Digestion and nitrogen metabolism of grass fed dairy cows

Abstract

Until recently, young, highly digestible grass was considered an ideal feed for dairy cows. However, research during the last decades has shown that the nutrient supply of grazing animals is insufficient for milk productions above c. 29 kg per day. Experiments in England and New Zealand have shown that the efficiency of protein utilization is relatively low and consequently, a high proportion of ingested nitrogen is excreted in urine and faeces. This reports the effects of grassland management and feeding strategies on the digestion and availability of nutrients from perennial rye- grass ( Lolium perenne ) in dairy cows.Chapter 1 is a literature review of the various factors affecting the composition and nutritional quality of grass. To facilitate high yields of dry matter and a high feeding value, grass is fertilized with high levels of nitrogen. Level of nitrogen fertilization, weather conditions and the duration between nitrogen application and harvesting date affect the content and quality of grass protein. Level of nitrogen fertilization and maturity also influence other components of grass, like carbohydrates and lipids.In Chapter 2 it is emphasized that high levels of nitrogen fertilization result in high concentrations of crude protein, which are easily fermented in the forestomachs. Thus, in grazing dairy cows, an important part of grass protein is fermented in the rumen and subsequently excreted in the urine. The actual efficiency of nitrogen utilization in cows grazing intensively managed pastures, is 15 to 25 %. We estimated that theoretically a 600 kg cow producing 25 kg milk per day can utilize dietary nitrogen with a maximum efficiency of 40 to 45%. This chapter discusses changes in grassland management and nutrition aiming at improvements of nitrogen utilization. When grass is the sole feed, efficiency of nitrogen utilization cannot be improved substantially without have a detrimental effect on animal performance. Supplementing the diet of grazing dairy cows with low protein, high energy feeds increases the efficiency of nitrogen utilization, mainly because it reduces nitrogen intake.Chapters 3 and 4 contain details of experiments using the nylon bag technique. Chapter 3 discusses the effects of grass maturation and rate of nitrogen fertilization on rumen degradability of organic matter and crude protein in fresh grass. These results were used to estimate the content of digestible protein entering the small intestine. Crude protein content and in situ degradability of organic matter and crude protein decreased with increasing grass maturity and with decreasing nitrogen application. With every 100 g per kg dry matter decrease in crude protein content, the estimated content of digestible protein entering the small intestine decreased by 19 g per kg dry matter, irrespective of how the crude protein content was manipulated.Chapter 4 discusses rumen degradability of crude protein and non-protein organic matter (- carbohydrates) of fresh and preserved grass, obtained in four nylon bag studies, and consequences for dairy cow rations. In Experiment 1, the effect of level of fertilization on the in situ degradation of fresh grass was studied. The second experiment focused on the effect of maturation on degradation of fresh grass. Experiments 3 and 4 dealt with the influence of maturation and dry matter content of grass silage and hay. Experiment 4 also included treatment with cell wall degrading enzymes. Fresh and preserved grass fertilized at high levels of nitrogen, contained large surpluses of fermentable nitrogen. In fresh grass the ratio of [soluble nitrogen]:[soluble carbohydrates] was lower than the ratio of [insoluble, degraded nitrogen]:[insoluble, degraded carbohydrates]. Therefore, it was concluded that ingredients with a low ratio of [insoluble, degraded nitrogen]: [insoluble, degraded carbohydrates] may be considered appropriate supplements to grass-based diets. In preserved grass the ratio of [soluble nitrogen]:[soluble carbohydrates] exceeded the ratio of [insoluble, degraded nitrogen]:[insoluble, degraded carbohydrates]. Wilting had no consistent effect on the [nitrogen]:[carbohydrates] ratio. Treatment with cell wall degrading enzymes resulted in a lower [soluble nitrogen]:[soluble carbohydrates] ratio. From these results it was concluded that silage-based diets require supplementation with ingredients rich in soluble carbohydrates.Chapter 5 reports an experiment in which the digestion and intestinal amino acid supply were studied in three rumen and duodenal cannulated lactating cows fed freshly cut grass. Grass was fertilized at levels of 275 or 500 kg of nitrogen/ha per year. High-nitrogen grass was fed in June and October; low-nitrogen grass in July and September. When low-nitrogen grass was fed, the digestibilities of organic matter and crude protein were lower than found with high-nitrogen grass. On low-nitrogen grass, the duodenal nitrogen flow expressed per unit of nitrogen intake was higher. The flow of amino acid nitrogen on low-nitrogen grass was slower in September, mainly because of reduced microbial protein synthesis attributed to slower organic matter degradation of low-nitrogen grass. Duodenal nitrogen flow per unit of nitrogen intake was inversely related to the nitrogen: organic matter ratio of the diet. Rate of nitrogen fertilization did not affect ruminal turnover of organic matter and neutral detergent fibre. Turnover and passage rates recorded in this experiment did not differ from reported data on cows fed winter rations at similar levels of dry matter intake.Changes observed in vivo in digestion and amino acid supply when fresh grass was partly replaced by concentrate mixtures (either maize starch or sugar beet pulp fibre) are presented in Chapter 6. Partial replacement of grass decreased crude protein digestibility. When high starch concentrate was fed, overall digestibility of neutral detergent fibre was lower than on the high fibre diet, mainly because of decreased ruminal digestion of neutral detergent fibre. With the high starch concentrate, 39% of the ingested starch escaped ruminal fermentation. Although less organic matter was fermented in the forestomachs on high starch concentrate, the duodenal amino acid nitrogen flow was higher than on the high fibre concentrate. The proportion of microbial protein was unaffected; thus, efficiency of microbial synthesis was estimated to be higher when high starch concentrate was fed.In the experiment reported in Chapter 7, six grazing dairy cows each fitted with a rumen cannula, were supplemented with high or low starch concentrates. The cows received 1 kg or 7 kg of high or 7 kg of low starch concentrate in two equal meals per day fed after milking. After a three-week adaptation period, samples were taken of grass and rumen fluid. Total sugar content of grass increased during daytime with the highest concentration directly before sunset. Patterns of ruminal pH values did not differ between treatments and were minimal at midnight. Volatile fatty acids and ammonia peaked at midnight. Supplementation with 7 kg of concentrate decreased rumen concentrations of ammonia and branched-chain volatile fatty acids. Acetate: propionate and non- glucogenic:glucogenic ratios of volatile fatty acids and percentage of milk fat tended to be lowest when the diet included 7 kg of high starch concentrate.Chapter 8 describes three ruminal fermentation studies carried out in combination with three feeding trials. These experiments were carried out to investigate the effect of partial replacement of grass by low protein feedstuffs on pH and concentrations of volatile fatty acids and ammonia in the rumen and on nitrogen excretion in milk, urine, and faeces by dairy cows. Feedstuffs tested were the high-starch and high-fibre concentrates used in the experiment reported in Chapter 6, maize silage, dried and ensiled pressed sugar beet pulp and high-moisture ear maize silage with or without husks. Partial replacement often increased dry matter intake, resulting in minor effects on nitrogen intake. Urinary nitrogen excretion ranged between 30 and 58% of nitrogen intake and decreased by 30 to 40% when grass was partially replaced. The reduction in urinary nitrogen excretion corresponded to a decrease of rumen ammonia. Faecal nitrogen output ranged between 25 and 30% of nitrogen intake and tended to increase with inclusion of low protein feed. Replacement by concentrate mixtures based on maize reduced milk fat content without changing rumen volatile fatty acid composition; for mixtures based on beet pulp, milk fat content remained unaffected.In the General Discussion (Chapter 9), the supply of aminogenic, glucogenic and ketogenic nutrients from grass is estimated from the obtained data. Supply of nutrients depends on total dry matter intake and on the composition of the dry matter.From the rumen evacuation data it was concluded that rumen fill is not a factor limiting grass intake. Possible limiting factors are discussed, like fermentation products (ammonia, volatile fatty acids) and the maximum capacity in chewing activity (grazing and ruminating).Relationships between crude protein content and the proportion of protein escaping from rumen fermentation were used to predict the supply of available protein in the small intestine. A curvilinear relationship was found between crude protein and predicted amount of available protein. Extrapolation suggested that a plateau level of c. 125 g available protein (DVE) per kg dry matter is reached at crude protein concentrations above 270 g per kg dry matter. This was higher than concluded in Chapter 5 from the in vivo results, where a maximum duodenal non-ammonia nitrogen flow was predicted when grass contains 225 g crude protein per kg dry matter. The nitrogen losses in the rumen increased linearly with crude protein content.The predicted supply of available protein based on in situ data agreed with in vivo data for grazing steers reported in literature, but was higher than the duodenal protein flow in dairy cows observed in our experiments. This discrepancy was mainly attributed to the low efficiency of microbial protein synthesis estimated in our experiments. Possible explanations discussed included the higher intake level and higher proportion of soluble carbohydrates and proteins in grass observed in our experiments compared to those reported in steers. Differences may also result from methodological errors. It was estimated that cows consuming 16 kg of grass organic matter and producing 27 kg of milk per day are in a large positive balance of glucogenic and ketogenic nutrients. However, comparing nutrient and energy requirements showed that the assumed nutrient requirements for maintenance and milk production are too low, whereas the nutrient supply was overestimated.An estimate was made of the effect of a decrease in nitrogen content in grass on the flow of nitrogen in grazing dairy cows, using a simplified model. Decreasing the nitrogen content in grass reduced nitrogen excretion in urine and faeces concomitant with an decrease in the proportion excreted in urine.The model was also used to predict the effects of partial replacement of grass by low protein concentrates. This resulted in a reduction in urinary nitrogen excretion with only minor changes in faecal nitrogen output. The efficiency of nitrogen utilization increased.It was concluded that the crude protein content in grass can be reduced to concentrations of c. 225 g per kg dry matter without significant negative effects on the supply of aminogenic nutrients. Yet even at that level, urinary ruminal and metabolic nitrogen losses will be significant. Since further decrease in protein content is correlated to a reduction in dry matter yield and in the nutritive value of grass, a further reduction of nitrogen losses should be established by partial replacement of grass by low protein feeds