Intake of conserved and grazed grass and performance traits in beef suckler cows differing in phenotypic residual feed intake

The objectives of the study were to quantify the phenotypic variation in residual feed intake (RFI) and productivity of beef suckler cows offered grass silage during pregnancy and, to determine the effect of RFI classification on herbage intake during lactation. Data were collected on 39 Simmental and Simmental x Holstein-Friesian cows. During the winter indoor feeding period cows were offered grass silage ad libitum daily and their individual DMI was recorded over 73 d. Cow LW, BCS, skeletal measurements, ultrasonic fat and muscle depth, visual muscularity score, rumen fermentation, total tract digestibility, blood metabolites and haematological variables, as well as feeding behaviour were measured. Phenotypic RFI was calculated for each animal as the difference between actual silage DMI and expected silage DMI. Expected silage DMI was computed for each animal by regressing average daily silage DMI on conceptus adjusted mean LW0.75 and conceptus adjusted ADG over a 73-d period. Within breed, cows were ranked by RFI into low (efficient), medium and high (inefficient) groups by dividing them into thirds. Overall mean (SD) values for grass silage DMI (kg/d), grazed grass DMI (kg/d), feed conversion ratio (FCR) kg DMI/kg of LW gain (kg/d) and RFI (kg DM/d) were 9.08 (1.15), 9.98 (1.86), 24.66 (29.61), 0.00 (1.00), respectively. The RFI groups did not differ (P > 0.05) in LW or ADG at any stage of measurement. Residual feed intake was positively correlated with silage DMI (r=0.87; P < 0.001) but not with grass DMI (r= -0.03; P > 0.05). Cows in the low RFI group consumed 14% (P < 0.05) and 22% (P < 0.001) less silage than those in medium and high RFI groups, respectively. Corresponding values for grazed herbage were 3% and 2% (P > 0.05). Cow RFI groups did not differ (P > 0.05) in BCS, ultrasonic backfat or rump fat thickness, visual muscle scores or total tract digestibility, cow milk yield, calving difficulty score, calf birth weight and ADG of calves did not differ (P > 0.05) between RFI groups. Overall, this study showed that low RFI cows were more biologically and economically efficient, as they consumed less feed during the indoor winter period and a similar amount of herbage during the grazing season compared to high RFI cows, without any compromise in growth, body composition or production traits measured. (c) 2013 Elsevier B.V. All rights reserved.