FEEDING SYSTEMS COMBINING PASTURE WITH CONCENTRATE AND TOTAL MIXED RATIONS FOR HIGH PRODUCING DAIRY COWS.

Studies were conducted to determine: 1) the effect of pasture allowance on substitution rate, pasture and total dry matter intake, milk production and composition, rumen digestion, and grazing behavior of unsupplemented and supplemented high producing dairy cows in early-mid lactation, and 2) the effect of feeding systems combining pasture and total mixed rations on dry matter intake, milk production and composition, body weight and body condition score changes, rumen digestion and grazing behavior of high producing dairy cows in early-mid lactation. Pastures grazed in the studies were based on smooth bromegrass (Bromus inermis L.) and orchardgrass (Dactylis glomerata L.). In the first study, twenty multiparous Holstein cows (4 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 DM/cow per d) and two concentrate supplementation levels (zero vs. 1 kg concentrate/4 kg 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 pasture/kg concentrate). Total DMI 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 milk/kg concentrate for the low and high pasture allowance, respectively. Concentrate supplementation reduced milk iv fat percentage but increased milk protein percentage. Rumen pH and NH3-N concentration were decreased by 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. This latter was quantitatively more important, explaining between 80 to 100% of the reduction in pasture DMI observed. In the second study, the effect of pasture allowance on the grazing behavior of unsupplemented or supplemented high producing Holstein cows was studied in two single reversal experiments. Ten multiparous cows (2 ruminally cannulated) were unsupplemented, and six multiparous cows (2 ruminally cannulated) were supplemented with a corn-based concentrate. Within each supplementation group cows grazed a bromegrass/orchardgrass-based pasture offered at two pasture allowances: 1) 25 kg DM/cow/d, and 2) 40 kg DM/cow/d. Recorders were used to measure grazing time and bites per d. Bite mass was calculated dividing the pasture DMI, measured by chromic oxide, by the total number of bites per d. With unsupplemented cows, pasture DMI (18.8 vs. 15.6 kg/d) and milk production (17.6 vs. 15.2 kg/d) were higher at high pasture allowance. The higher pasture DMI was a result of a longer grazing time and a greater number of bites after the p.m. milking. Rumen pH was lower (6.33 vs. 6.60) and total VFA concentration was higher (134.5 vs. 119.5 mmol/L) at high pasture allowance. With supplemented cows, pasture and total DMI (14.7 and 22.8 kg/d) and milk production (27.8 kg/d) were not affected by pasture allowance. Grazing behavior of supplemented cows was not affected by pasture allowance. Neither rumen pH (6.05) nor total VFA concentration (148.4 mmol/L) differed between low and high pasture allowance. Pasture allowance had important effects on grazing behavior, pasture DMI and milk production of unsupplemented cows, however v neither grazing behavior nor performance of supplemented cows were affected by pasture allowance. In the third study, forty-five Holsteins cows (15 primiparous, 30 multiparous) were used to compare three feeding systems combining pasture and total mixed ration (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 (DMI), using chromic oxide as a marker, was 21.6, 25.3, and 26.8 kg/d for PC, pTMR, and TMR, respectively. Milk production was highest for TMR (38.1 kg/d), lowest for PC (28.5 kg/d), and intermediate for pTMR (32.0 kg/d). All three treatments received ten bST injections, PC responded to six, pTMR to four, and TMR to nine administration of bST during the 21 wk trial; milk response averaged 13.8, 15.6, and 11.8% for PC, pTMR, and TMR, respectively. Cows fed pTMR and TMR had higher milk fat (3.33 vs. 3.13%) and true protein (2.97 vs. 2.82%) percentage than cows fed PC. Cows on PC gained less body weight and lost more body condition compared to cows on pTMR and TMR. Initial concentrations of plasma nonesterified fatty acids (NEFA) were higher in PC (302 μeq/L) than in pTMR (130 μeq/L) and TMR (225 μeq/L). Plasma urea nitrogen (PUN) was lower in both pTMR and TMR treatments (13.8 mg/dl) than in PC treatment (17.2 mg/dl). Performance was improved combining pasture and TMR compared to pasture plus concentrate, resulting in higher milk production, milk production persistency, milk fat and protein percentage, and no loss in body condition score. In the fourth study, six multiparous Holstein cows fitted with rumen cannulas were used to study the effect of these three feeding systems combining pasture and total mixed ration (TMR) on ruminal digestion in a 21-wk repeated measures experiment. Rumen ammonia nitrogen concentration was lower in both the pTMR and TMR treatments (10.2 mg/dl) than in vi the PC treatment (19.9 mg/dl). Rumen pH was not affected by treatments and averaged 5.87. Neither total volatile fatty acids concentration (137.5 mmol/L) nor main individual volatile fatty acids proportions (64.4, 20.6, and 12.0 mol/100 mol for acetate, propionate, and butyrate, respectively) differed among treatments. The combination of pasture with TMR reduced the total potentially degradable fraction of DM (85.5 vs. 82.3%) of pasture, and increased the lag time (5.4 vs. 6.5 h) and reduced the potentially digestible fraction (82.1 vs. 74.9%) of pasture NDF. Ruminal losses of ammonia nitrogen were reduced combining pasture and TMR, however no positive effects on rumen pH were observed. The combination of pasture and TMR also decreased the rumen digestion of pasture, indicating the presence of negative associative rumen effects.