We investigated the interactive effects of foragesource and forage particle size (PS) as a free-choiceprovision on growth performance, rumen fermentation,and behavior of dairy calves fed texturized starters.Forty-eight Holstein calves (42 ± 3 kg of body weight)were randomly assigned (n = 12 calves per treatment)in a 2 × 2 factorial arrangement of treatments with thefactors of forage source [alfalfa hay (AH) and wheatstraw (WS)] and forage PS [(AH: medium = 1.96 mmor long = 3.93 mm) and (WS: medium = 2.03 mm orlong = 4.10 mm), as geometric mean diameters]. Thetreatments were (1) AH with medium PS (AH-MPS),(2) AH with long PS (AH-LPS), (3) WS with mediumPS (WS-MPS), and (4) WS with long PS (WS-LPS).Regardless of forage PS, the preweaning starter intake,dry matter intake, metabolizable energy intake, weaningbody weight, and forage intake were greater for AHcalves than WS calves. Average daily gain, averagedaily gain/metabolizable energy intake, feed efficiency,and final body weight of the calves did not differ amonggroups. An interaction of forage source and forage PSinfluenced acetate, propionate, and acetate-to-propionateratio in the rumen on d 35, with the greatestacetate proportion and acetate-to-propionate ratio, butthe least propionate proportion for AH-MPS calvesthan the other calves. The total volatile fatty acid concentrationand the rumen proportions of propionate (d70), butyrate (d 35), and valerate (d 35) were greaterin AH-MPS calves than in AH-LPS calves. Calves fedAH had greater total volatile fatty acid concentration(d 35 and 70) and propionate proportion (d 70), butlesser ruminal proportions of butyrate (d 35 and 70),valerate (d 35 and 70), and acetate-to-propionate ratio(d 70) compared with calves fed WS. The ruminal valerateproportion (d 70) was greatest in WS-MPS calvesthan the other calves. An interaction of forage sourceand forage PS influenced preweaning standing time andstarter eating time, with the least standing time forWS-MPS calves and the gre
