Dietary preferences of cows offered choices between white clover and 'high sugar' and 'typical' perennial ryegrass cultivars

Abstract

An experiment was conducted to compare dietary preferences of cows offered simple choices between spatially separated monocultures of perennial ryegrass cvv. AberDawn (AD) and AberElan (AE), and white clover (WC) under strip grazing. AberDawn was bred to express high water soluble carbohydrate concentrations, whereas AE has typical levels. The proposed hypotheses were that cows would exhibit a partial preference for the ryegrass cultivar with a higher concentration of non-structural carbohydrates, and that there would be an interaction between the non-structural carbohydrate concentration of the ryegrass and strength of preference for WC. Non-lactating cows were offered a pasture allowance of about 20 kg DM each day for 9 days in 3 ‘choice’ treatments: AD + WC, AE + WC and AD + AE. All pastures had similar estimated metabolisable energy contents, but AD had a higher crude protein (136 v. 118 g/kg DM) and slightly lower neutral detergent fibre (480 v. 497 g/kg DM) concentration than AE, and non-structural carbohydrate concentrations were similar in both grasses. White clover had higher crude protein (236 v. 127 g/kg DM), and lower neutral detergent fibre (317 v. 489 g/kg DM) and non-structural carbohydrate concentrations (140 v. 183 g/kg DM), and contained less dead material (60 v. 242 g/kg DM) than the grasses. AberDawn contained less (P<0.05; 578 v. 698 g/kg DM) green ryegrass than AE. Pre-grazing pasture mass (1870 v. 2010 kg DM/ha), pasture allowance (23 v. 25 kg DM/cow), and residual pasture mass (1610 v. 1710 kg DM/ha) were lower for AD than AE, and for WC were lower (P<0.05) than for both grasses. Dry matter intake, intake rate and bite size were lower (P<0.05) on AD + AE than on treatments containing WC. Cows grazing AD + AE ruminated longer (P<0.05), ruminated more (P<0.05) boli, and had more (P<0.05) ruminating chews than those on treatments including WC. The cows on AD + AE had more (P<0.05) total chews than those on AD + WC or AE + WC, but grazing time was not always significantly different between choice treatments. It is suggested that the longer ruminating times and greater number of boli processed in cows grazing only grass may reflect the slower digestion rates of ryegrass compared with clover. Cows grazing the AD + WC or AE + WC choice treatments showed a partial preference for WC, spending more (P<0.001) time grazing and consuming more (P<0.001) WC than ryegrass. Within the AD + AE choice treatment, cows spent more (P<0.05) time grazing and consumed more (P<0.05) AE than AD. Using the mean of 2 methods of estimation, the partial preferences for AD + WC, AE + WC and AD + AE were 39 : 61, 41 : 59 and 45 : 55, respectively.