The effect of protein supplementation on nitrogen utilization in lactating dairy cows fed grass silage diets.

Abstract

The study set out to examine the effects of supplementing grass silage with various levels of protein concentration and degradability on dietary nitrogen (N) excretion in lactating dairy cows consuming at least 60% forage. Six Holstein/Friesian cows in early to midlactation were offered six diets comprising two levels of crude protein (210 and 290 g/kg DM) and three levels of protein degradability in the concentrate achieved using different amounts of untreated or formaldehyde-treated soybean meal. Despite a difference of almost 100 g/d in N intake, apparent fecal and milk N outputs were not significantly affected. Protein degradability also had no effect on N outputs in feces and milk. However, there was a major effect of both level and degradability of CP on urinary N output. Moreover, an interaction between level and degradability of CP was detected, such that the rate at which urinary N increases with increasing CP degradability was higher on the high-CP than on the low-CP diet. A low level of protein (150 g/kg DM in the diet) and medium to low rumen-degradable protein supplements provided a significant reduction in N excretion without compromising lactational performance (mean 24.8 kg/d), in terms of both milk yield and composition. This study also demonstrated that a high efficiency of N utilization could be achieved on low-CP diets (supplying less than 400 g N/d), with feces being the main route of N excretion, whereas an exponential excretion of urinary N was observed as N intake exceeded 400 g N/d.

Responses of milk urea nitrogen content to dietary crude protein level and degradability in lactatingHolsteindairy cows

“…Milk composition was not affected by increasing CP levels. The lack of responses in milk production and milk composition was consistent with others' observations that these did not change when dietary protein varied from 16.7 to 18.4% (Davidson et al, 2003), from 16.4 to 20.4% (Mulligan et al, 2004), from 16.4 to 18.0% (Wattiaux and Karg, 2004), and from 14.6 to 18.3% (Castillo et al, 2001). While some researchers observed significant differences in milk yield or composition with dietary protein from 13.1 to 17.0% (Frank and Swesson, 2002), it has been reported that milk production benefits from > 15% protein, but increasing the protein above 17% has no further effect (Groff and Wu, 2005), and dietary protein has only a low effect on milk fat and protein concentration (Sutton, 1989).…”

Relationship between efficiency of nitrogen utilization and isotopic nitrogen fractionation in dairy cows: contribution of digestion v. metabolism?

“…The efficiency with which feed N is converted into animal proteins, or efficiency of N utilization (ENU; animal N gain or milk N yield/N intake), is an important concern in livestock production, because it may impact on both farm economy (Godden et al, 2001) and environmental pollution (Castillo et al, 2001). However, in practice, assessing this efficiency is laborious, costly and not always possible for all animals, because it requires accurate measurements of individual feed N intake and animal body N gain or milk N yield over a long period of time.…”

Adaptations of hepatic amino acid uptake and net utilisation contributes to nitrogen economy or waste in lambs fed nitrogen- or energy-deficient diets

“…The current data suggest, therefore, that an alteration of the N metabolism to lowered nutrient supply occurs, but the magnitude differs whether either N or energy is supplied at 0.80 of the INRA recommendations. In dairy cows fed below maximal levels of metabolisable protein, only a small decrease in milk protein output was observed due to a more efficient use of absorbed AA (Castillo et al, 2001;Raggio et al, 2004). When feed energy supply is deficient; however, additional energy to support anabolism may be provided by two alternative processes: mobilisation of fat stores (Chowdhury and Orskov, 1997) and/or use of nutrients supplied in relative excess (e.g.…”

“…For instance, an increased metabolisable protein supply increased the net portal appearance (NPA) of AA (Blouin et al, 2002). However, the increases in metabolisable protein supply have increased (Blouin et al, 2002;Raggio et al, 2006) or had no effect (Metcalf et al, 1994;Castillo et al, 2001) on milk production and/or yield. Energy supplementation alone increased the net hepatic uptake of free AA SavaryAuzeloux et al, 2003), but a small milk protein yield response to the energy supplementation was observed (Rigout et al, 2003;Raggio et al, 2006).…”