3.2. Milk production and compositionThe principal production effects observed with increasing dietary CP were linear increases in yield of milk, 4%fat corrected milk (FCM) and milk protein content inwhich lactose, solids-not-fat and total solid content werenot altered among treatments (Table 3). Dairy cowsutilise feed CP with greater efficiency than otherruminants but still excrete about 2–3 times more N inmanure than they secrete in milk (Broderick & Reynal2009). Leonardi et al. (2003) showed that feeding the18.8% CP diets, 44% of the N excreted was excreted asurine and milk urea compared with 38% with the 16.1%CP diets. This increases milk production costs plusenvironmental N pollution. Moreover, there was nofurther improvement in milk fat content and feedefficiency beyond 16% CP (P > 0.05) and dietary CPnot utilised for production is lost in the urine, the mostpolluting form of excretory N (Argüello 2011; Castroet al. 2011). Similar feed efficiency at 12–18% CPsuggested that the small response in milk yield at 12–18% CP was due at least partly to the small increase inDM intake. The further increase in DM intake at 18% CPwas not accompanied by more milk and milk yield/DMintake declined. These results were in agreement withBroderick (2003), who reported that when dietary CP