title: Feed conversion efficiency in dairy cows: Repeatability, variation in digestion and metabolism of energy and nitrogen, and ruminal methanogens.
abstract: The objective was to study repeatability and sources of variation in feed conversion efficiency [FCE, milk kg/kg dry matter intake (DMI)] of lactating cows in mid to late lactation. Trials 1 and 2 used 16 cows (106 to 368 d in milk) grouped in 8 pairs of 1 high- and 1 low-FCE cow less than 16 d in milk apart. Trial 1 determined the repeatability of FCE during a 12-wk period. Trial 2 quantified the digestive and metabolic partitioning of energy and N with a 3-d total fecal and urine collection and measurement of CH4 and CO2 emission. Trial 3 studied selected ruminal methanogens in 2 pairs of cows fitted with rumen cannulas. Cows received a single diet including 28% corn silage, 27% alfalfa silage, 17% crude protein, and 28% neutral detergent fiber (dry matter basis). In trial 1, mean FCE remained repeatedly different and averaged 1.83 and 1.03 for high- and low-FCE cows, respectively. In trial 2, high-FCE cows consumed 21% more DMI, produced 98% more fat- and protein-corrected milk, excreted 42% less manure per kilogram of fat- and protein-corrected milk, but emitted the same daily amount of CH4 and CO2 compared with low-FCE cows. Percentage of gross energy intake lost in feces was higher (28.6 vs. 25.9%), but urinary (2.76 vs. 3.40%) and CH4 (5.23 vs. 6.99%) losses were lower in high- than low-FCE cows. Furthermore, high-FCE cows partitioned 15% more of gross energy intake toward net energy for maintenance, body gain, and lactation (37.5 vs. 32.6%) than low-FCE cows. Lower metabolic efficiency and greater heat loss in low-FCE cows might have been associated in part with greater energy demand for immune function related to subclinical mastitis, as somatic cell count was 3.8 fold greater in low- than high-FCE cows. As a percentage of N intake, high-FCE cows tended to have greater fecal N (32.4 vs. 30.3%) and had lower urinary N (32.2 vs. 41.7%) and greater milk N (30.3 vs. 19.1%) than low-FCE cows. In trial 3, Methanobrevibacter spp. strain AbM4 was less prevalent in ruminal content of high-FCE cows, which emitted less CH4 per unit of DMI and per unit of neutral detergent fiber digested than low-FCE cows. Thus lower digestive efficiency was more than compensated by greater metabolic efficiencies in high- compared with low-FCE cows. There was not a single factor, but rather a series of mechanisms involved in the observed differences in efficiency of energy utilization of the lactating cows in this study.
authors.affiliation: Department of Dairy Science, University of Wisconsin, Madison 53706.
authors.forename: C
authors.initials: C
authors.lastname: Arndt
authors.affiliation: US Dairy Forage Research Center, USDA-Agricultural Research Service, Madison, WI 53706.
authors.forename: J M
authors.initials: JM
authors.lastname: Powell
authors.affiliation: Department of Dairy Science, University of Wisconsin, Madison 53706.
authors.forename: M J
authors.initials: MJ
authors.lastname: Aguerre
authors.affiliation: Department of Computing and Biometry, University of Wisconsin, Madison 53706.
authors.forename: P M
authors.initials: PM
authors.lastname: Crump
authors.affiliation: Department of Dairy Science, University of Wisconsin, Madison 53706. Electronic address: wattiaux@wisc.edu.
authors.forename: M A
authors.initials: MA
authors.lastname: Wattiaux
mesh_terms: D000821:Animal Feed; D000818:Animals; D002417:Cattle / Q000502:physiology*; D004032:Diet / Q000662:veterinary*; D004043:Dietary Fiber; D004063:Digestion / Q000502:physiology*; D002149:Energy Intake; D004734:Energy Metabolism; D005243:Feces / Q000737:chemistry; D005260:Female; D007774:Lactation; D000455:Medicago sativa; D008892:Milk / Q000378:metabolism*; D008894:Milk Proteins / Q000378:metabolism; D009584:Nitrogen / Q000378:metabolism*; D012417:Rumen / Q000378:metabolism; D012820:Silage; D003313:Zea mays
chemical_list: D004043:Dietary Fiber; D008894:Milk Proteins; D009584:Nitrogen
keywords: dairy cattle; energy metabolism; feed efficiency; manure; nitrogen utilization