In ruminant nutrition, it is still quite common to look at the general crude protein level, with only marginal attention to the amino acid content. At the same time, high CP (and carbohydrates) is accused of the increased podal diseases and mastitis, and reduced fertility (Butles, 1998; Moretti, 1991). During the last decades, plasma urea nitrogen higher than 19 mg/dl was linked to negative effects on energy balance, progesterone production, and fertility. Furthermore, plasma urea and uterine pH are considered to be negatively correlated. High CP content in the diet led to high plasma nitrogen and, consequently, higher prostaglandins (PGF2α) production and reduced embryo survival.
Considering only the CP content of the diet is very risky. Nowadays it is well known that not all protein sources have the same amino acids bioavailability. I.e. we can have high CP but a deficiency in methionine (Met) depending on the protein sources. Lack in Met is strictly correlated with fertility problems.
Another important point is the difference between urea nitrogen and milk urea as well as between bulk milk urea and the individual values.
Blaming a generic protein excess for infertility is likely to be an easy way out so as not to investigate the much more complex etiologically typical causes of the “sub-fertility syndrome” that affects dairy cows.
Holstein cows last years’ genetic selection aimed to increase daily milk protein concentration and yield: since 2007 this value is higher than 3.4% and casein is now near 2.7% (Italian data). From a practical point of view, 40 kg of milk/day production means 1 kg/day of casein. Milk protein derives mostly from the metabolizable protein (the amino acids absorbed at the intestinal level) composed by rumen microbial protein, the protein that exceeds the rumen fermentation, and a small portion of tissue protein mainly deriving from the cellular turnover of the digestive system. During the first part of lactation, the udder is the first organ that uses amino acids. Even if ruminants can synthesize amino acids from different nitrogen sources, the so-called essential ones must be ingested with the diet. For ruminants methionine and lysine are defined as limiting amino acids: their lack results in lower protein production, even if the rest of the amino acids are present at high concentrations.
Ideally, the metabolizable protein is totally used by the animal and there is no milk urea excretion. In reality, deficiency in limiting amino acids, unbalanced amino acids intake, or deficiency of fermentable carbohydrates in the rumen are very common, causing low utilization of dietary amino acids and the increase of milk urea. Results from Italian studies proved that fresh cows diets are commonly deficient in amino acids: milk urea is always present, more than 40% of Holstein cows had milk urea up to 20 mg/dl at 5-60 days of lactation (Figure 1), while quite 6% had concentration higher than 36 mg/dl, concerning level for the animal health (Figure 2). Thus, rumen-protected essential amino acids supplementation is needed: they are not used by the rumen microflora, but available for the animal at the intestinal level.
Figure 1: percentage of animals with milk urea up to 20 mg/dl, indicated by year and days from calving (Italian data).
Figure 2: Percentage of animals with milk urea > 36 mg/dl, indicated by year and days from calving (Italian data).
Individual milk protein observation can be useful to detect amino acid deficiency: Italian studies show that more than 25% of Holstein cows have milk protein < 2.9% between 16 and 75 DIM, indicating a dietary amino acid deficiency. During the first days of lactation, the animal can lose up to 17 kg of body protein to face the amino acids high requirement: another biomarker of the dietary deficiency is the reduction of longissimus dorsii muscle thickness (Van der Drift et al., 2012). Every amino acid deficiency strongly affects casein production, energy balance, immune system function, and the quality of oocytes, follicles, corpus luteum, and embryos.
The Cornell Net Carbohydrate and Protein System (CNCPS®) and the commercially available CNCPS® software platforms can help nutritionists to regulate the protein intake of the animals with an accurate study of the amino acid profile balance. At the beginning of the lactation, when the cow is not already pregnant, the diet goal is to maximize the rumen microbial protein: CNCPS® is able to efficiently quantify the amount of this kind of protein. If climatic, environmental, and managerial conditions are favorable, the dry matter intake of a fresh high genetic value dairy cow can be higher than 28 kg/day. Thus, the rumen can produce over 1600 g of metabolizable protein of microbial origin, in addition to the vegetable-derived protein from the diet that by-pass the rumen (RUP): these protein sources lead, theoretically, to a 50 kg milk production with 3.3% of milk protein.
If cows produce less milk protein than expected, this is probably due to a deficiency in one or more limiting amino acids in the metabolizable protein (MP). The best amino acids profile, very similar to the milk one, is that of the microbial protein, while the rumen undegraded protein (RUP) is far from this. The best Lys:Met ratio is currently defined as 2.7:1 (around Lys 6.74-7.10% and Met around 2.30-2.52% of the MP). To fulfill Lys and Met animal requirements, the diet has optimized rumen fermentation to maximize the microbial protein. After that, it is necessary to balance the dietary intake with rumen-protected amino acids, especially Met and Lys. This is the only way to allow the exceptional genetic potential of the animals to be expressed in the best way and to obtain the highest production, fertility, and health standards possible.For more information: email@example.comOriginal article here.