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Submitted to: Journal of Dairy Science
Publication Type: Abstract Only Publication Acceptance Date: 3/10/2017 Publication Date: 5/5/2017 Citation: Zanton, G.I. 2017. Effects of experimental design and protein substitution strategy on production responses to feeding different levels of protein to primiparous dairy cows [abstract]. Journal of Dairy Science. 100(suppl 2):110. Interpretive Summary: Technical Abstract: Inferences about lactation responses to diet have been hypothesized to be affected by the use of change-over instead of continuous experimental designs. A direct test of this hypothesis has not been well studied. Additionally, when dietary protein level is changed it must occur through dilution with a lower protein ingredient, which is typically either a high-starch or high-fiber ingredient. The objective of this study was to evaluate the effects of reducing crude protein on lactation performance when dairy cows are fed diets continuously or according to a change-over experimental design. Fifty-four primiparous Holstein cows were randomly allocated to either a randomized complete block design (CONT; n=36, initial mean±SD: 129±36 days in milk (DIM), 580±40 kg body weight (BW), 44.1±3.2 kg milk), or to a replicated 3 x 3 Latin square design balanced for the effects of previous treatment (CHANGE; n=18, initial mean±SD: 129±35 DIM, 583±39 kg BW, 44.4±3.7 kg milk). Periods were 28 d in length with experimental measurements taken during days 22-28. Experimental designs were run concurrently with cows for the 2 designs managed and sampled identically with the exception of the diet changes that occurred for cows in CHANGE. Cows were milked 3x daily, received recombinant bovine somatotropin every 14 d, and were individually fed once daily a diet that was predicted to be either adequate (ADMP; 16.5% crude protein) or deficient in metabolizable protein (LOMP; 14.5% crude protein). Diets adequate in metabolizable protein contained expellers soybean meal. This was removed in LOMP and was replaced with a high-starch ingredient (STARCH; dry ground corn) or a high-fiber byproduct ingredient (FIBER; soyhulls). Contrasts for the effects of protein level (ADMP vs. LOMP) and carbohydrate source (STARCH vs. FIBER) were evaluated for both experimental designs, with P<0.05 considered significant and P<0.10 considered as trends. Dry matter intake (DMI) was greater for ADMP than LOMP in CONT, whereas DMI only tended to be greater in CHANGE (P<0.07). Yields of milk and milk protein were greater for ADMP in both designs. The yield of fat was greater for ADMP in CHANGE, but only tended to be greater for ADMP in CONT. Within CONT and between LOMP diets, carbohydrate source did not affect DMI, milk, or component yield; however, within CHANGE, cows fed STARCH produced greater yields of milk and tended to produce greater yields of milk protein. These results were directionally opposite from those observed in CONT where cow fed FIBER had yields that were numerically, though not significantly, greater. The inferences about MP status were generally similar for production measures for both experimental designs, whereas inferences about carbohydrate was affected by experimental design. |
