the Cornell Net Carbohydrate and Protein System Model and a recently developed nitrogen leaching index. Management options utilizing energy supplementation of grazing dairy cows could improve nitrogen efficiency within the cow, but would not necessarily reduce NO3-N leaching at the pasture scale if stocking rate was not controlled. The management option of using white clover to supply nitrogen to the pasture decreased NO3-N leaching, but produced less dry matter yield, which in turn reduced stocking rate. The economic returns of reducing NO3-N with these options need to be evaluated in light of milk prices and commodity and fertilizer nitrogen costs. At current prices and costs, the economic benefit from the energy supplementation options is substantial.

Supplementation of rumen-undegradable protein to the diets of early lactation Holstein cows on grass pasture.

The objectives were to evaluate supplements differing in rumen-undegradable protein (RUP) on the performance of early lactation cows on grass pasture. Twenty-four Holstein cows averaging 68 d of lactation and 39.8 kg/d of milk were rotationally grazed on predominantly Dactylis glomerata pasture for 8 wk. Cows were blocked according to parity, milk yield, and days of lactation and were randomly assigned to a grain mixture with a low or high RUP content. Pasture and the low and high RUP grain mixtures averaged 25.6, 14.7, and 13.7% crude protein and 4.0, 7.0, and 8.4% RUP, respectively (dry matter basis). Grain was fed twice daily at 1 kg/4 kg of milk, pasture provided all forage in the diet, and grain consumption was similar (8.9 kg/d per cow) for cows on both treatments. Total dry matter intakes estimated using chromic oxide were 19.9 and 20.9 kg/d for cows fed low and high RUP grain mixtures. Milk yields did not differ between treatments; means were 34.2 and 35.5 kg/d for cows fed low and high RUP grain mixtures. Multiparous cows tended to yield more milk (36.2 vs. 34.5 kg/d) and milk protein (1.06 vs. 0.98 kg/d) when fed the high RUP grain mixture. Concentrations of plasma urea N and nonesterified fatty acids were unaffected by treatment and averaged 18.7 mg/dl and 307 microeq/L, respectively. Results indicated that a supplemental grain mixture with a high RUP content did not alter milk yield of high yielding cows when pasture was the sole forage; however, milk protein yield tended to be greater for multiparous cows fed the high RUP grain mixture.

Synchronization of ruminal degradation of supplemental carbohydrate with pasture nitrogen in lactating dairy cows.

Twelve Holstein cows in early to midlactation (8 fitted with ruminal cannulas) were used to test the hypothesis that the synchronization of the rate of ruminal degradation of supplemental carbohydrate and N from fresh pasture would increase the amount of N retained for growth and milk production. A concentrate based on ground shelled corn was fed either at the time that pasture was fed at 0900 and 1700 h (synchronous) or 4 h after pasture was fed at 1300 and 2100 h (asynchronous). The crossover design included a 6-d adjustment period and a 10-d milk sampling period. Nitrogen balance was determined during the last 5 d of each period. Cows fed the synchronous diet had a lower mean ruminal pH. Peak ammonia concentration at 3 and 5 h after pasture feeding in the morning was reduced by approximately 33% of values obtained from cows fed the asynchronous diet. The diurnal pattern of blood urea N concentration was similar to that of ruminal ammonia, but mean daily concentrations of blood urea N did not differ between diets. Nitrogen retained for milk production and growth was not influenced by diet, and no differences in milk production, composition, or efficiency were observed. Based on changes in ruminal concentrations of ammonia, synchronous ruminal release of supplemental carbohydrate with pasture N appeared to improve the capture of ruminal N; however, these changes were transient and did not change the N status or performance of dairy cows.

Evaluation and application of the Cornell Net Carbohydrate and Protein System for dairy cows fed diets based on pasture.

This study evaluated the Cornell Net Carbohydrate and Protein System for dairy cows consuming diets based on pasture, assessed the sensitivity of the model to critical inputs, and demonstrated application opportunities. Data were obtained from four grazing experiments and four indoor pasture feeding experiments (25 dietary treatments) involving dairy cows in New Zealand and the US. The model provided a reasonably good estimate of changes in body condition score (r2 = 0.78; slope not significantly different from 1), estimated energy balance (r2 = 0.76; slope not significantly different from 1), blood urea N (r2 = 0.94; underprediction bias of 0.5%), microbial N flow (r2 = 0.88; slope not significantly different from 1), and milk production. The model underpredicted dry matter intake (r2 = 0.80; 13% bias) and overpredicted ruminal pH (r2 = 0.47; 1.7% bias). Predicted milk production was especially sensitive to changes in pasture lignin content, effective fiber, rate of fiber digestion, and amino acid composition of ruminal microbes. Milk production was first-limited by the supply of metabolizable energy when only high quality pasture was fed, but specific amino acids limited milk production when more than 20% of the diet consisted of a grain supplement. These results indicate that the Cornell Net Carbohydrate and Protein System can be used for dairy cows in a grazing system to make realistic predictions of performance.

Effect of intake of pasture on concentrations of conjugated linoleic acid in milk of lactating cows.

We examined the effect of intake of fresh pasture on concentrations of conjugated linoleic acid in milk fat. Sixteen Holstein cows were paired and divided into either the control group or the grazing group. The study involved initial, transition, and final periods. During the initial period, all cows consumed a total mixed diet. Cows in the control group were fed the total mixed diet throughout the study, and cows in the grazing group were gradually adjusted to a diet consisting of intensively managed pasture. Performance of cows in the grazing group was significantly reduced from that of cows in the control group during the final period (dry matter intake, 19% less; milk yield, 29.6 vs. 44.1 kg/d; and live weight, 40 kg less). During the initial period, when both groups were consuming a total mixed diet, concentrations of conjugated linoleic acid in milk fat were similar (X = 5.1 mg/g of milk fat). As the grazing group was gradually adjusted to pasture, concentrations of conjugated linoleic acid in milk gradually increased. During the final period, when cows in the grazing group were consuming a diet consisting of pasture only, conjugated linoleic acid concentrations in the milk fat were doubled (10.9 vs. 4.6 mg/g of milk fat). Furthermore, results showed the individual consistency of the milk fat content of conjugated linoleic acid over time but also demonstrated substantial variation among individual cows within treatment groups. Overall, this study indicated that the concentration of conjugated linoleic acid in milk fat is enhanced by dietary intake of fresh pasture.