The effect of carbohydrate source on ruminal fermentation and performance of Holstein lactating cows fed a diet containing slow-release urea

To achieve maximum microbial protein production and efficiency of feed protein utilization, the simultaneous availability of rumen degradability carbohydrate and nitrogen sources for ruminal microorganisms is essential. When using urea, due to its very high ruminal degradability rate, highly fermentable carbohydrate sources such as soluble sugars should be used to achieve maximum efficiency of nutrients utilization, microbial protein production and fermentation. On the other hand, considering the rate of nitrogen release in slow-release urea compared to urea, it seems that the type of carbohydrate source and its degradation rate should be considered when using this product in the diet of lactating cows, because it can affect the efficiency of nitrogen utilization in the rumen. Therefore, the aim of this study was to investigate the effect of carbohydrate source type on the performance of Holstein lactating cows fed a diet containing slow-release urea.

This experiment was performed in the 3 periods in a completely randomized design as repeated measurement using 9 Holstein lactating cows (average weight: 650± 50 kg, milk production: 33± 0.5 kg and DIM: 100± 25 days) in three 21-day periods. Experimental treatments were; 1. basal diet include Nitroza® (a source for slow- release urea, as control); 2. basal diet plus 2.7 % molasses as a replacement for corn grain and 3. basal diet plus 2.7% barley grain as a replacement for corn grain. To measure ruminal fermentation parameters, on the day 21 of each period, four hours after the morning meal, ruminal fluid was collected. Apparent digestibility of nutrients were measured by acid insoluble ash as internal marker. The milk production was recorded daily and on the day 20 of each period, milk produced from each animal was sampled at each meal to measure the milk composition. To determine the concentration of serum albumin, glucose, total protein, cholesterol, triglyceride and urea nitrogen concentration on day 21 of each period, 4 hours after the morning meal, blood samples were taken from the tail vein of each animal.

The results of the present study showed that the addition of molasses and or barley grain as a replacement for a part of corn grain had no significant effect on feed and nutrient intake (except for organic matter). Compared to the control group, replacing corn with molasses in a diet containing slow-release urea increased the amount of organic matter intake (P< 0.01). The results of independent comparisons showed that the addition of molasses compared with control and barley treatment reduced ADF intake, but increasing the amount of barley in the diet resulted an increase in the ADF intake (P< 0.02). Treatments had no significant effect on apparent digestibility of nutrients, ruminal fluid pH, total concentration of volatile fatty acids, molar ratio of propionate, butyrate, acetate, valerate, isovalerate, and acetate: propionate ratio and glycogenic:non-glycogenic fatty acids. Treatments had no significant effect on milk production and composition. Although, milk production in the cows fed diet containing molasses and barley grain was higher than that of control group. The serum concentration of albumin, glucose and urea nitrogen in was not significantly affected by the type of carbohydrate source. Compared to the control group, the addition of molasses caused a significant increase in serum total protein concentration (P <0.05). Economic calculations showed that the addition of molasses or barley grain to diets containing slow release urea increased net income.

The results of the present study demonstrated that the addition of carbohydrate sources with degradability higher than corn grain in dairy cow diets include slow release urea (Nitroza®) can improve nutrients utilization efficiency and increase farmer income.