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In the previous column, I have discussed the preference for direct selection for feed efficiency and have referred to residual feed intake as an effective way of selection for more efficient cows. In this contribution I continue with the efficiency argument, by showing other benefits associated with more efficient cows. These benefits were investigated in a study by Dr C. Arndt and co-workers, published in the Journal of Dairy Science, Volume 98 of 2015, pages 3938-3950, with the title: Feed conversion efficiency in dairy cows: Repeatability, variation in digestion and metabolism of energy and nitrogen, and ruminal methanogens.
The objective of their study was to investigate the repeatability and sources of differences in feed conversion efficiency (FCE), measured as kg milk per kg dry matter intake (DMI) of lactating cows in mid to late lactation. In Trials 1 and 2 they grouped cows, selected for high and low efficiency, in pairs of high- and low-FCE cows. In Trial 1 the repeatability of FCE during a 12-week period was determined. In Trial 2 the digestive and metabolic partitioning of energy and nitrogen (protein) in the body of the cow was quantified with a 3-day total faecal and urine collection and measurement of methane (CH4) and carbon dioxide (CO2) emissions. In Trial 3 selected ruminal microbes producing CH4, called methanogens, were studied in two pairs of ruminally cannulated high- and low-FCE cows. All cows received a TMR diet; the diet included 28% maize silage, 27% lucerne silage, 17% crude protein and 28% neutral detergent fiber (dry matter basis).
In Trial 1, the mean FCE remained different between pairs throughout the measured period of mid to late lactation and averaged 1.83 and 1.03 kg milk per kg DMI for high- and low-FCE cows, respectively. This shows that FCE is a repeatable trait. In Trial 2, high-FCE cows consumed 21% more DMI, produced 98% more fat- and protein-corrected milk, excreted 42% less manure per kg of fat- and protein-corrected milk, but per cow emitted the same daily amount of CH4 and CO2 compared with low-FCE cows. The percentage of energy intake lost in the faeces was higher (28.6 vs. 25.9%), but the urinary (2.76 vs. 3.40%) and CH4 (5.23 vs. 6.99%) losses were lower in high- than low-FCE cows. The higher loss in the faeces results because of the higher DMI in high-FCE cows, which results in more digesta passing through the digestive tract. High-FCE cows partitioned 15% more of energy intake toward net energy (utilisable energy) for maintenance, body gain and lactation (37.5 vs. 32.6%) than low-FCE cows. As a percentage of nitrogen intake, high-FCE cows tended to have greater faecal nitrogen (32.4 vs. 30.3%) and had lower urinary nitrogen (32.2 vs. 41.7%) and greater milk nitrogen (30.3 vs. 19.1%) than low-FCE cows. In Trial 3, Methanobrevibacter spp. strain AbM4 (which is a prime indicator of methanogens) was less prevalent in the 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 the lower digestive efficiency was more than compensated by greater metabolic efficiencies in high- compared with low-FCE cows. The results show that there is not a single factor, but rather a series of mechanisms involved in the differences in efficiency of energy utilization of high- and low-FCE lactating cows.