Increased concentration of water-soluble carbohydrate in perennial ryegrass (Lolium perenne L.).: Evaluation in dairy cows in early lactation

Twelve multiparous Holstein-Friesian dairy cows in early lactation were used to investigate the potential of using perennial ryegrass (Lolium perenne) with a high concentration of water-soluble carbohydrates (WSC) to increase the efficiency of milk production. Ad libitum access to one of two varieties of zero-grazed herbage was given continuously for 3 weeks: treatment High Sugar (HS), an experimental perennial ryegrass variety (Ba11353) bred to contain a high concentration of WSC, harvested in the afternoon; or Control, a standard variety of perennial ryegrass (cv. AberElan), harvested in the morning. All dairy cows also received 4 kg d(-1) of a standard dairy concentrate. Dairy cows given the HS diet treatment consumed 2(.)8 kg dry matter (DM) d(-1) more than Control dairy cows (P < 0(.)01), and the DM digestibility of the diet on the HS treatment was significantly greater than that of the diet on the Control treatment (0(.)75 vs. 0(.)72; s.e.d. 0(.)010; P < 0(.)05). Excretion of urinary purine derivatives (PD) tended (P < 0(.)1) to be higher from dairy cows on the HS treatment, implying increased microbial protein flow to the duodenum, although there was no significant difference in the apparent efficiency of rumen fermentation of either dietary nitrogen (N) or DM expressed as a ratio to urinary PD. Milk yields and milk composition were not significantly affected by dietary treatment, although true protein yields of milk were higher (P < 0(.)05) from dairy cows given the HS treatment. The proportion of dietary N excreted in urine was significantly lower from HS cows, although the values were low for both treatments (0(.)20 g g(-1) vs. 0(.)27 g g(-1); s.e.d. 0(.)020; P < 0(.)05). It is concluded that increased DM intakes by dairy cows given the HS treatment led to increased milk protein outputs. With a proportional decrease in urinary N excretion, the use of perennial ryegrass with a high WSC concentration, in the context of the harvesting regime used in this study, may help to reduce N Pollution from dairy systems into which it is incorporated.