Effect of barley and its amylopectin content on ruminal fermentation and nitrogen utilization in lactating dairy cows.

The effect of type of grain (corn vs. barley) and amylopectin content of barley grain (normal vs. waxy) on ruminal fermentation, digestibility, and utilization of ruminal ammonia nitrogen for milk protein synthesis was studied in a replicated 3 x 3 Latin square design trial with 6 lactating dairy cows. The experimental treatments were (proportion of dietary dry matter): CORN, 40% corn grain, NBAR, 30% normal Baronesse barley:10% corn grain, and WBAR, 30% high-amylopectin (waxy) Baronesse barley:10% corn grain. All grains were steam-rolled and fed as part of a total mixed ration. The NBAR and WBAR diets resulted in increased ruminal ammonia concentrations compared with CORN (8.2, 7.4, and 5.6 mM, respectively), but other ruminal fermentation parameters were not affected. Ruminal digestibility of dietary nutrients and microbial protein synthesis in the rumen were also not affected by diet. Corn grain had greater in situ effective ruminal dry matter degradability (62.8%) than the barley grains (58.2 and 50.7%, respectively), and degradability of the normal barley starch was greater than that of the waxy barley (69.3 and 58.9%, respectively). A greater percentage of relative starch crystallinity was observed for the waxy compared with the normal barley grain. Total tract apparent digestibility of dry matter and organic matter were decreased by WBAR compared with CORN and NBAR. Total tract starch digestibility was greater and milk urea nitrogen content was lower for CORN compared with the 2 barley diets. In this study, the extent of processing of the grain component of the diet was most likely the factor that determined the diet responses. Minimal processing of barley grain (processing indexes of 79.2 to 87.9%) reduced its total tract digestibility of starch compared with steam-rolled corn (processing index of 58.8%). As a result of the increased ammonia concentration and reduced degradability of barley dry matter in the rumen, the utilization of ruminal ammonia nitrogen for microbial protein synthesis was decreased with the barley diets compared with the corn-based diet. In this study, waxy Baronesse barley was less degradable in the rumen and the total digestive tract than its normal counterpart. The most likely reasons for these effects were the differences in starch characteristics and chemical composition, and perhaps the different response to processing between the 2 barleys.

Effect of carbohydrate source on ammonia utilization in lactating dairy cows.

This study was conducted to investigate the effect of dextrose, starch, NDF, and a carbohydrate (CHO) mix on utilization of ruminal ammonia in dairy cows. Four ruminally and duodenally cannulated Holstein cows (BW = 788 +/- 31 kg; 217 +/- 35 d in milk) were allocated to four treatments in a 4 x 4 Latin square design trial. Cows were fed an all alfalfa diet at 12-h intervals (DMI = 22.2 +/- 0.25 kg/d). Treatments were control, white oat fiber (NDF); corn dextrose (GLU); cornstarch (STA); and a CHO mix (25% of each): apple pectin, GLU, STA, and NDF (MIX). Carbohydrates were introduced intraruminally during feeding at 20% of dietary DMI. Ruminal ammonia was labeled with (15)N. Ruminal pH was the highest for NDF followed by STA and MIX and GLU (P < 0.001). Ruminal ammonia concentration and pool size were decreased by GLU and STA compared with NDF (P < 0.001 and P = 0.03, respectively). Acetate, isobutyrate, isovalerate, and total VFA concentration in the rumen were decreased (P = 0.009 to 0.001), and butyrate was increased (P < 0.001) by GLU compared with the other CHO. Microbial N flow to the duodenum was decreased (P < 0.05) by NDF compared with the other CHO, and the flow of microbial N formed from ammonia was greater for STA compared with GLU and NDF (P = 0.04 and 0.03, respectively). Urinary N loss was decreased (P = 0.05) by GLU and STA, but overall (feces plus urine) N losses were not affected (P = 0.73) by treatment. Milk urea concentration was lowered by GLU and STA compared with NDF and MIX (P = 0.002). The proportion of bacterial N synthesized from ammonia in the rumen was greater with STA than with NDF and MIX and was least for GLU (P = 0.02). Irreversible ammonia loss and flux were lower (P = 0.09 and 0.02, respectively) for GLU than for STA and NDF. As a percentage of the dose given, cumulative secretion of (15)N ammonia in milk protein was greater for STA than for GLU or NDF (P = 0.01 and 0.001, respectively). This experiment demonstrated that provision of readily fermentable energy can decrease ammonia concentrations in the rumen through decreased ammonia production (GLU), or through enhanced uptake of ammonia for microbial protein synthesis (STA). Rapidly fermentable energy in the rumen decreased ammonia production and flux, but the overall efficiency of ammonia utilization for milk protein synthesis was only increased by enhancing ruminal microbial ammonia uptake.

Effect of dietary crude protein level and degradability on ruminal fermentation and nitrogen utilization in lactating dairy cows.

The objectives of this experiment were to investigate the effects of two ruminally degradable protein (RDP) levels in diets containing similar ruminally undegradable protein (RUP) and metabolizable protein (MP) concentrations on ruminal fermentation, digestibility, and transfer of ruminal ammonia N into milk protein in dairy cows. Four ruminally and duodenally cannulated Holstein cows were allocated to two dietary treatments in a crossover design. The diets (adequate RDP [ARDP] and high RDP [HRDP]), had similar concentrations of RUP and MP, but differed in CP/RDP content. Ruminal ammonia was labeled with 15N and secretion of tracer in milk protein was determined for a period of 120 h. Ammonia concentration in the rumen tended to be greater (P = 0.06) with HRDP than with ARDP. Microbial N flow to the duodenum, ruminal digestibility of dietary nutrients, DMI, milk yield, fat content, and protein content and yield were not statistically different between diets. There was a tendency (P = 0.07) for increased urinary N excretion, and blood plasma and milk urea N concentrations were greater (P = 0.002 and P = 0.01, respectively) with HRDP compared with ARDP. Milk N efficiency was decreased (P = 0.01) by the HRDP diet. The cumulative secretion of ammonia 15N into milk protein, as a proportion of 15N dosed intraruminally, was greater (P = 0.003) with ARDP than with HRDP. The proportions of bacterial protein originating from ammonia N and milk protein originating from bacterial or ammonia N averaged 43, 61, and 26% and were not affected by diet. This experiment indicated that excess RDP in the diet of lactating dairy cows could not be efficiently utilized for microbial protein synthesis and was largely lost through urinary N excretion. At a similar MP supply, increased CP or RDP concentration of the diet would result in decreased efficiency of conversion of dietary N into milk protein and less efficient use of ruminal ammonia N for milk protein syntheses.