Effects of alfalfa and cereal straw as a forage source on nutrient digestibility, rumen microbial protein synthesis, and lactation performance in lactating dairy cows

Abstract Text: This study was conducted to investigate nutrient digestibility, rumen microbial protein (MCP) synthesis, and lactation performance when alfalfa was replaced with cereal straw in the diet of lactating cows. Forty-five multiparous Holstein dairy cows were individually fed, blocked based on days in milk (164 ± 24.8 d) and milk yield (29.7 ± 4.7 kg), and randomly assigned into one of three treatments. Three isonitrogenous diets with a forage-to-concentrate ratio at 45:55 contained similar concentrate composed by ground corn grain, wheat bran, soybean meal, and cottonseed meal and 15% corn silage, with three forage sources (DM basis): 23% alfalfa hay and 7% Chinese wild rye hay (AH); 30% corn stover (CS); and 30% rice straw (RS). The trial lasted for 14-week. The rumen MCP was estimated using purine derivatives and creatinine in urine, and metabolizable protein was the sum of the intestinally absorbable MCP plus intestinally absorbable dietary protein estimated by modified 3-step procedure. Apparent nutrients digestibilities were measured with acid-insoluble ash as internal marker. The data of animal performance are summarized in Table 1. Cows fed AH had higher MCP yield (P < 0.05) and metabolizable protein (P < 0.01) than those fed RS and CS, which may be attributed to the higher content of soluble carbohydrates in AH. Total volatile fatty acids concentration in ruminal fluid collected by an oral stomach tube was higher in AH than in CS and RS. Apparent digestibilities of nutrients were higher in AH than in CS and RS (P < 0.05). In summary, when cereal straw was used to replace alfalfa as a main forage source for lactating cows, the shortage of easily fermented energy may reduce the rumen MCP synthesis, resulting in lower milk protein yield, and lower nutrient digestibility may restrict milk production.

Table 1. Dietary composition and lactation performance in dairy cows
Item	Treatment1			SEM	P-value
	AH	CS	RS
Composition, , % of DM
Crude protein	16.7	16.2	16.0
Neutral detergent fiber	31.1	36.3	36.9
Non-fibrous carbohydrate	40.6	36.0	34.6
Dry matter intake, kg/d	18.0	18.2	18.0	0.18	0.64
Milk yield, kg/d	23.5a	19.4b	20.8b	0.52	<0.01
Milk composition, %
Protein	3.30a	3.20ab	3.10b	0.055	0.05
Fat	4.21	4.20	4.19	0.077	4.21
Lactose	4.89a	4.84a	4.70b	0.026	<0.01
Total solid	13.7a	13.5ab	13.2b	0.15	0.13

^a,b Values with different letters differ significantly (P< 0.05).

Keywords: forage source, lactation performance, ruminal microbial protein