REVIEW

Nutritional Strategies to Improve Nitrogen use Efficiency by Ggrazing Dairy Cows

Estrategias Nutricionales para Mejorar la Eficiencia de uso del Nitrógeno en Vacas Lecheras en Pastoreo

Juan Pablo Keim¹*, and René Anrique¹

¹Universidad Austral de Chile, Facultad de Ciencias Agrarias, Independencia 641, Valdivia, Chile. "Corresponding author (juan.keim@uach.cl).

Grazing dairy cows are characterized by converting a low proportion of dietary N into milk N. This low N use efficiency (NUE) is mainly due to an excessive supply of crude protein (CP) of pasture with regard to animal requirements. Excess N in the diet has negative effects for the animal; it alters the organoleptic characteristics of milk and pollutes the environment through N excretions. The aim of this review was to analyze the available information on nutritional strategies to improve NUE by grazing dairy cows, such as diluting the dietary N through supplementation, synchronizing ruminal carbohydrate (CHO), and N fermentation rates, and using pastures with a high water soluble carbohydrates (WSC) content. There is a beneficial effect of carbohydrate supplementation on NUE, with an increase in milk yield (MY) of 0.83 ± 0.34 kg milk kg^-1 supplement and a reduction of ammonia N (NH₃-N) in the rumen. A better synchrony between CHO and rumen N does not improve MY and NUE. However, a positive effect on NUE was identified as a result of increasing the WSC/CP ratio. The use of cultivars with high WSC content increases the dry matter intake (DMI), MY, and milk N production, with no consistent effect on NUE.

Key words: Concentrate supplementation, ruminal synchrony, high sugar ryegrass, crude protein, milk production.

Las vacas lecheras en pastoreo se caracterizan por convertir una baja cantidad del N dietario en N lácteo. Esta baja eficiencia en el uso del N (NUE) se debe principalmente al excesivo suministro de proteína cruda (CP) de la pradera en relación a los requerimientos del animal. El exceso de N en la dieta tiene efectos negativos para el animal, altera las características organolépticas de la leche y contamina el medio ambiente a través de las excreciones de N. El objetivo de este trabajo fue analizar la información disponible sobre estrategias nutricionales para mejorar EUN en vacas lecheras a pastoreo, tales como diluir la concentración de N dietario a través de la suplementación, sincronizar la tasa de fermentación de carbohidratos (CHO) y N en el rumen, y utilizar praderas con alto contenido de carbohidratos solubles (WSC). Se observa una relación beneficiosa entre la suplementación con CHO y la NUE, con un aumento en producción de leche (MY) de 0.83 ± 0.34 kg leche kg^-1 suplemento y menor cantidad de N amoniacal (N-NH₃) en el rumen. La mejora en la sincronía entre CHO y N a nivel ruminal, no genera los efectos esperados en PL y EUN. Se observó un efecto positivo sobre la NUE al mejorar la relación WSC/CP. La utilización de cultivares con un mayor contenido de WSC incrementa el consumo de MS, MY y la cantidad de N lácteo, mientras que la NUE no mejora consistentemente.

One of the objectives of animal nutrition is to provide the amount of nutrients that the animal needs to achieve a certain level of production. Any deficiency will translate into a lower production, while an excessive nutrient supply involves losses that will increase production costs and can also affect the animal and the environment (Hristov and Jouany, 2005; Pacheco et al., 2008).

Grazing systems, compared to confinement systems which provide a balanced ration, reach lower levels of production and nutritional efficiency. An example of this situation is the low conversion of dietary N into milk N, ranging between 13 and 31% in grazing systems and 40 to 45% under confinement systems with balanced rations (Delagarde et al., 1997; Verité and Delaby, 2000). A low N use efficiency (NUE) is basically due to the incapacity of animals to build significant amounts of protein reserves, being necessary to adjust protein supply according to protein requirements (Hoekstra et al., 2007). Energy is the main limiting nutrient for productivity under grazing conditions, while amino acids supply usually exceeds animal requirements (Kolver et al., 1998).

Dietary N which is not converted into milk N has negative effects on the animal, reducing energy availability due to the energy spent in the conversion of ammonia (NH₃) into urea (Pacheco et al., 2008); besides excess dietary N it is associated with reproductive problems (Butler, 1998), lower dry matter intake (DMI; Cosgrove et al. , 1999), risk of intoxication due to excess nitrates (Bolan and Kemp, 2003), undesirable odors or flavors in milk (Bendall, 2001) and environmental pollution, mainly through excretion of urinary N (UN; Pacheco et al., 2008). Due to the negative effects related to low NUE, it is necessary and challenging to develop strategies that allow increasing NUE in dairy systems. Under confinement conditions there are several approaches to optimize the proportion of dietary N converted into milk N (Verité and Delaby, 2000; Castillo et al., 2001; Hristov and Jouany, 2005; Flachowsky and Lebzien, 2006; Hoekstra et al., 2007), unfortunately there is still uncertainty about the pathways that may improve NUE under grazing dairy systems.

This review is focused on analyzing factors that explain the low NUE of grazing systems in temperate regions, and the nutritional strategies that have been evaluated to improve the conversion of dietary N into milk N, such as reducing N content of pastures, lowering N content of the diet through supplementation, improving ruminal synchrony between energy and protein, and the use of Lolium perenne L. cultivars with a high sugar content.

Nitrogen metabolism
Nitrogen utilization in ruminants occurs through a series of processes that take place in the rumen and the animal. While rumen bacteria can metabolize a variety of N sources, animals need amino acids for their metabolism (Pacheco and Waghorn, 2008). Therefore, the requirements of both microorganisms and host must be taken into consideration when formulating diets.

In ruminants, N arrives to the small intestine as microbial protein, ruminal undegraded protein (RUP) and as endogenous protein, and collectively contribute to metabolizable protein (MP; Schwab et al., 2005). Microbial protein is highly digestible and is characterized by a balanced amino acid profile, while in RUP the amino acid composition resembles that of the precursor feed (NRC, 2001). Total digestible protein in the small intestine corresponds to MP composed by amino acids and peptides that are absorbed and made available for animal metabolism (Lapierre et al., 2005), while excess MP is deaminated by the liver and excreted as urea through the urine (Bohnert et al., 2002). Therefore, as N intake increases urinary N increases considerably with no effect on fecal N, being urinary N the main determinant of the negative relationship between N intake and NUE (Figure 1).

Microbial protein is the result of microbial metabolism and in most cases represents the main source of the amino acids absorbed by the intestine (70 to 100%) of the host animal (Schwab et al., 2005). Microbial protein is synthesized mainly from NH₃ (38 to 80%) generated from protein degradation (Pacheco and Waghorn, 2008) and peptides, when protein is rapidly degraded in the rumen (Hristov and Jouany, 2005; Nolan and Dobos, 2005).

The amount of microbial protein synthesized depends on the nutritional quality of the feed, intake level and retention time of solids and liquids in the rumen (NRC, 2001; Hristov and Jouany, 2005; Nolan and Dobos, 2005) and may potentially reach 50 g per 100 g CP ingested (Pacheco et al., 2008). In terms of nutritional factors influencing microbial protein synthesis it is important to consider the nature of energy and N sources, among them; microbial growth and the efficiency of NH₃ utilization are highly determined by the availability of carbohydrates (CHO) in the rumen (Newbold and Rust, 1992; Heldt et al., 1999). Therefore, the control of factors associated to CHO availability in the rumen, such as the amount supplemented, its source and degradability, and the synchronization between the rate of release of energy and N in the rumen could help improving the efficiency of ruminal NH₃ and dietary N utilization (Hristov and Jouany, 2005).

The concentration of ruminal NH₃ increases when intake of rumen degradable protein (RDP) is high compared to fermentable energy, favoring NH₃ absorption and transport to the liver for urea synthesis (Castillo et al., 2001; Kulling et al., 2001). Urea is released into the blood stream and excreted through the urine, normally allowing toxic effects associated to excessive ammonia to be avoided (Lapierre et al, 2005). The detoxification process has an energy cost of 4 moles of ATP per mol of urea synthesized (McBride and Kelly, 1990), which is equivalent to 7.17 kcal ME g^-1 N as urea (Tyrell, 1970; cited by Pacheco and Waghorn, 2008) or to 3.3 kcal ME g^-1 urea.

In lactating dairy cows, between a 60 and 90% of ingested N is eliminated through feces and urine depending on N intake and production level (Flachowsky and Lebzien, 2006). Urinary N excretions up to 500 g d^-1 have been measured with high N intake (Steinshamn et al., 2006; Pacheco and Waghorn, 2008).

Pasture as a source of nitrogen
In dairy systems based on high quality pastures (2.75-2.9 Mcal ME kg^-1 DM), energy supply is regarded as the main limiting factor for milk production, allowing to sustain production levels of up to 27 L cow^-1 d^-1, while protein is not limiting with production levels up to 35 L cow^-1 d^-1 (Kolver et al., 1998).

The N contained in forages is found as proteins, peptides, free amino acids, nucleic acids, nitrates, and secondary metabolites (Goswami and Willcox, 1969). True protein (TP) normally ranges between 70 and 90% of total CP (Tamminga, 1986) and RDP (Pacheco and Waghorn, 2008). The RUP content in fresh pasture is generally low and with values ranging between 11.7, 8 and 3% CP for a permanent pasture with 27% CP, adjusted for rumen passage rate of 8, 5 and 2% h^-1, respectively (Anrique et al., 2008). Also RUP increases slightly with longer grazing intervals (Hoekstra et al., 2007).

CP content and N fractions of pastures are variable and influenced by factors such as botanical composition and season of the year (Fulkerson et al., 1998; Aufrere et al. , 2003), also by physiological stage and agronomical managements (Peyraud and Astigarraga, 1998). Increasing N fertilization tends to increase CP and non protein N (NPN) content in the plant, and also degradability of N fractions in rumen, therefore decreasing the contribution of RUP (Nolan and Dobos, 2005). Pastures undergoing intensive N fertilization have reached CP levels higher than 300 g kg^-1 DM (Pacheco et al., 2008) with NPN concentrations from 240 to 420 g kg^-1 total N (Goswami and Willcox, 1969), thus increasing the risk of negative effects on the animal and the environment and achieving a low NUE (Cosgrove et al, 1999; Bendall, 2001; Bolan and Kemp, 2003; Pacheco et al., 2008). The low NUE obtained under grazing conditions is basically due to the high dietary N intake, coupled with a high concentration of soluble protein and an imbalance and asynchrony between protein and energy supplies to the rumen (Nolan and Dobos, 2005; Hoekstra et al., 2007; Pacheco and Waghorn, 2008), and represents a loss of N and energy potentially available for productive purposes (Pacheco et al., 2008).

NUE of grazing dairy cows ranges between 13 and 31% (Verité and Delaby, 2000), with higher efficiency being observed with low CP content in the ration and high milk productions (Bach et al., 2000; Pacheco et al., 2008). As it has been possible to store in the milk up to 45% of the ingested N under experimental conditions (Delagarde et al., 1997), we believe that it may be possible to increase NUE in grazing dairy systems by developing nutritional strategies to reduce N intake and optimize microbial protein synthesis.

Reducing N intake from pasture based diets. A reduction of N content in pastures may be achieved through agronomic management such as selecting forage species with lower N content, which is a long term process, or by reducing the amount of N supplied as fertilizer or by increasing grazing intervals (Peyraud and Astigarraga, 1998). However, reducing N fertilization can decrease forage yield and longer grazing intervals should reduce the nutritional quality of the pasture. On the other hand, providing supplements with a low CP content is an alternative that allows diluting dietary N concentration (Pacheco et al., 2008), may not affect milk production and may increase stocking rate on the farm.

The purpose of supplementation to improve NUE should be focused on decreasing dietary CP content, avoiding an excess of CP available in the rumen, and increasing the energy supply to help converting dietary N into microbial protein (Hristov and Jouany, 2005; Pacheco et al., 2008).

A higher synthesis of microbial protein would reduce N excretions only if there is enough energy to allow the differential of absorbed amino acids to be retained in milk (Flachowsky and Lebzien, 2006; Pacheco et al., 2008) and if the animal has the genetic potential to produce a higher amount of milk protein (Chagunda et al., 2009).

The use of concentrates with low CP and high NSC contents has improved NUE (Keady et al., 1998), while using protein supplements increased considerably the N excretion through the urine and reduced NUE (Mulligan et al., 2004). Reducing CP content in the diet from 180 to 160 g kg^-1, through ration formulation, made possible to maintain milk production, reduce N intake by 10 to 15 percentage units and N excretion by 13 to 20 percentage units (Satter et al., 2002), indicating the possibility to increase NUE by the reduction of CP in the diet without affecting milk production.

Table 1 summarizes the results from studies that analyzed the effect of using different CHO sources on productive and metabolic responses of grazing dairy cows. Pasture was the main component of the diet except for treatments in which the cows received up to 10 kg of concentrates. The CP content of the supplements ranged between 121 and 260 g kg^-1, and an average NUE of 24 ± 4.8% (minimum 15.4 and maximum 32.9%: n = 52) was observed. Based on 91 diets evaluated in different countries, Castillo et al. (2000) observed an average NUE of 28%.

Table 1. Effects of reducing the CP content of the diet by carbohydrate supplementation on NUE and milk production in grazing dairy cows.

The results summarized in Table 1 also show that the ruminal NH₃-N decreases with energy supplementation (Van Vuuren et al., 1993; Berzaghi et al., 1996; Carruthers and Neil, 1997; O'Mara et al., 1997; Jones-Endsley et al., 1997; Reis and Combs, 2000; Bargo et al., 2002), which suggests a higher NH₃-N utilization for microbial protein synthesis (Berzaghi et al., 1996; Carruthers and Neil, 1997; O'Mara, 1997).

Since animals do not have the ability to store the excess of ingested N (Hoekstra et al., 2007), as N intake increases, milk and fecal N concentrations remain relatively constant, while the amount of urinary N excreted increases proportionally (Steinshamn et al., 2006) reaching levels closer to 50% of total ingested N (Hristov and Jouany, 2005). In the reviewed studies, the amounts of urinary and fecal N vary with CHO supplementation, with positive (Jones-Endsley et al., 1997; Robaina et al., 1998; Reis and Combs, 2000; Walker et al., 2001), neutral (O'Mara, 1997; Bargo et al., 2002) or negative responses (Berzaghi et al., 1996; Bargo et al., 2002). This may be caused by a number of factors determining the N losses, such as the CP concentration of the supplement and the ration, a possible increase of the total DMI caused by the supplementation, an increase of the amount of available energy for microbial protein synthesis, and a possible decrease of DM digestibility as a consequence of a lower ruminal pH. If concentrate supplementation does not dilute CP concentration of the diet, it may not decrease the urinary N excretions, because the surplus of ingested N regarding N requirements would not be reduced. Furthermore, if DMI increases and CP concentration of the diet remains constant, higher amounts of daily urinary N excretions may be expected.

It has been observed that in supplemented dairy cows there was an increase of the proportion of fecal N compared to urinary N (Van Vuuren et al., 1993; Carruthers and Neil, 1997; Bargo et al., 2002; Mulligan et al., 2004; Steinshamn et al., 2006). As mentioned above, fecal N should remain relatively constant, but could increase due to a higher DMI and a lower N digestibility. When the MP meets animal requirements, increasing microbial protein synthesis or RUP supply may have no effect on NUE, but may produce an increase in the proportion of fecal N in relation to urinary N (Flachowsky and Lebzien, 2006; Pacheco et al., 2008), which corresponds to the indigestible fraction of MP and RUP at intestinal level. When higher amounts of circulating N are converted into fecal microbial N, via fermentation in the posterior digestive tract, a transference of potential urinary N to fecal N, an environmentally less labile source of N, could be achieved (Gressley and Armentano, 2007) and therefore the potential impact on the environment may be reduced.

The effect of CHO supplementation on pasture diets has been also analyzed using continuous culture fermenters, evaluating ruminal digestion and N metabolism (Bach et al., 1999; Bargo et al., 2003; Wales et al., 2009). Ruminal NH₃-N decreased as a consequence of supplementation. The amount of microbial N produced (g d^-1) kept constant, however, with low and medium substitution rates (kg pasture kg^-1 concentrate) there was an increase in the proportion of microbial N in relation to total N flow (Bargo et al., 2003). In other CHO supplementation experiments, it has been found a significant increase in bacterial N synthesis (Bach et al., 1999), or no response (Wales et al., 2009) compared with a pasture only diet. In order to avoid a negative effect on the OM digestibility and efficiency of microbial protein synthesis (g N kg^-1 digested OM), it has been suggested that grain inclusion should not exceed 24% of total diet (Wales et al., 2009). At higher levels the efficiency of microbial N synthesis tends to decrease with supplementation (Bach et al., 1999; Bargo et al., 2003).

From a productive point of view, concentrate supplementation of grazing dairy cows causes an increase in milk production (Berzaghi et al., 1996; Carruthers and Neil, 1997; Robaina et al, 1998; Ries and Combs, 2000; Walker et al, 2001; Bargo et al, 2002; Mulligan et al., 2004; Steinshamn et al., 2006), however, the response observed has been variable with a media of 0.83 ± 0.34 (n = 23) kg milk kg^-1 supplement and lower than in confinement, limiting the use of this alternative to the relationship between price paid per liter of milk, the cost per kilogram of supplement and the animal response. The highest milk production in supplemented grazing dairy cows usually occurs due to an increase in total DMI and digestibility of the diet. However, there are several factors affecting the milk response (MR) like herbage allowance, amount and type of supplement, and production level and stage of lactation of the cow. Higher herbage allowances are related to higher substitution rates (SR; SR = (pasture DMI in unsupplemented treatment - pasture DMI in supplemented treatment)/supplement DMI), a small increase in total DMI and therefore a lower MR to concentrate supplementation. The milk response per kilogram of concentrate decreases as the amount of concentrate increases, being lower above 3-4 kg concentrate d^-1 when pasture quality and quantity are not limiting. Finally, cows in early lactation have a higher MR than cows in late lactation.

In summary, supplementation with energy sources and low protein content increases NUE in grazing dairy cows, due to a decrease in the CP content of the diet and a possible higher microbial protein synthesis. Another beneficial effect is that it decreases the proportion of urinary N in relation to fecal N, since the last one has a lower impact on the environment. Supplementation decreases pasture intake per cow, allowing increasing the stocking rate, and a higher milk yield per cow. So it would allow producing more milk protein per surface unit with lower losses to the environment (Verité and Delaby, 2000). However, the milk response to supplementation, the price paid per liter of milk and the cost per kilogram of supplement must be considered to avoid a negative effect on the profitability of the dairy farm.

Improving ruminal synchrony between CHO and N. The concept of ruminal synchrony proposed by Johnson (1976) establishes that ruminal NH₃ utilization and microbial protein synthesis would be maximized if there is a synchrony between the availability of energy and N in the rumen (balanced amounts at the same time). This could be achieved by changing the CHO or N sources, changing the feeding patterns (time of supplementation regarding grazing) or the feeding frequency (Cabrita et al., 2006). A better ruminal synchrony could be achieved under grazing systems, when the CHO sources have a degradation rate of 13 to 14% h^-1, because this is similar to the RDP degradation rate of the pasture (Van Vuuren et al., 1990), although degradation rate of the pasture will vary with different circumstances (Aufrere et al., 2003).

Table 2 shows the results obtained in experiments aiming at improving ruminal synchrony between CHO and N, either by changing the fermentation rates of CHO and N through modification of the diet ingredients (Shabi et al, 1998; Casper et al, 1999; Gehman et al, 2006) or changing the feeding patterns (Kolver et al., 1998; Trevaskys et al., 2004; Lund et al., 2008).

Table 2. Effects of applying supplementation strategies, to optimize carbohydrate and nitrogen synchronization, on NUE and milk production of grazing dairy cows.

Casper et al. (1999) evaluated the supplementation with corn (Zea mays L.) or barley (Hordeum vulgare L.) plus soybean (Glycine max Merr.) meal or extruded soybean meal to improve ruminal synchrony between CHO and N, while Gehman et al. (2006) determined the effect of supplementing grazing cows with corn, barley or citrus pulp. None of these studies could determine any clear effects on NUE when modifying the degradation rate of CHO and CP, however, Casper et al. (1999) observed a tendency towards a higher NUE when supplementing with barley and soybean meal (synchrony due to the fast degradation of CHO and CP) and corn with extruded soybean meal (synchrony due to the lower degradation rate of CHO and CP), and determined lower amounts of ruminal NH₃-N (barley plus soybean meal), which indicates a better utilization of N for the synthesis of microbial protein. Meanwhile, milk production was higher in diets supplemented with corn as a consequence of a higher DMI. Total intake can decrease when supplementing with large amounts of rapidly degradable CHO sources, due to a decrease of OM digestibility and a lower ruminal passage rate (Chamberlain and Wilkinson, 1996). Gehman et al. (2006) determined a higher NUE when supplementing with corn (C) or barley (B) rather than when using citrus pulp (CIP), even though considering the synchrony the supplementation with B or CIP should increase NUE, since their degradation rate (13 a 14% h^-1) would be expected to allow an optimum ruminal NH₃ utilization by grazing dairy cows. Experimentally, a disadvantage of modifying the ingredients of the diet is that there are effects that could be attributed to ruminal synchrony, when they would actually be a consequence of factors linked to the supplemented ingredient (Hristov and Jouany, 2005). In order to identify effects that are a consequence of better ruminal synchronization, it is recommended to change the feeding patterns (Newbold and Rust, 1992), specifically, the moment when the supplement is offered to the animals in relation to the grazing event.

From the studies of Kolver et al. (1998), Trevaskys et al. (2004), and Lund et al. (2008) we define a synchronized diet as the one where the highest proportion of rapidly degradable CHO was supplemented before grazing to match the energy supply (from the degradation of the CHO in the supplement) and NH₃ content at the rumen (from the degradation of CP of the pasture). In the three studies there was a decrease in N intake for diets with a better synchrony, but only Trevaskys et al. (2004) found a slight increase of NUE (+ 1.1%), as a consequence of a lower CP content in the pasture of the treatment with better synchrony (308 vs. 328 g kg^-1). There were no differences in MY and urea N levels. The effects of synchrony in MY and N excretion are not clear (Newbold and Rust, 1992; Hristov and Jouany, 2005) and less important than expected (Cabrita et al., 2006). A proof of this is that Kaswari et al (2007) did not obtain significant differences between isoenergetic and isoproteic diets with different synchrony indexes for ruminal N-NH₃ content, microbial protein synthesis, and metabolizable energy flow. Factors explaining the lack of response with synchronizing degradation of CHO and N, may be that ruminants have the capacity to recycle the excess N into urea and supply it to the rumen during periods of deficit, remaining available for microbial protein synthesis when slowly degradable CHO are fermented and release energy (Marini et al., 2004; Lapierre et al., 2005), also ruminal bacteria can recover rapidly from N shortages (Newbold and Rust, 1992), so that their metabolic functions are not affected.

The lack of response to a higher synchrony in cows fed pastures with high CP content (more than 200 g kg^-1 on DM basis) would be caused by an excess of MP in relation to the requirements of dairy cows (for a milk production of around 30 L d^-1), therefore the animal would not be able to convert the higher MP supply, generated by a better synchrony, into milk N (Trevaskys et al., 2004). Cows with a higher genetic merit for protein synthesis could store a higher amount of consumed N in milk (Chagunda et al., 2009). Therefore selecting cows with higher milk protein production would allow depositing the excess of MP at the intestinal level into milk N.

Increase of water soluble carbohydrate concentration of the pasture
Effect of agronomic management. A higher WSC content in the forage allows to decrease N intake and urinary N, because there is an inverse relationship between sugar content and CP of the forages (Tas et al., 2006b). Therefore, the beneficial effects of forages with a high WSC content should be a consequence of a higher energy supply at the rumen and a lower N intake (Taweel, 2006).

It is possible to improve the WSC/CP ratio by reducing N fertilization (Taweel, 2006), increasing grazing intervals (Peyraud and Astigarraga, 1998), using selected cultivars with high sugar content (Miller et al., 2001), or adjusting the pasture intake according to the daily patterns of WSC and CP contents of the plant (Trevaskys et al., 2004). WSC concentration in grasses is higher at sunset than during the morning (Rutter et al., 2004; Hristov and Jouany, 2005) as a consequence of sugar accumulation (especially sucrose) due to photosynthesis, finding higher levels at the moment when photosynthesis and respiration rates are equal. Meanwhile CP content is higher during the morning than at sunset, because there is a negative relationship between WSC and CP content (Cosgrove et al., 2007).

In theory, maximizing animal DMI at sunset would allow to improve NUE. Forage samples that were collected at sunset have a wider WSC/CP ratio than those collected at the morning (Trevaskys et al., 2004). Trevaskys et al. (2004) determined the effect of offering a daily strip of pasture to dairy cows in the morning or the afternoon, observing an increase in DMI, MY, milk N content and NUE when the cattle entered the pasture in the afternoon (Table 3). Nevertheless, due to the high CP content of the pasture (328 and 268 g kg^-1 DM, in the morning and afternoon, respectively), NUE remained within low standards (13.9 and 16.3% for cows entering the field in the morning and afternoon, respectively).

Use of high sugar varieties. Miller et al. (2001) evaluated milk production in late lactating dairy cows fed a perennial ryegrass pasture, selected for a high WSC content (HS), or a standard cultivar. They observed a higher digestible DMI (+ 1.5 kg d^-1), greater MY (+2.7 kg d^-1) and NUE (+ 7%), and a lower proportion of urinary N (10%) in cows fed the HS cultivar. Two years later in a similar study with early lactation cows, Moorby et al. (2006) determined a higher WSC content (243 vs. 161 g kg^-1 DM), higher digestibility and a lower NDF content for the HS cultivar as compared to the control, resulting in a higher pasture (15.3 vs. 13.1 kg DM d^-1) and total DMI (18.8 vs. 16.6) and a non significant tendency to a higher milk yield (+ 2.3 kg d^-1). However there were no differences in NUE, but values (average of 36.7%) are very high for grazing cows, which was attributed to the low CP content of the pasture (average 101.5 g kg^-1 DM). In this experiment urinary N decreased 7% and fecal N increased 14% for cows fed HS, which would lessen the environmental impact associated to N excretions for cows fed HS cultivars.

Taweel et al. (2005) evaluated an HS perennial ryegrass with lactating dairy cows (126 ± 21 DIM). The HS cultivars had 20% more WSC than standard cultivars and, as a different from Miller et al. (2001) and Moorby et al. (2006) findings, their NDF and CP contents were similar, so the observed responses could be attributed to the increase in WSC content. There were no significant differences for pasture DMI, total DMI, digestible DMI, MY, and NUE, while MUN and ruminal NH₃-N tended to be lower in cows fed the HS cultivar. In a subsequent study Taweel et al. (2006) did not observe differences in DMI, MY, and NUE in early lactation cows (75 ± 7 d) fed with HS and standard cultivars. The reason behind the lack of response with feeding HS cultivars, observed in these two studies, could be that an increase in WSC content of around 20% would not be enough to increase palatability and forage digestibility (Taweel et al., 2005; 2006). Although there was a difference in WSC content between HS and standard cultivars, the energy and CP content were similar, so the only difference between HS and standard cultivars was the ruminal CHO rate of degradation. As discussed above, the effects off modifying CHO and N degradation i.e. ruminal synchrony does not increase milk yield neither NUE. Even though MUN was lower for HS varieties, indicating a possible higher microbial protein synthesis, the CP levels of the diets were sufficient to satisfy animal requirements, so that the difference of microbial protein would not be used for productive purposes.

Tas et al. (2006a) determined a higher WSC content for HS cultivars (210 to 260 g kg^-1 for the first and second season of evaluation, respectively). WSC content decreased and the CP concentration increased during the second season, attributed to a higher N fertilization of the pasture. During 2002, DM and N intake, milk and protein production in early lactation cows was higher for HS cultivars, however, during 2003 there were no differences between HS and standard cultivars. MUN content was lower for the HS treatment during the two seasons. The increase in WSC content at the expense of the CP and NDF concentrations did not show the expected differences between cultivars for NUE during the two seasons (24.1 and 24.6% during 2002; 19 and 20.9% in 2003). Tas et al. (2006b) evaluated the productive response in early lactation dairy cows fed with six cultivars of perennial ryegrass that had different WSC content. The cultivars with higher WSC content showed a lower CP and NDF content, however, there were no differences in DMI and MY. There were no significant differences in NUE between cultivars.

The information in Table 3 allowed us to determine a high correlation (r² = 0.75) between the WSC/CP ratio and NUE (Figure 2), which tends to increase linearly (NUE = 9.329 * WSC/CP + 16.57) as the WSC/CP ratio increases, in other words, to improve NUE, it would be necessary to increase the WSC concentration and, simultaneously, decrease the CP content.

In summary, the use of HS cultivars increases DM intake (Miller et al., 2001; Moorby et al., 2006; Taweel et al., 2006), milk production (Miller et al., 2001; Moorby et al., 2006; Tas et al., 2006a; Taweel et al., 2006) and milk N (g d^-1). However, NUE improved consistently only in one study (Miller et al., 2001), while MUN decreased only in one experiment (Taweel et al., 2005). A higher milk production was observed when cows were fed with HS pasture, this was associated with a higher DMI but not to a better nutrient utilization (Miller et al., 2001; Trevaskys et al., 2004; Moorby et al., 2006; Tas et al., 2006a; Taweel et al., 2006). A higher, but not significant, NUE average was observed when cows consumed HS cultivars, when comparing with values obtained with cows fed conventional cultivars (24.7 ± 4.8% vs. 20.9 ± 5.9%), however, this happened mainly because of the low CP content of the pastures evaluated, without an important effect of the increase in WSC content of the plant. Therefore, it would be expected that with permanent pastures such as those present in Southern Chile, characterized by a high CP content during autumn and spring (Anrique et al., 2008), NUE might be lower to those values described, despite the use of HS cultivars and close to those observed by Trevaskys et al. (2004).

CONCLUSIONS

A low NUE is observed in grazing dairy cows, which is attributed to the high CP content of most pastures in temperate areas during autumn and spring. The most efficient strategy to improve NUE and reduce urinary N under these conditions seems to be to reduce dietary CP content through supplementation with low-protein concentrate feeds. The supplementation allows increasing animal stocking rate and milk protein production per surface unit and decreases the loss per product unit. A better ruminal synchrony has not meant a higher NUE under grazing conditions, while the use of HS cultivars shows inconsistent results.

In order to improve NUE it is necessary to increase the WSC/CP ratio of the pasture plants. Therefore, a viable alternative would be to combine practices such as carbohydrate supplementation and to offer pasture during the afternoon. Additionally, genetic selection of dairy cows, orientated towards a higher concentration of milk protein, is important for cows to increase milk N retention.

ACKNOWLEDGEMENTS

To Dr. Luis Latrille for the valuable comments. Financial assistance provided by Dirección de Investigación y Desarrollo (DID)/Universidad Austral de Chile is gratefully acknowledged.

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Received: 16 February 2011. ]]> Accepted: 10 August 2011.