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Limited research is available that evaluates animal productivity from increasing levels of pasture supplementation to dairy confinementtype diets. Two 8week field studies (fall of 2004, F2004; and spring of 2005, F2005) and an in vitro continuous culture study were conducted to examine animal performance under different combinations of total mixed ration (TMR) feeding and highquality pasture grazing. Such feeding systems are increasingly referred to as a partial mixed ration (PMR). Cows were assigned to either an all-TMR diet (100T, no access to pasture) or one of the following three PMR diets: 1) 85% TMR-restricted (85T) 2) 70% TMR-restricted (70T) and 3) 55% TMRrestricted (55T). Actual diets during the fall trial became 59% TMR and 41% pasture, 68% TMR and 32% pasture, and 79% TMR and 21% pasture instead of the formulated 55T, 70T, and 85T, respectively. The corresponding actual diets during the spring trial became 65% TMR and 35% pasture, 79% TMR and 21% pasture, and 89% TMR and 11%, respectively. Cows on the PMR diets had access to pasture (annual ryegrass) and grazed as a single group for 7 h/d between a.m. and p.m. milkings. In F2004, the TMR feeding system maximized total dry matter intake (DMI) and milk production, but 4% fatcorrected milk (FCM), milk fat yield and protein yield did not differ among diets. In S2005, FCM was greatest for treatments 85T and 100T, and lowest for treatment 70T, with 55T yielding an intermediate value. Milk fat and protein yields were greatest for treatments 85T and 100T compared with those diets that included the greatest amount of pasture. Greatest DM intake from pasture was associated with the greatest concentration of conjugated linoleic acids (CLA, a group of fatty acids with potent cancerfighting properties) in milk. Also, concentrations of saturated fatty acids in milk fat were greatest for cows consuming TMR. Gross feed efficiencies (kg FCM per kg DMI) were similar for all diets in F2004, but in S2005, cows consuming PMR exhibited enhanced gross feed efficiencies compared to cows consuming TMR exclusively.
Continuous culture fermentation allowed for the assessment of ruminal fermentation patterns, intermediates, and end products of fermentation to better understand the production responses obtained from the two field trials. Ruminal variables examined included ruminal culture pH, methane (CH4) production, ammonia nitrogen (NH3-N) concentration, volatile fatty acid (VFA) production, and microbial biomass production. Increasing the amount of forage added to the fermentors altered the molar proportions and daily production of VFA. Treatment 55T tended (P = 0.08) to yield the greatest amounts of total VFA (108.0 mmol/d), followed by treatment 85T (98.7 mmol/d). Methane production was greatest (P < 0.05) for the allTMR diet (42.5 mmol/d) and lowest for 70T (16.6 mmol/d). Reduced (P < 0.05) apparent and true DM degradabilities were reported for the treatments that exhibited the lowest pH values (5.65 and 5.68 for 70T and 100T, respectively). Increasing the amount of forage offered to continuous cultures resulted in an increase (P < 0.05) in microbial DM flow and decreased CH4 production. Although ruminal NH3N concentration was similar for all diets, increasing the amount of forage resulted in improved N capture by ruminal microorganisms. These studies suggest that in PMR feeding systems such as the ones tested pasture can be as high as 41% without affecting overall lactation performance when compared to an allTMR ration.