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Submitted to: American Dairy Science Association Abstracts
Publication Type: Abstract Only Publication Acceptance Date: 4/21/2016 Publication Date: 7/8/2016 Citation: Zanton, G.I. 2016. Effect of experimental design on production responses in high-producing dairy cows fed two levels of metabolizable protein [abstract]. Annual Meeting of American Dairy Science Association, July 19-23, 2016, Salt Lake City, Utah. Interpretive Summary: Technical Abstract: Results from studies on lactation responses to diet have been hypothesized to be affected by the use of change-over (CHANGE) instead of continuous (CONT) experimental designs; a direct test of this hypothesis has not been well-studied. The objective of this study was to evaluate the effects of reducing metabolizable protein (MP) on lactation performance when dairy cows are fed diets continuously or according to a change-over experimental design. Forty-six multiparous, Holstein cows were fed a common diet for 14 d and then randomly assigned to a randomized completed block design (CONT; n=34, initial mean±SD: 2.9±0.9 parity; 84±30 d in milk; 720±59 kg body weight (BW); 58.1±6.4 kg milk/d) or a balanced 2-treatment, 4-sequence, 4-period change-over design (CHANGE; n=12: 2.7±1.0 parity; 90±32 d in milk; 709±78 kg BW; 56.7±3.9 kg milk/d). Cows were fed once and milked thrice daily, received recombinant bovine somatotropin every 14 d, and received a diet that was predicted to be either adequate (ADMP) or deficient (LOMP) in MP for 112 d or changing according to sequence for four 28-d periods. Base diet was formulated to contain (% dry matter [DM]) 37.5% brown midrib corn silage, 16% alfalfa silage, and 41% concentrate. Treatments differed by adding 5.5% expeller soybean meal to the base diet for ADMP or 5.5% soyhulls for LOMP. By design, chemical composition differed with 16.6 vs. 14.7% crude protein, 28.2 vs. 31.1% neutral detergent fiber, and 27.6 vs. 27.5% starch for ADMP vs. LOMP, respectively. Production data were analyzed for study days 22-28, 50-56, 78-84, and 106-112, corresponding to 21 d of adaptation and 7 d of measurement for cows in the change-over design. Statistical analysis was conducted using the mixed procedure of SAS as either a randomized complete block design or as a balanced change-over design in which differences with P<0.05 were considered significant. Response to dietary treatment was not different for BW, BW change, and dry matter intake, irrespective of experimental design. Milk and protein yield and milk urea N concentration were reduced with LOMP compared to ADMP for both designs. Fat yield response to diet was not different for CONT while fat % increased with LOMP. In contrast, fat yield was significantly reduced for cows fed LOMP in CHANGE with no significant difference in fat % for this design; significant carry-over effect differences were observed. Under the conditions of this experiment, inferences on the response to different levels of MP were affected by the experimental design and the production variable of interest. |
