Corn supplementation on milk urea nitrogen content of dairy cows grazing on temperate annual pasture

: The excretion of urinary nitrogen (N), one of the most important environmental contaminants from livestock systems, is highly correlated with milk urea N content. The objective of this research was to evaluate the use of different types of corn supplementation on milk urea N in grazing dairy cows. Twelve Holstein × Jersey lactating dairy cows were divided into six uniform groups according to milk production, lactation stage and live weight. Treatments were compared according to a 3 × 3 replicated Latin square experimental design, with three periods of seventeen days (twelve days to adaptation and five to measurements). The experimental treatments were exclusively grazing (G); grazing + supplementation with 4.2 kg DM of corn silage (CS) and grazing + supplementation with 3.2 kg DM of ground corn (GC). The pasture used was annual ryegrass (Lolium multiflorum L.) and white oats (Avena sativa L.). The milk protein production increased 65 g/day in the GC treatment group compared to the G and CS groups. The supplemented dairy cows showed lower milk urea N (-2.8 mg/dL) than unsupplemented cows, but the N utilization efficiency (g N output in milk/ g N intake) did not change between treatments (average = 0.26). Additionally, there was a relationship between milk and plasma urea nitrogen concentrations (R 2 = 0.64). In conclusion, for dairy cows grazing annual temperate pastures, corn ground supplementation increased milk protein production and reduced the excretion of milk urea N, whereas corn silage reduced the excretion of milk urea N without affecting milk protein production.

Nitrogen (N) deposition on terrestrial ecosystems has increased significantly during the last decades, causing soil acidification, water eutrophication, decreases in biodiversity, and increases in greenhouse gas emissions. The most important gases involved in these processes are ammonia (NH 3 ) and nitrous oxide (N 2 O). A large portion of these gaseous emissions come from animal N excretion. In mammals, the main form of N excretion occurs as milk, feces, and urine urea. Following its synthesis, the urea equilibrates rapidly throughout body fluids, including milk and urine. Consequently, studies have shown that is possible to evaluate the urinary N excretion and N utilization efficiency using the plasma urea N and, in practical situations, using the milk urea N (NOUSIAINEN et al., 2004).
The N utilization efficiency can be affected by the relationship between N and energy in the diet. In the case of ruminants that are fed fresh forages, ruminal N losses are often high due to the unbalanced level of high degradable N and the low fermentable energy in the forage. This leads to an 1 Programa de Pós-graduação em Ciência Animal, Universidade do Estado de Santa Catarina (UDESC), 88520-000, Lages, SC, Brasil. E-mail: henrique.ribeiro@udesc.br. * Corresponding author.

ABSTRACT:
The excretion of urinary nitrogen (N), one of the most important environmental contaminants from livestock systems, is highly correlated with milk urea N content. The objective of this research was to evaluate the use of different types of corn supplementation on milk urea N in grazing dairy cows. Twelve Holstein × Jersey lactating dairy cows were divided into six uniform groups according to milk production, lactation stage and live weight. Treatments were compared according to a 3 × 3 replicated Latin square experimental design, with three periods of seventeen days (twelve days to adaptation and five to measurements). The experimental treatments were exclusively grazing (G); grazing + supplementation with 4.2 kg DM of corn silage (CS) and grazing + supplementation with 3.2 kg DM of ground corn (GC). The pasture used was annual ryegrass (Lolium multiflorum L.) and white oats (Avena sativa L.). The milk protein production increased 65 g/day in the GC treatment group compared to the G and CS groups. The supplemented dairy cows showed lower milk urea N (-2.8 mg/dL) than unsupplemented cows, but the N utilization efficiency (g N output in milk/ g N intake) did not change between treatments (average = 0.26). Additionally, there was a relationship between milk and plasma urea nitrogen concentrations (R 2 = 0.64). In conclusion, for dairy cows grazing annual temperate pastures, corn ground supplementation increased milk protein production and reduced the excretion of milk urea N, whereas corn silage reduced the excretion of milk urea N without affecting milk protein production. Key words: nitrogen excretion, ground corn, corn silage, grazing.
Dall-Orsoletta et al. inefficient utilization of forage N to microbial growth, increasing N-NH 3 absorption and urinary N excretion (PEYRAUD & DELAGARDE, 2013), which might be mitigated by using supplements containing fermentable carbohydrates, such as ground corn and corn silage. Additionally, it has been shown that energy supply has a role in regulating post-absorptive N utilization and efficiency by increasing milk protein synthesis (RIUS et al., 2010).
In south Brazil, annual ryegrass and oat are the major fresh forage species used in grazing dairy systems. These species contain high CP and, consequently, energy supplementation may be an alternative to improve the efficiency of forage protein utilization. However, the relationship between feeding strategies commonly used in dairy systems in the south of Brazil and N excretion needs to be better studied. The aim of this study was to evaluate the effect of corn silage and ground corn supplementation for dairy cows grazing annual temperate pastures on milk urea N excretion and to quantify the relationship between milk urea N and plasma urea N in this condition.
All data presented in this research were measured concomitantly in an experiment in which the objective was to assess the CH 4 emissions (DALL-ORSOLETTA et al., 2019). Thus, a more detailed description of experimental procedures can be reported there. Briefly, the experiment was performed from August to November 2016 in Lages, SC,Brazil (50.18ºW,27.47ºS; 920 m altitude) in accordance with the regulations of the Santa Catarina State University Ethical Committee (Protocol no. 43.73.09.08.16). Twelve Holstein × Jersey multiparous cows were separated into 6 homogeneous groups according to milk production (23.3 ± 6.9 kg/d), days in milk (101 ± 57.6 days), and body weight (492 ± 76.8 kg). Each group was randomly assigned to treatment sequences according to a replicated 3 × 3 Latin square design. The experimental period lasted 17 days and consisted of 12 days of adaptation and 5 days of measurement. Treatments consisted of cows with access to annual ryegrass + oat without supplementation (G) or with different energy supplementation -ground corn (GC) and corn silage supplementation (CS). The amount of ground corn was limited to 4 kg/ day to avoid drastic modifications on ruminal environment, and the corn silage was calculated to offer the same net energy lactation (NE L ) provided by the ground corn supplementation. The NE L of supplements was estimated using the equations published by INRA (2007). The quantities offered were 4.2 and 3.6 kg (as DM basis) of corn silage and ground corn, respectively.
The ryegrass (Lolium multiflorum cv. Barjumbo) and oat (Avena sativa cv. FUNDACEP -FAPAR 43) pastures were sown in May 2016 on a total area of 6.5 ha. The area was split into six paddocks, two per treatment, and was grazed by the same two cows during each period. The grazing method was strip grazing, and the herbage allowance (HA) was adjusted to obtain the same post-grazing sward height among treatments. The area allocated daily to each treatment group was calculated by estimating the pregrazing herbage mass (LANTINGA;et al. 2004). The HA above ground level in the G, CS, GC groups were 35, 30.8 and 34.6 kg DM/cow/day, respectively.
The cows were milked twice daily (7 a.m. and 3:30 p.m.). Over the last 5 days of each period, individual milk production was recorded, and individual milk samples were collected to determine the milk urea N concentrations. The herbage DM intake was estimated in each group by determining the difference between the pre-and post-grazing herbage mass values. The milk urea N was determined by infrared spectrophotometry (Dairy Spect FT, Bentley Instruments Inc, Chaska, Minnesota, USA). The plasma urea N was determined by individual blood samples. Samples were collected into vacutainer tubes without anticoagulant on the morning of the first (day 13) and last days (day 17) of each experimental period by jugular venipuncture. Tubes were centrifuged (2,000 × g for 10 min), and the plasma was then separated and stored at -20 ºC until it was analyzed for urea.
The ruminal degradable protein (RDP) balance was calculated from the difference between the supply of the truly digestible protein when the degradable nitrogen limits microbial growth (PDIN) and the supply of the truly digestible protein when the available energy limits microbial growth (PDIE) (INRA, 2007). For details about grazed herbage and supplement sampling and analyses see DALL-ORSOLETTA et al. (2019). The CP, NDF and ADF contents of grazed herbage averaged 212, 447 and 236 g/kg DM, respectively. The grazed herbage had a good nutritive value; on average, the NE L was 1.67 Mcal/kg DM and 107 g of truly protein digestible in the small intestine per kg of DM. For the corn silage and ground corn, the CP values were 61 and 85 g/kg DM, the NE L values were 1.53 and 1.82 Mcal/ kg DM, and the truly digestible protein values in the small intestine were 65 and 87 g/kg DM, respectively.
The dependent variables were subjected to an analysis of variance using the function PROC MIXED in the software SAS (version 9.3, SAS Institute, Cary, NC, USA) with a model that included the random effect of square, the random effect of cow nested in square, the random effect of period, and the fixed effect of treatment.
The milk production and milk protein production increased in the GC cows compared to the G and CS groups ( Table 1). The supplemented dairy cows showed lower milk and plasma urea N than unsupplemented dairy cows, but the N utilization efficiency (g N output in milk/g N intake) did not change between treatments. In a comprehensive review, CASTILLO et al. (2000) showed that N efficiency utilization in dairy cows decreased exponentially when N intake is greater than 400 g/day. In the current work, the N intake of G and CS treatments was approximately 400 g/day, increasing to 482 g/day in GC cows. Thus, it is logical to assume that cows receiving ground corn supplementation did not reduce N efficiency utilization because they improved their milk protein production, which was probably a consequence of greater fermentable energy intake. In contrast, cows receiving corn silage had lower metabolizable protein supply but maintained their milk protein production by reducing the N urinary excretion, as indicated by reductions in milk and plasma urea concentrations when compared to G cows.
The plasma urea N concentration recorded in this research was linearly related to the milk urea N concentration (Figure 1), and both were reduced when the RDP balance was improved (Figure 1b). The relationship between plasma and milk urea N reflects the rapid equilibration of blood urea into milk, and a positive linear coefficient is expected when values of milk urea N were not greater than 25 mg/dL (BURGOS et al., 2007). In the current study, milk urea N did not exceed 22 mg/dL. Additionally, both corn silage and ground corn supplementation improved the RDP balance, reducing plasma and milk urea N. These results may be, at least partially, explained because temperate herbage grasses are known to have protein contents usually greater than 140 g/kg DM, a value that is considered the limit for ruminal nitrogen loss (PEYRAUD & DELAGARDE, 2013), increasing plasma and milk urea N concentrations. In this study, the Table 1 -Dry matter intake, milk production, protein balance, milk and plasma urea N concentrations of dairy cows grazing annual ryegrass (Lolium multiflorum cv. Barjumbo) and oat (Avena sativa cv. FUNDACEP -FAPAR 43) without supplementation or supplemented with corn silage or ground corn.