NLMIXED in R

Compare NLIN vs NLMIXED

Grass_new<-groupedData(y~x|Trial,
                       data=Grass2,
                       FUN=mean,
                       labels=list(x="Time", y="DMres"))      
                       
nlin<-nlsList(y~f1(x,U,D,Kd), 
              data=Grass_new,
              start=list(U=19.7,D=52,Kd=0.035))
              
nlmixed<-nlme(y~U+D*exp(-(Kd*x)),
                data=Grass_new,
                fixed=list(U+D+Kd~1), 
                random=pdDiag(U+D~1), 
                method="REML",
                start=c(20,50.88,0.02698))
                
plot(comparePred(nlin, nlmixed, level=1))
coef(nlmixed, level=1)